Notes on Anti-lock brakes from an e-mail posting. Any advice regarding brakes is use with discretion and at your own risk. If in doubt seek advice from a GM dealer.
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Hello;
I recently replaced a rear brake caliper on my 89 Trofeo.
The anti-lock light came on after this installation.
The anti-lock does not work - checked that in a skid test on icy
patch.
I did a brake fluid change, and bled the brakes all around. This
did nothing to solve the problem.
Is there any possibility that the caliper has to be a special type?
Picked it at the the local Parts America store...
Or am I missing something obvious here?
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I have not seen a reply to this so here goes.
Essentially the hydraulic part of the brake system and the antilock part of the system are separate. In other words bleeding the brakes has nothing to do with the antilock. The antilock works by operating one (or more) of three valves that can reduce (but never increase) the pressure in one of three lines, the two front lines and the single rear brake line (though there are in fact two lines to the rear brakes, just one valve...)
This means that a fault is in the electronics of the system most likely, or possibly one of the valves at the front. My guess is that you have damaged the wiring on the hub in changing the caliper.
The antilock sets fault codes. You can extract the fault codes using the flashing lamp method (anti-lock lamp), similar to the service engine soon light.
The rear caliper is identical to calipers I have changed that do not have antilock in every respect that I can identify from the drawings. (Except, of course, that it has the ABS speed sensor built into the hub. This cannot be serviced separatly.)
To get the fault codes, ignition off. Ignition ON. Note if anti-lock lamp goes out after 4 seconds. Continue irrespective of if it did or did not. Ignition OFF. Ground pin H of the ALDL. Ignition ON (don't start engine). Anti-lock lamp lights for four seconds, then blinks the 1-2 code then each stored trouble code. A 1-2 code is not a fault code, it indicates proper operation of the system. Codes are two digit, e.g. 22. If you cannot extract codes this way, you might have a significant problem. The system will not display the trouble codes using the anti-lock lamp under these conditions (for good reasons). You MUST diagnose with the proper scan tool. Codes auto clear after 100 ignition cycles with no code set. You can clear most codes by jumping pin H to ground three more times after the four second time out once in diagnostics. This must be completed before ten seconds elapse. Confirm success by switching the ignition off, removing the jumper and starting the whole process again. You should have just the 1-2 code. To make this process clearer, jumper pin H to ground with the ignition off. Ignition on, wait approx. 4 seconds for lamp to go out. Disconnect ground link, wait a about 1 second, put link back, wait 1 second, remove link and so on three times, slowly, but within 10 seconds.
Pin A of the ALDL is a ground pin.
HTH
Codes.
|
21 |
Right front wheel speed sensor |
|
22 |
Right front wheel toothed wheel frequency error |
|
25 |
Left front wheel speed sensor |
|
26 |
Left front wheel toothed wheel frequency error |
|
31 |
Right rear wheel speed sensor |
|
32 |
Right rear wheel toothed wheel frequency error |
|
35 |
Left rear wheel speed sensor |
|
36 |
Left rear wheel toothed wheel frequency error |
|
41 |
Right front solonoid valve fault |
|
45 |
Left front solonoid valve fault |
|
55 |
Rear solonoid valve fault |
|
61 |
Motor pump circuit fault |
|
63 |
Solonoid valve relay fault |
|
71 |
EBCM fault (sounds like bad news to me) |
|
72 |
Serial data fault (scan tool fault?) |
On the first 8 codes, the first (21, 25, 31, 35) code indicates a faulty sensor, broken wiring or such like. The second code (22, 26, 32, 36), indicates that the speed being reported is implausable, if you unscrewed one of the front sensors but left it wired in, you would get the frequency error. If you disconnect the wiring from a sensor, you will get the sensor error.
If codes 41, 45, 55, 63 are current, you cannot extract them by the lamp flash method. I don't know if this would work, but if you disconnect pin 11 from the Hydraulic Modulator Unit (HMU), possibly the lamp would be able to do this? Pin 11 grounds the ABS lamp too, there is a relay inside the HMU that switches from ground to +Batt which with a diode can bring the ABS lamp on (when switched to ground, the ignition off position). Check also all the fuses and fusible links, (fuses 3, 7, 14; links S and Z).
As a second observation, there is a connection between the Electronic Brake Control Module (EBCM) and the other car electronics on circuit 800. One wonders what communications there are between say the BCM and the EBCM. I suspect that there are none, and in fact the only role is when in diagnostics with a Tech tool, the Tech tool "talks" to the EBCM via the same diagnostic highway (circuit 800) as is used for the BCM, P/ECM and the other units. Certainly the Tech tool can display sensor speed as reported by each sensor while the car is driving. It must use circuit 800 to do this.
To make full diagnosis of the anti-lock brakes I stongly recommend that you obtain a copy of the workshop manual. GM will sell you one.