ABS Brakes work better when I pull the fuse!

[noelb]

I was nearly home today and decided to pull the ABS fuse to see if the brakes worked any better without it. The difference was chalk and cheese. The brakes work so much better with the fuse pulled. Also the pedal would go to the floor with ABS but with fuse pulled the wheels would lock.

Surely there must be some way of getting similar braking capacity with the fuse in and the ABS working.

What is wrong?

ano

[redbird2]

do you have ABS light on with the fuse in?

[DA BIG ONE]

What t-case?

[noelb]

Standard T/case (not Auto trac) ABS light is not on with the fuse in. Ignition on selftest OK.

After the last post I jacked up LHF wheel and disconnected the sensor and tested with DVM to see what sort of reading I could get. But how fast do you need to spin the wheel because the reading gets higher the faster you spin the wheel. I was able to get a reading higher than 350Mv but from memory I don't remember how high the readings got. (my memeory is clouded with info read whilst surfing the WEB)

Next day tested the brake and for the first time, since I have had the vehicle, I could actualy hear squeeling type noises from the from front wheels when ABS activated. The braking is much improved. Now this is what I am used to with ABS, but why all of a sudden a difference.

I decided to do some Surfing (WEB) to see if any other similar problems or solutions are mentioned.

I found an interesting post that mentioned what a private mechanic did when presented with a GM ABS truck exhibiting the same symptoms as mine and other vehicles I have read about on this forum. The owner of this vehicle had previously taken the vehicle to a GM dealer with no joy. (Maybe this is your problem too Robyn)

Apparently according to the mecxhanic, the ABS system has an automatic Air purging system that sometimes does not work. He removed the Master Cylinder and substituted the push rod for one that was 3/16 inch longer. They then drove the vehicle until the brakes started to lock up. They then changed back to the original pushrod and hey presto the brakes worked properly.

I am wondering whether I still had some air in the system that wasn't purged. (I have had the brake bled more than Twice by a GM dealer with no difference in braking capacity) When I removed the fuse and got the brakes to lock up perhaps this has purged the air from the system causing the brakes to operate correctly when the fuse was replaced.

Robyn I would be interested to see if, after you tried this, you noticed a difference with the ABS on your truck.

I think I might be on to something but is any of this new information to any of you?

The only other thing I can think of is that there was a faulty connection where I unpluged the sensor. I don't think this is the case though as my vehicle would have been experiencing other symptoms.

[Robyn]

The brakes on my 94 Burb are firm and feel fine but the stopping power is not there.

Robyn

[noelb]

Robyn,

The stopping power wasn't there either for mine a couple of days ago, but for some reason it's there now. If you push real hard on the pedal, whilst stationary, engine running, does the pedal go to the floor on your truck? I can't say that I have tested mine since brakes have improved but it would disappear to the floor under hard braking even after having the brakes bled twice by GM.

Just curious. I'm trying to nail this one down to something.

[DmaxMaverick]

Hydroboost systems will allow the pedal to go to the floor with enough foot pressure. You should hear and feel some hydraulic growling/hissing during the process.

ABS makes diagnosing MC problems difficult. You could have a bad MC, being masked by the ABS activity.

Have a look at THIS THREAD. Different truck model, but the operation is the same.

[Dihrdbowti]

I pulled the fuse on my ABS in the Yukon when it felt like the ABS was coming on during normal braking. It worked fine after that. I took it to a tire shop (that works on brakes) and had it checked out. They said that the ABS computer was getting a signal that the wheels were locking up. They couldn't be sure where the problem was. I brought it to the stealer who diagnosed it as the wheel hub. They explained the hub was wearing out and taking the tolerances for the wheel sensor were way out causing the ABS to engage. I had the hub replaced and low and behold no more ABS problem..

[ccatlett1984]

swapping out a plunger would just kill the seals in the MC, since it would travel further than the other plunger and might hit some junk in the bore, also you have to completely bleed the brakes when you swap a MC, let alone bench bleed it.

if you have squishy brakes the internal lining of the soft brake lines may have collapsed, I went to SS braided lines on my truck and OMG what a different. huge change in pedal firmness. and to my knowledge there is no self bleeding mechanism in the ABS system, you just have to be careful when you bleed them so the system doesnt activate, so you can get all the air out.