PDA

View Full Version : 1994 Blazer - ABS Bypass - is it possible



Nate99
03-24-2006, 09:41 AM
My brother in-law has a '94 full size Blazer 4wd, and the ABS controller has crapped out on him. The stealership want $1300 to replace, so obviously we're not going that route.

Has anyone here deleted the ABS from an OBS truck before? I would imagine that the ABS was optional in 1994, so swapping out parts shouldn't be too difficult, but if anyone knows what all might be involved, apart from scrapping the controller module and plumbing new brake lines from the proportioning valve to the calipers/wheel cylinders, I'd like to hear your thoughts.

Thanks for the help.

wilbilt
03-24-2006, 10:08 AM
I believe ABS was standard equipment beginning with the RWAL in '88. That's a pretty simple system and bypassing it would just involve one brake line.

If the '94 has 4WAL, it would be more involved to bypass it, as it has a boost pump integrtated with the control unit to assist the vacuum booster.

Nate99
03-24-2006, 10:58 AM
So would the 4WAL vaccum booster not be functional without the boost pump and related hardware? bending up new brake lines wouldn't be too difficult, but if we had to get a different booster and an adjustible proportioning valve to, in effect, make this a custom system, I'm not sure that my brother in-law would go for it. He's not much of a car guy himself, so reliability is paramount.

wilbilt
03-24-2006, 11:14 AM
I don't know if the MC/booster are specific to 4WAL trucks, but I suspect they would be as it is an engineeered system.

I wouldn't be surprised if similar trucks were sold in less-developed countries with standard braking systems, due to service and reliability issues. I don't know how you would find out if the standard US components would work as a standalone system, though.


It might be hard to find that information due to liability issues these days...:(

1badgmc
03-24-2006, 11:25 AM
I know on my '91 when my ABS box died, I just unplugged it and tossed it in the dumpster. Solved that problem.

Nate99
03-24-2006, 11:36 AM
I know on my '91 when my ABS box died, I just unplugged it and tossed it in the dumpster. Solved that problem.

We'll try that first.

On an empty street.

Seriously though, was your '91 a 4WAL or just the rears?

1badgmc
03-24-2006, 11:41 AM
Rear only.

You can just unplug the harness and see what happens. :D

lmadore
03-24-2006, 11:52 AM
Actually, my 95 K1500 w/ 4WAL has not been functioning due to a dead controller for several years. All that has to be done is unplug the harness.

Nate99
03-24-2006, 12:11 PM
Actually, my 95 K1500 w/ 4WAL has not been functioning due to a dead controller for several years. All that has to be done is unplug the harness.

When the controller initially went out did you get really poor braking performance?

kenman1717
03-24-2006, 12:25 PM
your 94 would have 4wal, and if you elimintate the abs, your wheels will just lock up when you touch the brake pedal, no biggie unless you intend to stop from any given speed.

Whifflebat
03-24-2006, 12:35 PM
your 94 would have 4wal, and if you elimintate the abs, your wheels will just lock up when you touch the brake pedal, no biggie unless you intend to stop from any given speed.


:think: They won't lock up unless you're under hard braking... all abs does is cycle the braking pressure very very quickly to prevent the wheels from locking up in hard braking situations, not all the time.

kenman1717
03-25-2006, 06:36 AM
:think: They won't lock up unless you're under hard braking... all abs does is cycle the braking pressure very very quickly to prevent the wheels from locking up in hard braking situations, not all the time.

the reason i said that, was cuase i drove a blazer a while back that they had unhooked the abs, cause it quite, and everytime you touched the pedal the wheels would lock up, this was just like backing out of a parking spot, with no cars around, you wouldn't try to stop just keep it slow, didn't matter the pressure, they just locked, and the whole vehicle just rocked back and forth.

Whifflebat
03-25-2006, 07:22 AM
That wasn't because of the ABS, other things were wrong. Bad rear axle seals come to mind.

wilbilt
03-25-2006, 08:01 AM
My brother in-law has a '94 full size Blazer 4wd, and the ABS controller has crapped out on him. The stealership want $1300 to replace, so obviously we're not going that route.


Just to clarify, you are talking about the hydraulic control unit and not the electronic control unit, correct? I assumed you meant the hydraulic unit at that price.

One of my wife's relatives has a '97 or so K2500 with a bad hydraulic unit. The "ABS" light is on all the time, and it has a low pedal. He continues to drive it like that, but it scares the cr*p out of me. It's much worse than my '88 when the RWAL unit was bad.

Frankenbiker
03-25-2006, 10:07 AM
Federal Motor Vehicle Safety Standards require that the vehicle's "foundation" brake system be somewhat separate from the ABS system as a safety feature. This means that the brakes are designed so that if the ABS system craps out, the vehicle can still be safely and normally stopped; It "defaults" to a "no-ABS" condition. You don't have ABS function at full-limit braking, which will result in wheel lock ("and possible loss of vehicle control") at full-limit braking, but everyday stopping function, by law, must not be effected by ABS failure.

Instead of dropping $1300 at the dealer, just unplug the ABS controller. This "deletes" the ABS, functionally. Then root around in one or more local junkyards for a new controller. I'd think you could get one for as cheap as $100 (just a guess) if you looked around enough.

-blaine

wilbilt
03-25-2006, 10:34 AM
Federal Motor Vehicle Safety Standards require that the vehicle's "foundation" brake system be somewhat separate from the ABS system as a safety feature. This means that the brakes are designed so that if the ABS system craps out, the vehicle can still be safely and normally stopped; It "defaults" to a "no-ABS" condition. You don't have ABS function at full-limit braking, which will result in wheel lock ("and possible loss of vehicle control") at full-limit braking, but everyday stopping function, by law, must not be effected by ABS failure.



I agree with that in the sense of an electrical failure, but a hydraulic failure is still a hydraulic failure. On the RWAL trucks, it is common to get crud trapped in the isolation/dump valve, which is part of the ABS system. You can unplug the controller and get rid of the light, but you still have a low pedal.

The 4WAL hydraulic controller is much more complex, and if it fails (internal leak, etc.), you have the same result....no ABS...but no brakes, either.

Pauly
03-25-2006, 12:46 PM
I dont thinkthe low pedal is due to the ABS hydraulics though. I dont see how an ABS malfunction either electronic or hydrulic can cause a low pedal?

MAybe I am misiong something here?

peace
Pauly

wilbilt
03-25-2006, 02:29 PM
I dont thinkthe low pedal is due to the ABS hydraulics though. I dont see how an ABS malfunction either electronic or hydrulic can cause a low pedal?

The simplest example to give here would be the RWAL isolation/dump valve as used on '88-93(?) trucks. It consists of two solenoid valves and a chamber containing a piston backed by a spring (accumulator).

It is connected inline with the rear brake line and normally does nothing. The brake pressure flows through it like it's not even there. If the ABS electronic controller dies, you could disconnect it and still have normal brakes.

In the case of a rear-wheel lockup, the first thing that happens is that the controller closes the isolation valve. This prevents any increased line pressure to the rear brakes. If the wheels are still locked a few milliseconds later, the controller then opens the dump valve, and lets some pressure bleed from the rear brakes, "dumping" it into the accumulator. When the wheels start spinning again, the controller resets and waits for it to happen again.

OK, so lets say there's somebody (everybody) that never changes their brake fluid. Let's say a piece of crud from the dirty MC reservoir makes it's way down into the isolation/dump valve and lodges in the dump valve orifice, keeping it from closing completely. This creates an internal leak in the rear brake circuit, so that every time the brakes are applied, fluid is "leaking" into the accumulator instead of being directed to the rear brakes. This causes a pedal that goes to the floor unless you pump it several times (filling the accumulator).

Unfortunately, there is not always time to pump the brakes several times when faced with a panic stop situation.

That's a simple RWAL example of an internal leak. Now consider the 4WAL, which has 3 circuits (RF, LF, and the rears), 3 sets of isolation/dump valves. and 3 accumulators. How many internal leaks are possible now due to dirty fluid, etc.?

someotherguy
03-25-2006, 10:02 PM
I absolutely agree with just unplugging it. The ABS control unit on my '94 burb is bad; it just thumps one time after a cold start, then it gives up and sets the ABS warning light. I didn't even bother unplugging it, because it gives up on the ABS after that little "self-check" thump. Then, when I was swapping to a tach cluster, I just left the warning bulb out.

By the way, you might want to do some real diagnostics on that unit. It might be something a little more simple than the controller; keep in mind the dealer wants to sell you on the most expensive parts and labor possible. I went through ALL the diagnostic steps on mine and finally came to the point that the flowchart says yeah; the control unit is bad. (The actual name for it is the Brake Pressure Modulator Valve, or BPMV for short.)

On the 96-up you can pull one of the maxi-fuses under the hood to kill the ABS. I did this on my '96 dually wrecker because the ABS unit was fried as well. Practically every single ABS code you could get, my truck had. SOooo. out comes the fuse. :D

If you want to bother troubleshooting your ABS system, I can pull up the flowchart w/the Mitchell OnDemand system; my bud has it. The trick with it is he has to print it, then I scan it. The software pretty specifically prevents you from copying the text, e-mailing, or anything. Just view & print. And it's damn expensive software!

Richard

ccreddell
03-25-2006, 10:26 PM
Yeah, I hate the ABS on mine (even tho its working). Its the prime reason I havent been able to drive it for almost two years (accident-the rears didnt help much and the front was locked up). But I'm slowly getting it back together, should be driving it by the end of April.

matt trakker
03-29-2007, 09:21 PM
Sorry about digging up an old thread, but this is what a mechanic wants to do to my 93 Blazer. I have a low pulsating pedal right now and my ABS is unplugged, since the warning light came on about a year ago. I hated the ABS anyway. I've done all I can on my own to figure this out, and I am NOT going to a dealer so they can screw me.
He tried bleeding my brakes and hardly any fluid would come out of the rears, and very little from one of the brake calipers up front. He seems to think it's the ABS unit, the big aluminum one I have disconnected. By the way, most of my braking system is new parts, save for the calipers which he says are fine, and the master cylinder. This problem feels liks something is stuck on though.
He told me it wouldn't be much to just plumb lines and bypass the whole thing, and this guy is pretty good, knows his stuff, but I just don't know what to believe right now after reading this. My brakes are intermittently stuck on and he thinks this is whats causing it....I dunno.:crazy:

matt trakker
03-31-2007, 08:21 AM
No one's tried this?

bimmerd00d
08-06-2007, 02:59 PM
When the controller initially went out did you get really poor braking performance?

It did on our '94 K1500 Suburban with 4WAL

matt trakker
08-06-2007, 08:52 PM
I ended up swapping in a salvaged brake unit, it was 200 bucks for the unit, versus about 1600 for a new one. The ABS works fine now, but I still have some issues that need to be worked out with my brakes.