VSSB/Speedo messed up since new tune

[93ChevyTBI]

I had new 4.10's put in my truck a couple months ago so that needed a new ratio put in on the tune and well as my VSSB needing to be altered. A friend of mine put 2 DIP switches on my VSSB and we calibrated it to where it's dead on. Here's the problem though: when I start it up, the speedo moves with RPM...even while in park. When I'm driving it ticks/jerks around all the time. He said his does the same thing. Is the ratio in the tune, the VSS/DRAC, for adjustment of gears and tires causing this or is it the DIP switches. Or could the VSSB bord be fried?

thanx for your help

[Blue71]

Did it do it before you added the switchs?

I did the update with the switchs and had no problems, whent from 3:43 to 4:56's

There is also a setting in the ecm constants RPM/VSS ratio that also needs to be set, But should not effect your speedo readings.

I would check the board over.

[93ChevyTBI]

No, it did not do this before the DIP swithces were installed. And we did change the ratio in the PCM to account for the tires and gear change. Seeing that they were at the same time, I don't know which it is. If the speedo was off, it would be the calibration, but it's dead on when it's not jerking around so I don't think it's the calibration. I'm thinking my VSSB is fried but hoping otherwise.

[pfk]

......
There is also a setting in the ecm constants RPM/VSS ratio that also needs to be set, But should not effect your speedo readings.




What is this setting for? and how do you calculate your new ratio?

[pfk]

I had new 4.10's put in my truck a couple months ago so that needed a new ratio put in on the tune and well as my VSSB needing to be altered. A friend of mine put 2 DIP switches on my VSSB and we calibrated it to where it's dead on. Here's the problem though: when I start it up, the speedo moves with RPM...even while in park. When I'm driving it ticks/jerks around all the time. He said his does the same thing. Is the ratio in the tune, the VSS/DRAC, for adjustment of gears and tires causing this or is it the DIP switches. Or could the VSSB bord be fried?

thanx for your help

First thing to check would be to make sure that the solder job on the dip switches is ok (ie no splashed solder or anything) and that your dip switch setting are in fact correct.

Second would be check your distributor. If I remember correctly there is a tsb for speedo jumping with rpm due to worn out distributor bushings (I'm looking for it as I'm typing)

If you could borrow a stock vss buffer from somebody, you could verify it pretty easily however I can't see how it could get screwed up just by the dip switches themselves (solder error or dip switch setting errors aside)

[Blue71]

Had it in some of my notes from one of HaulnA$$ post.

Here it is.

Quote
Be sure to change your "RPM/VSS ratio" constant when you change gears and/or rear tire sizes. The PCM needs this value to be correct to properly calculate MPH. It has nothing to do with your speedo, that is controlled independantly by the DRAC module. The formula for calculating the ratio is ((Tire Rev's/mile) X Rear end Gear Ratio)/60. To correct the speedo, you need to change some jumpers in the DRAC (Digital Ratio Adapter Controller) module. I have a file that explains how to do this if you need it. end Quote

[93ChevyTBI]

First thing to check would be to make sure that the solder job on the dip switches is ok (ie no splashed solder or anything) and that your dip switch setting are in fact correct.

Second would be check your distributor. If I remember correctly there is a tsb for speedo jumping with rpm due to worn out distributor bushings (I'm looking for it as I'm typing)

If you could borrow a stock vss buffer from somebody, you could verify it pretty easily however I can't see how it could get screwed up just by the dip switches themselves (solder error or dip switch setting errors aside)
I had it out last weekend just to look for anything obvious. Didn't find anything. However, now that I think about it, I did use a pen to push all the switches down harder to make sure they were engaged fully (the DIP switches are rocker type, not simple push up/down) and there was some play in a couple of them. At the same time, I remember my speedo behaving better. Could I push them down and superglue them? LOL!!!! that sounds so cheap/trashy just typing it :dunce: Seriously though, it's either glue the switch down or get new switches put in. I'm worried about the glue, epoxy, JB weld!!, leaking through the switch though.

what do you think??..bad switches?

[93ChevyTBI]

We (me and 94Blazer383)did the RPM/VSS ratio when he did my tune. I don't recall the exact number off the top of my head though. Is that what my problem sounds like....the RPM/VSS ratio?? We'll give it a shot and look into it then.

thanx for the help so far.

I gotta say something now...I'm sitting here reading all the posts and I see all these people online right now. It's freaking 1:00 am in the morning/late Sunday night. Are we all night owls or what?? I think it's hilarious.

[pfk]

I had it out last weekend just to look for anything obvious. Didn't find anything. However, now that I think about it, I did use a pen to push all the switches down harder to make sure they were engaged fully (the DIP switches are rocker type, not simple push up/down) and there was some play in a couple of them. At the same time, I remember my speedo behaving better. Could I push them down and superglue them? LOL!!!! that sounds so cheap/trashy just typing it :dunce: Seriously though, it's either glue the switch down or get new switches put in. I'm worried about the glue, epoxy, JB weld!!, leaking through the switch though.

what do you think??..bad switches?

I think it could be due to that,,,, easiest way for you to get around it would be just to solder jumpers in place of the switches if they are not easily available to you.

Heres a link tbi chips drac mod (http://www.tbichips.com/drac/) lol, go figure, I just looked at mine and they are all screwed up :D

[pfk]

Dammit, I think I just figured out what happens if your jumper settings are not correct :( it appears that it may fry your ecm

As per my other posts, I have been having major issues with datalog errors. I first made some headway by swapping the ecm, and just tonight realised that by unplugging the drac module, I got rid of all errors all together. While searching for problems with the truck jacked up on the hoist, I have be driving approx 70 mph while checking voltages etc. My new ECM just started doing what my old one was doing, just before I noticed that I "mirroired" the jumpers on my drac, but literally mirroired, ie I put all the jumpers the same on each side instead of opposite from each other :(

I guess I'll find out tomorrow when I get another ecm if it will all work ok or not.

[HaulnA$$]

What is this setting for? and how do you calculate your new ratio?The RPM/VSS ratio constant is how the transmission side of the code calculates MPH. It is independant of the MPH reported to the engine side of the code. basically, it works like this. The DRAC puts out two VSS signals to the PCM. One is a 2000 pulse/mile signal that is calibrated by the jumpers in the DRAC and is used by the engine code as a MPH input and shows up in the engine ALDL datastream. This same 2000 pulse/mile signal also drives the speedometer. The second DRAC output to the PCM is a 40 pulse/mile signal that is used by the transmission code to calculate MPH. It is calibrated by changing the RPM/VSS ratio. The only way to tell if it is correct is to datalog the transmission ALDL datastream.

The way I calibrate the DRAC and RPM/VSS ratio is with a GPS unit and Datamaster since with Datamaster I can datalog engine and transmission data at the same time. I calibrate the DRAC so that the MPH in the engine data matches the GPS then I calibrate the RPM/VSS ratio so that the transmission MPH matches the GPS and the engine data. A modified DRAC with switches and an emulator make it a lot easier to do. HTH

[93ChevyTBI]

so the VSSB/DRAC CANNOT fry your ecm like PFK sugested? that's a load off my mind.

Haulin', what do you think is going on with my speedo? As mentioned in first post, it starts going up and down with RPM even while in park. In drive it's very jerky....might try to post a clip of it if I can.

[pfk]

I did in fact burn up my 2nd ecm, confirmed this morning with another one. I can not however confirm that my incorrect DRAC jumper settings were the direct cause until I get rid of the errors. (I am still working on it)

[93ChevyTBI]

I did in fact burn up my 2nd ecm, confirmed this morning with another one. I can not however confirm that my incorrect DRAC jumper settings were the direct cause until I get rid of the errors. (I am still working on it)
dude, what are you doing to them?:whacko:

seriously, they are old as sin now...maybe hitting their life expectancy? I hope it doesn't boil down to "the more you mess with them, the shorter they live".

By the way...exactly how do you know when your ecm is fried?

[pfk]

Once my ecm dies, I get instant errors in datamaster with every touch of the gas pedal. When I put in a known good ecm, those errors go away. I left it alone so far today, too much work to do, but I'll post back when I figure out whats wrong with my speedo circuit.

[93ChevyTBI]

Dang!!!!! that sounds an awful lot like mine....but it runs fine. Yours running fine or just datalogging and speedo messed up?

[pfk]

Speedo is actually fine on mine, and it still runs "fine" but it is lacking after the ecm failure.

If you're using datamaster, start a log, leave it in park, and tap the gas pedal to max 10% tps over and over again, look at how many errors you get. I get one just about every touch of the pedal. With the good ecm, I get none in park.

2nd, disconnect your DRAC module, put it in second and go fo a drive while logging. It won't shift at all but I have no errors when I am driving with the good ecm and no DRAC.

I wasn't necessarily implying that your ecm is burnt, only what I know has happenned in my case. I'm dying to get back to the truck to get it figured out once and for all but I've fallen behind due all the time I have wasted on it.

[93ChevyTBI]

Well, I still haven't gotten it fixed. I hate to beat a dead horse here but something odd happened today on the interstate. As soon as I turn on cruise control...within a second....the speedo drops all the way down past 85mpg and lays there for a couple seconds before it start bouncing up and down from 78-85. I'm doing 75 mph just so you know. Why the hell would the cruise affect it like this?
I'm thinking 1 of 4 things.....
1.) bad DRAC calibration (really don't think this is it though)
2.) bad RPM/VSS ratio constant (maybe)
3.) fried ECM (maybe)
4.) fried VSS board

I don't think the cruise is at fault b/c the speedo acts up sometimes when it's off. That's the kicker..it always acts up on start up...sometimes keeps acting up, sometimes calms down and works fine. I'm stumped.

Haulin: Dude, your expertise would be greatly appreciate.

By the way...how much would a new ECM cost?

[pfk]

Well, I still haven't gotten it fixed. I hate to beat a dead horse here but something odd happened today on the interstate. As soon as I turn on cruise control...within a second....the speedo drops all the way down past 85mpg and lays there for a couple seconds before it start bouncing up and down from 78-85. I'm doing 75 mph just so you know. Why the hell would the cruise affect it like this?
I'm thinking 1 of 4 things.....
1.) bad DRAC calibration (really don't think this is it though)
2.) bad RPM/VSS ratio constant (maybe)
3.) fried ECM (maybe)
4.) fried VSS board

I don't think the cruise is at fault b/c the speedo acts up sometimes when it's off. That's the kicker..it always acts up on start up...sometimes keeps acting up, sometimes calms down and works fine. I'm stumped.

Haulin: Dude, your expertise would be greatly appreciate.

By the way...how much would a new ECM cost?


Your buffer setup is different than mine I think, however, since you did not have any problems before the mod, I would suggest trying to borrow one from somebody to troubleshoot with.

Once you have ruled out the buffer as the problem, post back and I will upload some diagrams to photobucket for you.

computers are actually quite cheap, I don't know which service # you are using but my 2 cost me $50 each (used).

I ordered a scope module for my scanner today so I can track down my problem. I have done everything I possibly can with what I have and have found nothing to this point. I'll be very happy once it is finished so I can move on to programming.

[HaulnA$$]

Well, I still haven't gotten it fixed. I hate to beat a dead horse here but something odd happened today on the interstate. As soon as I turn on cruise control...within a second....the speedo drops all the way down past 85mpg and lays there for a couple seconds before it start bouncing up and down from 78-85. I'm doing 75 mph just so you know. Why the hell would the cruise affect it like this?
I'm thinking 1 of 4 things.....
1.) bad DRAC calibration (really don't think this is it though)
2.) bad RPM/VSS ratio constant (maybe)
3.) fried ECM (maybe)
4.) fried VSS board

I don't think the cruise is at fault b/c the speedo acts up sometimes when it's off. That's the kicker..it always acts up on start up...sometimes keeps acting up, sometimes calms down and works fine. I'm stumped.

Haulin: Dude, your expertise would be greatly appreciate.

By the way...how much would a new ECM cost?There is almost no way that it is the RPM/VSS Ratio constant or the PCM. Neither has anything to do with the speedometer which leads me to my first question. Have you datalogged the engine and/or the transmission data and if so, what does the data say? I suspect the data in the engine datastream might mimmick your speedo. I suspect that your VSSB is bad. I have a modified VSSB that I use to tune with (there is a pic posted somewhere on here). I wired in seven SPDT switches so that way the two banks of jumpers always stay complemented to each other. I'm not sure what happens when you put two banks of DIP switches in and start flipping them. Get a new VSSB from a junkyard and try it. Lastly, the best wat to set the VSSB and RPM/VSS ratio is with a GPS. HTH

Edit: Here is the link with the pic of the modified VSSB. http://www.fullsizechevy.com/forums/general-discussion/performance/256313-installing-my-4-10s-rpm-vss-ratio-setting.html

[93ChevyTBI]

ok...I'll get me a new VSSB and try it.I'll post back when that's done.

Anyone have any ideas why the cruise control would affect like it does?

[pfk]

ok...I'll get me a new VSSB and try it.I'll post back when that's done.

Anyone have any ideas why the cruise control would affect like it does?

the buffer feeds the cruise with the "speed" info, if there is a bad connection in the buffer, it's possible that the voltage drop accross the circuit is affecting the speedometer.

[93ChevyTBI]

First of all...please don't no one flame me for this...I feel bad enuff posting this for everyone to see. The VSS has 2 plugs going into it...since it's been a while since DIPs were installed, I don't remember if I unplugged 1 or 2 plugs. The big 9 pin switch is plugged in but that's it. The 4 pin does not have a plug going into it. Is that the problem? Brian @ tbichips said it was the problem but I have taken the PCM out, the cassette deck out, probed around as far as my fingers can go and I cannot find the "missing" 4 pin plug....if that's what's needed. I went to a junk yard this morning and found a switch where the 4 pin WAS hooked up...so that's what's started this question. Anyone have any ideas where that 4 pin plug is, where it comes out of, etc.?

[pfk]

The 2nd plug is not used on all applications. My 95 has only one plug and here's a pic of a 93 diagram that shows the single wire for the cruise

http://i188.photobucket.com/albums/z60/pfk_ca/partialbufferdiagram.gif

[93ChevyTBI]

my truck is a 1993 but it must be a late model 1993 b/s a lot of stuff on it is 1994 style...the computer, the brake light switch, and now I'm guessing the VSS. Am I wasting my time looking for that plug? It's 100 degrees, done drank 3 beers while out there and really ready to break tha dash in half looking for it. If I'm barking up the wrong tree, I'll just stop and try another junk yard next week. By the way...what does the 4 pin plug go to?

thanx for all your help so far PFK!!

[pfk]

here's a pic of the 94 buffer

http://i188.photobucket.com/albums/z60/pfk_ca/94buffer.gif

it shows a c4 connector as well (meaning there should be a connector) however I do not remember seeing one myself (I work on them quite often) mind you I'm not always behind the dash and my memory isnt the greatest :D

here's a pic of the 95 like mine

http://i188.photobucket.com/albums/z60/pfk_ca/buffer.gif

[HaulnA$$]

First of all...please don't no one flame me for this...I feel bad enuff posting this for everyone to see. The VSS has 2 plugs going into it...since it's been a while since DIPs were installed, I don't remember if I unplugged 1 or 2 plugs. The big 9 pin switch is plugged in but that's it. The 4 pin does not have a plug going into it. Is that the problem? Brian @ tbichips said it was the problem but I have taken the PCM out, the cassette deck out, probed around as far as my fingers can go and I cannot find the "missing" 4 pin plug....if that's what's needed. I went to a junk yard this morning and found a switch where the 4 pin WAS hooked up...so that's what's started this question. Anyone have any ideas where that 4 pin plug is, where it comes out of, etc.?Not having the 4 pin plug connected would not cause the symptom you describe. If I'm not mistaken, the 4 pin plug is only used if you have ABS. My '94 C1500 does not use that connector and it does not have ABS but my '95 S-10 TBI has ABS and uses the 4 pin connector. If you don't have ABS then don't worry about trying to find the wires.

[93ChevyTBI]

Not having the 4 pin plug connected would not cause the symptom you describe. If I'm not mistaken, the 4 pin plug is only used if you have ABS. My '94 C1500 does not use that connector and it does not have ABS but my '95 S-10 TBI has ABS and uses the 4 pin connector. If you don't have ABS then don't worry about trying to find the wires.
My truck does not have ABS. Well, that's a done deal on that issue. I'll go put my dash back together now. Now back to square one.....getting a new VSS next weekend..maybe this week if I can get home early enuff.

thanx for the info guys

[93ChevyTBI]

well, just got back from 2 junk yards and didn't get anything. Found 4 VSS's but they all had just 1 bank 9and that dang 4 pin plug plugged in!!). Weird thing is mine is a 1993 w/ 2 banks. A friend of mine on here has a 94 and his has 1 bank.

Question: Can I use a 1 bank VSS and install DIP switch on it or must I get a VSS w/ 2 banks?

[pfk]

Dammit, I think I just figured out what happens if your jumper settings are not correct :( it appears that it may fry your ecm

As per my other posts, I have been having major issues with datalog errors. I first made some headway by swapping the ecm, and just tonight realised that by unplugging the drac module, I got rid of all errors all together. While searching for problems with the truck jacked up on the hoist, I have be driving approx 70 mph while checking voltages etc. My new ECM just started doing what my old one was doing, just before I noticed that I "mirroired" the jumpers on my drac, but literally mirroired, ie I put all the jumpers the same on each side instead of opposite from each other :(

I guess I'll find out tomorrow when I get another ecm if it will all work ok or not.


I just found my way back to this thread from a link in another, and felt I should update it a little.

I later found my problem with my truck and got it fixed up, I also went back and tried my pcms again, all of them work fine.

Just to be clear with what I am trying to say, if you screw up as I did with your drac mod, you will likely not fry your pcm. I can not explain why changing the pcm in my case made any difference at all only that that is what I saw at the time.

[93ChevyTBI]

I'll udate as well. Mine has gotten ALOT better....not perfect though. I'm pretty sure that after unplugging the VSSB several times, I inadvertantly caused it. Think about it...when you try to pull the plug out while pushing that thing in so it'll release, what are you really pulling on?? THE WIRES!!! I went back a couple weeks ago, just making sure everything was connected and was pushing the wires in real hard and problem almost went away. It still happens on start up and every now and then while cruising but not 90% of time like it was doing. I might pull all the wires and put new connections on them to see if that helps....like someone mentioned...a voltage drop may have caused it.

Anyone need a cluster now?

[94Blazer383]

I'll udate as well. Mine has gotten ALOT better....not perfect though. I'm pretty sure that after unplugging the VSSB several times, I inadvertantly caused it. Think about it...when you try to pull the plug out while pushing that thing in so it'll release, what are you really pulling on?? THE WIRES!!! I went back a couple weeks ago, just making sure everything was connected and was pushing the wires in real hard and problem almost went away. It still happens on start up and every now and then while cruising but not 90% of time like it was doing. I might pull all the wires and put new connections on them to see if that helps....like someone mentioned...a voltage drop may have caused it.

Anyone need a cluster now?


Did you ever figure this out? I think I have an extra Drac in the top of my garage. I also have an extra ECM, which I dint think thats what the problem is. Sounds like you are getting warm with the wire problem.

I cant believe you guys would doubt my soldering abilities.....j/k

http://i105.photobucket.com/albums/m232/tapyz250/Drac%20Module/IMGA0115.jpg

[93ChevyTBI]

The soldering was not the issue I don't think. I think it's the wires going into the plug itself...not the VSSB. That's what you pull on when when you try to unplug it

[93ChevyTBI]

hate to beat a dead horse but I just got done messing with my truck for an hour or so. Chilling with a Bud Light Lime right now. Speedo is still messed up. I started the truck up and it was bouncing around whenever I hit the throttle. I went under the truck and unplugged the VSS sensor and guess what?? The dam! thing was still bouncing around...with the sensor unplugged!! That totally baffles me. So I plugged it back and hopped inside the truck while it was still running and unplugged the VSSB behind the glove box. Needle never moved...which was kinda obvious since no connection was there. I swapped in another VSSB watching the speedo needle very closely and as soon as the VSSB plug was halfway in there, the needle was start moving again. Same story with my original VSSB/DRAC.

So....the issue is not with VSS sensor (wasted $100) at all. Can anyone help me pinpoint one of two areas? Would the problem lie between VSS and VSSB or between VSSB and wherever it goes, like PCM?

I really DO NOT want to take the dash apart to fix this. I've took the ashtray out today and probed behind it while watching the needle to see if I was hitting any loose wires..no luck. Already tried new cluster, new VSSB, new VSS sensor. What the hell could it be?

[pfk]

hate to beat a dead horse but I just got done messing with my truck for an hour or so. Chilling with a Bud Light Lime right now. Speedo is still messed up. I started the truck up and it was bouncing around whenever I hit the throttle. I went under the truck and unplugged the VSS sensor and guess what?? The dam! thing was still bouncing around...with the sensor unplugged!! That totally baffles me. So I plugged it back and hopped inside the truck while it was still running and unplugged the VSSB behind the glove box. Needle never moved...which was kinda obvious since no connection was there. I swapped in another VSSB watching the speedo needle very closely and as soon as the VSSB plug was halfway in there, the needle was start moving again. Same story with my original VSSB/DRAC.

So....the issue is not with VSS sensor (wasted $100) at all. Can anyone help me pinpoint one of two areas? Would the problem lie between VSS and VSSB or between VSSB and wherever it goes, like PCM?

I really DO NOT want to take the dash apart to fix this. I've took the ashtray out today and probed behind it while watching the needle to see if I was hitting any loose wires..no luck. Already tried new cluster, new VSSB, new VSS sensor. What the hell could it be?

It sounds like the speedo is picking up transient voltage from the secondary ignition system.

Did you check the distributor bushings for play, or the routing of the plug wires? If a spark is jumping to the main harness, it could show up on the speedo. Double check all of your ground connections as well (the one beside the battery is the one used for a lot of the dash)

[93ChevyTBI]

"Secondary ignition system"...I'm gonna go out on a limb and guess you mean an MSD. All I have ignition-wise is a Blaster coil. The plug wires are not routed at all....used zip ties to keep them away from the motor...some do touch each other. How bad is that? Ground connection beside the battery....dunno about that one but will pop the hood and check. Sorry but I don't have enough know-how to check the bushings on the dizzy.

[pfk]

"Secondary ignition system"...I'm gonna go out on a limb and guess you mean an MSD. All I have ignition-wise is a Blaster coil. The plug wires are not routed at all....used zip ties to keep them away from the motor...some do touch each other. How bad is that? Ground connection beside the battery....dunno about that one but will pop the hood and check. Sorry but I don't have enough know-how to check the bushings on the dizzy.


"Secondary ignition system" means everything on the output side of the coil (Coil wire,Cap,rotor, wires, plugs)

A lot of the time the wires drop behind the distributor and lie on top of the main engine harness as well as the harness going to the alternator on the pass side (alternator could be another culprit as well)

It is very common for corrosion to form on the ground connection to the fender beside the battery. One of the other symptoms that will show up is the gauges will move quite a bit when the turn signals are on.

If you have the distributor cap off and grab the rotor, it should move slightly up and down and rotate a little, but should not move laterally.

[93ChevyTBI]

God, I hate electrical systems!

I'll check everything you said in the morning....especially about the gauges moving with turn signals are on...that's interesting. Did I mention that turning on cruise control amplifies the issue? It'll drop to 85+ mph and hand there for up to 10 seconds.

thanx again man

[93ChevyTBI]

"Secondary ignition system" means everything on the output side of the coil (Coil wire,Cap,rotor, wires, plugs)

A lot of the time the wires drop behind the distributor and lie on top of the main engine harness as well as the harness going to the alternator on the pass side (alternator could be another culprit as well)

It is very common for corrosion to form on the ground connection to the fender beside the battery. One of the other symptoms that will show up is the gauges will move quite a bit when the turn signals are on.

If you have the distributor cap off and grab the rotor, it should move slightly up and down and rotate a little, but should not move laterally.
Well, it's not the coil wire, plug wires, alternator wire, or the grounds at the fender well. I rerouted the wires at the dizzy to make sure none of them were crossing. I'll check the dizzy cap and rotor shortly. In meantime, any other ideas?

[94Blazer383]

Well, it's not the coil wire, plug wires, alternator wire, or the grounds at the fender well. I rerouted the wires at the dizzy to make sure none of them were crossing. I'll check the dizzy cap and rotor shortly. In meantime, any other ideas?

Maybe your are getting something coming from the Tach wire? I know that your truck has a tach in the dash and mine does not. Thats one difference that I can think of. On your coil you should have 2 plugs. They should both have a red and white wire coming out of them. One pair should go to the back of the distributor and the other should run inside the truck. If you could disconnect the white wire that runs inside the truck it will disable your tach. I may just be pissing in the wind...:dunno:

You should make sure that all your grounds to the engine block are good.

[93ChevyTBI]

I'll check the coil again to see if that helps. I pulled my Haynes manual out to see if I could find anything in it. The VSSB plug contains a light blue wire that is supposed to be the connection between the gauge and the VSSB. I'm gonna take my dash apart again to trace this thing down to see if that the problem.....keeping my fingers crossed.

Your idea about the coil picking up interference from the tach is valid upon start-up. But when driving, there is no correlation..tach could be at 2000 and speedo may be jumping around from 45-80.