Help! ABS Acting Funny

[tchort]

Hi fellas, I am in need of your expertise once again. Basically, the ABS system is no longer going through it's self-test at 4 mph as it should. It still works if I hit the brakes hard, and there are no warning lights on the dash.

A bit of background: I had some bodywork done recently. The driver's side floorpan was busted. In the process, the moron cut 3 of the 4 brake lines that are routed on the driver's side, going to the ABS unit. Well, sh*t happens; they fixed the lines and bled the brakes. There are no leaks.

Now, as I mentioned, the ABS no longer goes through its self-test. Also, the brakes feel mushy. Like, there's more pedal travel before they bite.

Any thoughts?

 

[Hib Halverson]

If the ABS is not self-testing at 4 mph, then it's not working.

When you say there are "no warning lights on the dash"...do you mean neither the ABS Ative or the Service ABS lights come on during bulb check after you key up or....do you mean that the ABS Active light does not come on?

 

[tchort]

What do you mean by "ABS Active" and "Service ABS"? Are there two separate warnings lights for ABS on at '88 Vette? If so, I'll have to check which ABS light comes on during bulb check, but one does for sure.

During bulb check, I see all lights I normally see, which includes an ABS light and a "Servive Engine" light. What I meant to say is that neither the "ABS light" or the "Service Engine" light come on while the car is running to signal a problem.

Why do you say that if the ABS system isn't going through the self-test, it's not working? If I slam on the brakes, I can hear it and feel it pulsating the pedal. It also stops the tires from continuing to slide once it engages.

 

[Hib Halverson]

I say that because I've never heard of a C4 ABS won't turn on the service light when it fails to self test.

If you are feeling pedal pulsing and hearing the noise behind the seat, then obviously, the system is working. Whether or not it's working correctly might be the question.

During bulb check see if the ABS service light is coming on.

 

[tchort]

Yes, the amber ABS light does come on during bulb-check, then goes out like normal. But, like I said, the system does NOT go through its normal self-test at 3-4 mph. What gives?

Could a little bit of air in the brake lines cause this?

 

[tchort]

So... does anyone have any ideas what could be causing the ABS to fail to do its initial self-diagnosis, but still function under extreme braking conditions?

 

[Hib Halverson]

I am really puzzled by this one. It's hard for me to imagine an ABS that fails to self-test the motor but, otherwise, seems to work normally.

Just let me ask...are you really sure the unit is not self-testing? Is it possible you've just missed hearing it? Try a road test with the door over the well behind the driver seat open.

 

[tchort]

Well, I am the king of obscure when it comes to this car. You may remember the cooling fan wild goose chase I was dealing with several months ago. Anyway, I will try as you suggest. I gotta say though, that sound is very distinct and easy to hear...

I've got another theory that I thought of this morning: The FSM says that the system is designed to do its self-test at a very low speed in case it detects an actual braking condition calling for activation of the ABS system, requiring an interruption of the test. Thus, if something is telling the ABS computer that brakes are being applied hard enough to require activation of the ABS, the self-test will be interrupted and will not occur.

Good so far?

So, based on my reasoning above, what is there on this car that could malfunction in such a way as to send erroneous signals to the ABS computer?

Could it be a badly adjusted parking brake that causes a constant but weak application of the rear brakes? (I say "weak" because it's not enough to give the impression that the brakes are engaged when I'm driving the car.)

Could it be something else, like a sensor at one of the wheels?

 

[tchort]

Just let me ask...are you really sure the unit is not self-testing? Is it possible you've just missed hearing it? Try a road test with the door over the well behind the driver seat open.

Ok, I must be getting deaf in my old age. If my ears are this bad at 29, by the time I'm 60 I'll be so deaf I won't be able to hear MYSELF!! hnoes

So I went out to the car, popped open the storage compartment, took out the tray o' crap and the piece of insulation that was underneath it. Left the storage door open, started the car, moved it a little and ... :bash:bash:bash VOILA, the sound of ABS going through its self-test!!!!!

So, Hib, I stand corrected. Thanks for bearing with my dumba**.

The only thing I can say in my own defense though, is the sound of the self-test is much MUCH quieter than it used to be. Would new brake fluid cause the ABS system to operate with less noise?

 

[John Robinson]

Now, as I mentioned, the ABS no longer goes through its self-test. Also, the brakes feel mushy. Like, there's more pedal travel before they bite.



I think you should bleed the brakes again. A mushy pedal sounds to me like the system still has air in it. Until you have all the air out of the brake system you really can not judge what the ABS is doing.

If you have a mechanic (other then the one who cut the brake lines) have them bleed the brakes.

 

[LT4man]

I agree wholeheartedly with John. If you have anything BUT the pedal you used to have, have the brakes bled!

The one time you NEED them, too late if the pedal is anything but good.

SAVE THE

 

[tchort]

Yeah, the plan was to bleed the brakes again anyway because I figured the mushy brakes is a separate problem from the ABS system.

BTW, nice ride lt4man. Hopefully next year, I can join you guys at CruiseFest.

 

[LT4man]

The same fluid runs through the ABS as runs through the calipers and brake lines.

I know my hearing ain't what is used to be! But, what is?

Thanks for the compliment about my Vette. I need to take some new pics and post 'em.

If you don't mind, I will have some of the fun you are going to miss this year at CF08!

SAVE THE

 

[Hib Halverson]

So I went out to the car, popped open the storage compartment, took out the tray o' crap and the piece of insulation that was underneath it. Left the storage door open, started the car, moved it a little and ... :bash:bash:bash VOILA, the sound of ABS going through its self-test!!!!!

(sigh)

I feel better, now.
My work is done, here.