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1994 Blazer - ABS Bypass - is it possible

This is a discussion on 1994 Blazer - ABS Bypass - is it possible within the Technical / Maintenance forums, part of the General Discussion category; My brother in-law has a '94 full size Blazer 4wd, and the ABS controller has crapped out on him. The ...

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    Registered User Nate99's Avatar
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    1994 Blazer - ABS Bypass - is it possible


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    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.
    Last edited by Nate99; 03-24-2006 at 09:42 AM.
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    There is no "t" in Dexron wilbilt's Avatar
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    Re: 1994 Blazer - ABS Bypass - is it possible

    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.

    1988 C-1500 Silverado. Olympic White. RCLB, 5.7 TBI, 700R4.

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    Re: 1994 Blazer - ABS Bypass - is it possible

    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.
    2005 GMC Sierra 1500 CC - Sold

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    There is no "t" in Dexron wilbilt's Avatar
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    Re: 1994 Blazer - ABS Bypass - is it possible

    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...

    1988 C-1500 Silverado. Olympic White. RCLB, 5.7 TBI, 700R4.

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    Re: 1994 Blazer - ABS Bypass - is it possible

    I know on my '91 when my ABS box died, I just unplugged it and tossed it in the dumpster. Solved that problem.
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    Re: 1994 Blazer - ABS Bypass - is it possible

    Quote Originally Posted by 1badgmc
    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?
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    Re: 1994 Blazer - ABS Bypass - is it possible

    Rear only.

    You can just unplug the harness and see what happens.
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    Re: 1994 Blazer - ABS Bypass - is it possible

    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.

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    Re: 1994 Blazer - ABS Bypass - is it possible

    Quote Originally Posted by lmadore
    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?
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    Re: 1994 Blazer - ABS Bypass - is it possible

    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.
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    Re: 1994 Blazer - ABS Bypass - is it possible

    Quote Originally Posted by kenman1717
    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.

    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.
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    Re: 1994 Blazer - ABS Bypass - is it possible

    Quote Originally Posted by Whifflebat
    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.
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    Re: 1994 Blazer - ABS Bypass - is it possible

    That wasn't because of the ABS, other things were wrong. Bad rear axle seals come to mind.
    Rest in Peace Jason Payne

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    Re: 1994 Blazer - ABS Bypass - is it possible

    Quote Originally Posted by Nate99
    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.

    1988 C-1500 Silverado. Olympic White. RCLB, 5.7 TBI, 700R4.

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    Re: 1994 Blazer - ABS Bypass - is it possible

    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
    Last edited by Frankenbiker; 03-25-2006 at 10:08 AM.

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    Re: 1994 Blazer - ABS Bypass - is it possible

    Quote Originally Posted by Frankenbiker
    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.

    1988 C-1500 Silverado. Olympic White. RCLB, 5.7 TBI, 700R4.

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    Re: 1994 Blazer - ABS Bypass - is it possible

    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?

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    Re: 1994 Blazer - ABS Bypass - is it possible

    Quote Originally Posted by Pauly
    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.?

    1988 C-1500 Silverado. Olympic White. RCLB, 5.7 TBI, 700R4.

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    Re: 1994 Blazer - ABS Bypass - is it possible

    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.

    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

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    Re: 1994 Blazer - ABS Bypass - is it possible

    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.




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