ABS light, no codes

[sntrym95]

95 GMC Yukon 2wd 4 door, 350 Auto. ABS light is on, comes on bright when key is turned, dims after a few seconds. Brake light comes on briefly then goes off. Brakes work fine.

Tried to pull codes using instructions here: http://www.aa1car.com/library/abs_kelseyhayes.htm

The only code I get is 12, which is expected. Outside of a real code reader, is there anything else I can try?

[ChevyTech]

If it ever activates the anti-lock when you think it should not look up this bulletin.

I would remove the front speed sensors and clean the mounting surface just as a maintenance item.

Make sure the front wheel bearings are not loose.

Bulletin No.: 02-05-25-006B
Date: January 05, 2006
TECHNICAL
Subject:
Antilock Brake (ABS) Activation At Low Speeds (Clean Wheel Speed Sensor Mounting Surface)

Models:
1999-2000 Cadillac Escalade
1995-1999 Chevrolet Silverado (Old Style)
1995-2000 Chevrolet Suburban, Tahoe (Old Style)
1995-2003 Chevrolet Astro Van, Blazer, S10
1995-1999 GMC Sierra (Old Style)
1995-2000 GMC Yukon, Yukon XL (Old Style)
1995-2001 GMC Envoy, Jimmy
1995-2003 GMC Safari Van, Sonoma
1995-2001 Oldsmobile Bravada

[sntrym95]

Thanks ChevyTech. I was hoping you would weigh in on this one. Front bearings are good. I'll pull the brakes and check the sensors.

One other item of note is that the speedometer reads a little faster than TTS Datamaster says. i.e. speedo=66, PCM=60.

I'm still in the process of trying to figure out if the DRAC was altered, the rearend was swapped, or something else. There were 32x11.5 tires on it when I got it, and I'm returning it to stock 29" tires (235/75 or 255/70).

[ChevyTech]

1995 model was the last year for a separate DRAC.

The speedometer gets its signal from the DRAC and so does the PCM so they should match. It could be lag time in the datatmaster readings, or its calculation.

The Anti-lock system can not be very picky about front to rear speed differences because the front wheel sensors do not get the benefit of the signal modification by the DRAC, for different axle ratios, but the system still works.

NOTE: Some codes may only show up with a scan tool and will not blink out a trouble code. I know the later RWAL modules were that way. I would try reading brake codes with a scan tool.

[sntrym95]

It must be the calculation because I had the cruise set. Trying to get my hands on a scan tool that will read ABS codes. None of the software packages I have for OBD1 will read ABS codes.

Does the front wheel sensor data only go to the ABS module or does the PCM read those as well?

[ChevyTech]

Originally Posted by sntrym95

Does the front wheel sensor data only go to the ABS module or does the PCM read those as well?

Only to the ABS module.

[sntrym95]

Quick update: One mechanic could not read any codes. Possible bad ABS module, but will have someone else try.

Speedometer is off a tad as GPS and PCM both read same. Need to see about getting that recalibrated if possible.

[ChevyTech]

If you want to tweak the calibration, I can give you a new jumper configuration for the DRAC module. That will change the speed signal going out of the module.

If you want me to do this, give me the three letter code on the DRAC module and the current jumper configuration.

Some people like to remove the jumpers in the DRAC and install dip switches. Other people just move jumpers with a soldering iron.

[sntrym95]

Yeah I've looked into modifying it. I'm thinking it's been messed with although I haven't pulled it. I also think there might be 3.73 gears even though the RPO code is GU6 since it runs higher RPMS than my truck at the same speed. I've got some more investigating to do, like spinning the tires and counting revs of the driveshaft, etc.

Update on the ABS - the box is bad, so I'll be hunting one down at the local pull a part this week.

Thanks for the help , ChevyTech. I'll keep you posted on the speedo/gear situation.

[sntrym95]

So I jacked up the Yukon and spun the tire (two rotations for the open diff). 3.42 gears. So the computer has the right speed, GPS agrees, and the gears are correct. So the speedo is off. I can handle that.

I looked at the DataMaster logs I had taken from my truck and the Yukon, and the RPM at the same speed (60 mph) is the same, so maybe the tach is off by a mark or two. At any rate, I think I'm good. Thanks again ChevyTech.

[ChevyTech]

You're welcome!

[sntrym95]

I finally got around to puling the DRAC since something is still not quite right. The jumpers have not been modified. They read 00010101010111 where 0 is open and 1 is closed. The 3 letter code is PYD.

From what I can tell, this is correct for 235/70/15 tire, which the Yukons were not equipped with to my knowledge, only the trucks. I'm running 255/70/15 tires, so the jumper config should be 00100101011011 according to my calculations based on the info here: http://tbichips.com/?page_id=504.

I may hit the junkyard to find another one that is either configured the way I need or one that I can modify so I don't disable my vehicle.

[sntrym95]

So before I went to the boneyard, I decided to pull the VSSB from my 95 truck (same engine, tranny, and gears) to check the pin arrangement. It was the same. So I put in the Yukon to see if it made any difference, which it did not. So that tells me the VSSB (DRAC) is working correctly in the Yukon. So the computer is correct, the VSSB is working correctly, and the GPS agrees, but the speedo and tach do not.

I'm starting to think it's either one of the goofy electrical ground things going on that makes it just a little off, or the instrument cluster is just off. I'll probably just go pick up one at the junkyard to see if it reads the same. Should be fun trying to find a 95...

[ChevyTech]

Two DRACs reading the same leads me to believe the problem is the speedometer. In my opinion, everything in the instrument panel GM put in these trucks is low end, low quality, and low accuracy.

[sntrym95]

Thanks again ChevyTech. I might swap the cluster from my truck over just to see. Then get one from the junkyard and figure out how to get the odometer correct