ABS Issues

[zig]

I posted this on another forum and got no responses. hopefully someone can offer some guidance here.

For such a simple system, this ABS is kicking my butt. I got the car about a month ago and has had the Service ABS light come on every time I drive it. Sometimes within the first mile and sometimes after a few miles. I pulled the codes and it has the three listed in the title. It may have more but I understand it can only hold three at any one time. I have cleared the codes multiple times and all three codes eventually come back. The startup self test runs, I hear the pump and feel the brakes when I start off. No ABS light ever after the startup test. Tested the accelerometer. 2.7 volts on level ground. When I rotate it the voltage changes to 5.0 volts in one direction and .5 volts in the other. Will assume that is good. Haven't seen that written on a Corvette site but did see those voltages written on a Porsche site that said they used an early model Bosch ABS system.

I'll start with Code 21 - Have tested the RF sensor and it shows 1140 ohms. Went back to the body harness connector and measure resistance there and got the same. Went to the EBCM connector and measure the circuit and got the same. Wires are good. Not shorted to ground or power and the sensor shows good resistence.

Code 32 and 36 - Have verified the wires are good all the way back to the EBCM connector. Bearings are not bad on either side. Toothed wheels are clean on both sides. Pulled the sensor and cleaned them (they were both dirty around the magnets). Cleaned the sensor seating surfaces and reseated the sensors with sealant and 86 in lbs of torque. Still set the codes. Jacked the rear and put it on jack stands. Ran it in first gear up to about 12-13 mph(I don't recommend doing this but I'm desperate). Measured .80-.85VAC on the left side and .95-1.0VAC on the right. Put a piece of paper between the sensor and hub to increase the air gap and decrease the signal level on the right side. Got them within .05VAC. Codes still set again after I cleared them. Not sure what the air gap between the sensor and the toothed wheel is supposed to be. If anyone has an idea or knows what the signal level is supposed to be at a given speed, I'd like to write that in my FSM.

Only thing I cannot do is check the ECBM wheel speeds since I don't have a Tech1/1A. I have TunerPro v5 and ALDL to USB cable but haven't been able to establish comms with the EBCM using it.

Only thing I can think is the EBCM is flakey but I haven't read anyone having problems like this. Any other ideas?

Mike

[tpr60]

I've had a couple GM trucks that set abs codes because of hub/bearing assy. being worn/loose. Since c4 has a problem with hubs after hard use, I'd be checking them next.

[tomtom72]

I don't know much, but I have an abs cartridge for a snap-on MT2500 scanner. I bought it as it is supposed to be specifically for the early GM 3 channel ABS system. I could send the cartridge if you know anyone with a MT2500 you could borrow the scanner from. The best is if you know some one with a Tech 1 or 1A or even a Master Tech machine.

sorry that's all I can contribute.


Tom

[scottfab]

Originally Posted by tpr60

I've had a couple GM trucks that set abs codes because of hub/bearing assy. being worn/loose. Since c4 has a problem with hubs after hard use, I'd be checking them next.

This looks to be a great place to start, hubs and slop and/or cleaning around the sensors looking for ferrous material near the sensors.

By the OP description this is going to be an intermittent issue. Always fun. I'd stay focused on the first error that comes up, 21.
Good luck.
You have an opportunity to add to the collective knowledge here when you find the cause. Take pictures

[zig]

tpr60 - Hubs are good and I can't detect any play in them. Car only has 12,000 miles on it and is tight as a drum with no rattles, clunks or vibrations.

Tom - Don't know of anyone off hand but I'll start asking around. Got a couple of people in small repair shops that may have one.

Scott - Intermittent is never good and I think you're correct. One of the reasons I wanted to connect to teh EBCM was to monitor the speed signals to see if the signals were dropping out. Maybe a corroded wire internal to the insulation. Had a batterry cable that was corroded like that and would intermittently kill the battery driving down the road. I'll take some pictures when I pull everyhting apart again but the hubs, toothed wheels ans sensors looked nearly new when I put them back on.

Mike

[Franke]

Zig, after reading these posts and thinking a bit of all you've done it would seem to me that you have at least 3 - probably more codes stored. Since the 3 codes you can read are from three seperate wheel sensors and are frequency related etc, i'm thinking the common item to all is the ECBM. I don't think you are having toothed wheel issues or mag pickup issues. Can you monitor battery voltage at the EBCM when the failure occurs? Can you verify good ground at the EBCM? If that checks out I would try another EBCM if one is to be had. IMHO

[zig]

Originally Posted by Franke

Can you verify good ground at the EBCM? If that checks out I would try another EBCM if one is to be had. IMHO

All the times I have recommended checking the ground and I failed to follow my own advice. The grounds I have checked while I had the plenum off were good but you never know. I'll find the ground point location in the FSM and check it. The frequency errors and possible bad EBCM make sense to me also. If not a bad ground, it may be a bad reference signal in the EBCM. I have cleared the codes about 10 times and only those three set. And I've actually had the two codes for the rear set and no the 21 for the front. I'm areeing more and more with a bad EBCM.

[zig]

Just ordered an EBCM from Shawn (Sparky) at CorvetteNutz. He has been parting out a 91 ZR-1 with low miles and I have received some great parts and customer service in the not so distant past. Highly recommended if you need parts.

[scottfab]

Originally Posted by zig

All the times I have recommended checking the ground and I failed to follow my own advice. The grounds I have checked while I had the plenum off were good but you never know. I'll find the ground point location in the FSM and check it. The frequency errors and possible bad EBCM make sense to me also. If not a bad ground, it may be a bad reference signal in the EBCM. I have cleared the codes about 10 times and only those three set. And I've actually had the two codes for the rear set and no the 21 for the front. I'm areeing more and more with a bad EBCM.

It's a long shot but have you pulled the connector to the EBCM and plugged back in? And have you pulled the codes from the ECM lately?
Could be a hint there too. Sorry to say this sure smells like an issue that going to stink for a while unless you can get hold of some parts to swap out.

[zig]

Pulled the connector a couple times. All clean and shiny. I have been working with my TunerPro software so I connect a few times a week. No codes. Looking for a Tech 1A now. Seems they are most mandatory.