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ABS questions

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Grumpyhead123

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Austin, TX
My 91 Suburban has a primitive ABS system on the rear circuit. I know it's there but not how it functions. There is an electronic control box next to the master cylinder (yellow box) and what looks like an additional proportioning valve (white box) under the master cylinder.
The two big questions are how does the system detect wheel spin since there are no sensors on the rear wheels and can I just remove the entire old system, re plumb all the lines and install a new proportioning valve like this https://www.summitracing.com/parts/SUM-G3906
Thanks,
David IMG_1630.jpeg
 
Grumpyhead123 said:
My 91 Suburban has a primitive ABS system on the rear circuit. I know it's there but not how it functions. There is an electronic control box next to the master cylinder (yellow box) and what looks like an additional proportioning valve (white box) under the master cylinder.
The two big questions are how does the system detect wheel spin since there are no sensors on the rear wheels and can I just remove the entire old system, re plumb all the lines and install a new proportioning valve like this https://www.summitracing.com/parts/SUM-G3906
Thanks,
David View attachment 305424
Click to expand...
1) no idea how it works
2) yes you can replumb it

3) the system will still be looking for the controller after you remove it. I suspect the ABS light will stay on. It’s probably illegal in CA to remove it because, well, it’s CA
 
I believe the system uses the output of the Vehicle Speed sensor on the transfer case to look for "unrealistic" changes in rear wheel speed. There is a VSS wire running to that control box.

The system is easily removed, and will not trigger a light if it's fully stripped out. I pulled the system out of my 1990 K5 and have no problems, and low fluid still triggers the light.

No new parts are needed, you can delete the solenoid block and run the rear line directly to the combo valve.

I have some photos on my build thread: https://ck5.com/forums/threads/ugly-truckling-build.315442/page-15

-Rob
 
I deleted the craptastic and on my 91 as well. Pretty simply I went to the boneyard and got the lines from a pre-abs truck from the master to older style proportioning valve is got from the yard. Only thing we had to do was cut and reflare the line to the rear as the tube nut on the rear line didn't match up sizewise. Pretty simple to swap the nut and reflare. Rebleed the brakes and call it a day. Then throw the abs garbage in the trash where it belongs.
 
Never had an ABS equipped squarebody. Does the ABS actually work decently? Or is the issue that being old at this point is doesnt work as before and its an easier fix to just delete?
 
muddybuddy said:
Never had an ABS equipped squarebody. Does the ABS actually work decently? Or is the issue that being old at this point is doesnt work as before and its an easier fix to just delete?
Click to expand...
The abs on a squarebody was just for the rear wheels. Meaning it's not as effective as 4wheel abs. The biggest problem is that all the main components are discontinued from GM and not supported by any aftermarket parts house/supplier. If you have one that works, great. If it stops working peel it off and chuck it in the can. One point of note though, if it's not functioning and not leaking, one could leave the parts in place and it won't impact the regular brakes.

In my case, the prop valve was leaking and also not allowing fluid to flow to the rear circuit. This made bleeding the rear impossible. So off it went.

Oh, and if you think you can use the rear wheel abs stuff off of an early GMT400 truck, you can't. Different part numbers and zero cross compatibility to the squarebody stuff. Very similar, but not the same.
 
RivMan said:
I believe the system uses the output of the Vehicle Speed sensor on the transfer case to look for "unrealistic" changes in rear wheel speed. There is a VSS wire running to that control box.

The system is easily removed, and will not trigger a light if it's fully stripped out. I pulled the system out of my 1990 K5 and have no problems, and low fluid still triggers the light.

No new parts are needed, you can delete the solenoid block and run the rear line directly to the combo valve.

I have some photos on my build thread: https://ck5.com/forums/threads/ugly-truckling-build.315442/page-15

-Rob
Click to expand...
I have this system on my '90 K5. I don't think it is doing anything. I swapped a 2002 AAM 10.5 with discs in. It needs an adjustable prop valve, way too much rear brake. Bought a willwood one, hoping to replace the ABS valve with it. I do have a good inverted flare tool so this shouldn't be an issue. I did a VSS upgrade on the swapped NP205 to make the ECM happy, so all the pieces are there like factory. When you stripped out the ABS parts did you just unplug the harness that feeds the module? I have no warning lights on the dash and would like to keep it that way.
 
HankScorpio said:
I have this system on my '90 K5. I don't think it is doing anything. I swapped a 2002 AAM 10.5 with discs in. It needs an adjustable prop valve, way too much rear brake. Bought a willwood one, hoping to replace the ABS valve with it. I do have a good inverted flare tool so this shouldn't be an issue. I did a VSS upgrade on the swapped NP205 to make the ECM happy, so all the pieces are there like factory. When you stripped out the ABS parts did you just unplug the harness that feeds the module? I have no warning lights on the dash and would like to keep it that way.
Click to expand...

I removed the box, and stripped all of the wires pinned to it out of the engine harness.

-Rob
 
Yep. Unplugged the module and pruned the wiring out. No abs light on mine after the retrograde.
 
Bent77 said:
the ABS light will stay on. It’s probably illegal in CA to remove
Click to expand...

Believe it or not, There is almost zero regulation and enforcement for passenger car brakes in California. The only rule is the brake lights have to work. hence the famous brake light inspection. You can buy a car from the junkyard with four tires and an engine, have a hole in the floor and you stop your car with your feet like Fred Flintstone - it's 100% legal as long as the brake lights work
 
fordsucks! said:
Believe it or not, There is almost zero regulation and enforcement for passenger car brakes in California. The only rule is the brake lights have to work. hence the famous brake light inspection. You can buy a car from the junkyard with four tires and an engine, have a hole in the floor and you stop your car with your feet like Fred Flintstone - it's 100% legal as long as the brake lights work
Click to expand...
It's actually brakes and lights.
No brake lights.
You need to get the brakes inspected by an authorized shop.
It's a joke though, I bought a car from the junkyard put new brakes on it and went to test, the guy put 50% brake pads.
I asked him if he looked at them and he said yeah it doesn't matter, 50% is good enough. :doah::surepal:
 
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