ABS pump fubar? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 07-11-2015, 03:13 AM   #1
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Default ABS pump fubar?

So... long before the brake lines on the '04 Grand Prix GTP Comp G turned into peat moss, I was having a problem with the brakes... under hard braking, the front right wheel would lock up intermittently (gee, thanks ABS) and I noted that the pads wore faster on that side. Then the ABS computer started to complain. I took it to have the ABS computer read and had a bad ABS speed sensor on the left front. So, both front hubs got replaced with the best hubs I could source... either Moog or Federal Mogul. Car still wanted to lock up the front right wheel... and then ABS started complaining again... this time a rear speed sensor. I replaced both rear hubs with good ones. Then the ABS computer went nuts and the errors indicated the ABS module was bad... I removed the ABS module from the ABS pump and shipped it to be repaired since its a $800+ part.

I got the ABS module back, installed it and while swapping the rack and pinion due to a pinhole leak in one of its lines, I brushed a brake line and it immediately started leaking. Subsequent investigation showed most of the brake lines where ready to leak... so nearly all the lines have been replaced except the few sections of line with special bits like flexible steel line and armorguard. Due to rust issues and a broken bleeder, the front rubber lines and rear calipers have also been replaced.

I also observed that the system is split diagonally, instead of front - rear. So I suspected that the reason the front right wheel was locking up is that the other diagonal part of the system was not working... and since its the master cylinder that divides the system I replaced that, suspecting that there was an internal leak.

Unfortunately, due to life, money, weather, etc., these repairs have occurred over the last two years and that car has only moved a few thousand miles.

Today... after fighting and fighting with these brake lines, they finally bled without leaking. I already noticed that the new master cylinder is leaking out the rear, so that'* going to need replacing with 0 miles on it. Unfortunately, after taking the car out for a test drive, I am totally unhappy with the soggy petal and the fact that the front right wheel STILL F'ING LOCKS UP! Then the ABS computer freaked out.

To recap... the only brake parts not replaced... front calipers, pads, and rotors, rear pads, rotors and rubber lines, about 6 feet or steel line, the power booster, the ABS pump section and the brake petal. I've replaced the master cylinder, ABS module, 15 feet of steel line, front rubber lines, rear calipers.

When bleeding the system, it did seem like the LF and RR bleeders did not have the flow that the LR and RF bleeders did... but the only thing left in the hydraulic system is the ABS pump... which I assume is super special because of the Comp G package. Also, even though the master cylinder is full of fluid, the car is reporting "Low Brake Fluid", so this is just one more reason to replace the brand-new master cylinder. I'd try to get better brake feel by bleeding the car some more, but if I have to replace the ABS pump and master cylinder (again), I'd rather wait.

Any thoughts on my situation or logic here?

BKNJ
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Old 07-11-2015, 12:12 PM   #2
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Kind of hard to diagnose this, without ABS codes.......

The ABS pump, is not a pump, it'* a series of solenoids....the other half is the controller....your pressure is created by the MC....

Have you tried to go to a dealership?

What you need is a road test, with a Tech II scanner attached, and record a snapshot while braking normally, and in ABS mode.......then replay the snapshot of all wheels, looking at speed data(I graph them on a computer, looking for spikes of misinformation)....

Like I said, without codes, really hard to diagnose......one thing I have not seen replaced, is a wiring harness.....

Sometimes an intermittent problem is diagnosed as a speed sensor, when it fact, it is the wiring harness to it.....in some cases, you have water intrusion, and the harness AND sensor have to be replaced.....
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Old 07-11-2015, 04:15 PM   #3
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Well, I still have to finish buttoning up the car for an extended drive. Right now I have no front sway bar, a few bent-wrong brake lines without clips, zip tied sway bar links and somewhat loose subframe bolts. I was only able to do a short test drive... and now that I no longer have any leaky unions, I can put most of the car back together and probably take it to the local Advanced, as I know their scanner can read ABS codes.

Honestly, I'm not sure its going to produce any codes, as the ABS computer only freaked once during my test and after restarting the car, it remained quiet. The only warning being the incorrect "Fluid low" alert.

Good heads up on the wiring harness. I do believe the harness looks a little questionable were it branches off to the small connector in the rear of the ABS pump (I understand this is not really its proper name, but it seems like what people like to call it... just like "cable modems" which are not really modems.). I had forgotten about that. Can I test that lead with a voltmeter and get any meaningful idea if it has a break?

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Old 07-12-2015, 12:34 PM   #4
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There is only one way you can really test the circuits to each wheel, and that is with a Tech II scan tool......you take a snapshot of the data as you manually turn the wheel....if ok then you do it while moving the harness to see if you get a "blip" in the data......

But you need a code for a starting point.....as for the fluid level sensor, if the level is ok(above the fill line on reservoir), disconnect the harness to the level sensor on the reservoir and see if the light goes out....if it does, bad sensor....
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