ABS issue, '98 GMC Sonoma ZR2

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  #1  
Old 12-03-09, 06:04 PM
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ABS issue, '98 GMC Sonoma ZR2

My sons GMC has no codes, with the ABS fuse out the brakes work fine, with the fuse in the ABS pump fires almost every time that he stops no matter how easy he stops. The pump seems to fire at 7 mph. I have removed the front wheel sensors and cleaned them, no change. I took it to an inspection station and asked that they diagnose it, they told me that the computer says it was the driver side wheel sensor. I replaced the sensor, no change.

It feels like the rear wheels are pulsing when the pump fires but it's really hard to tell.
 
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  #2  
Old 12-14-09, 05:07 PM
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Make sure the sensor gap is correct (most systems all you have to do is push the sensor in as far as it will go and leave it at that). Check to see if there is any tooth wheel damage on the left front (drivers side). Loose wheel bearings will also cause abs problems. Unplug the abs computer harness from the abs computer and inspect it for damaged or corroded pins. Check the alignment of the sensor to the tooth wheel. Worst case just leave the abs computer unplugged and just pump the brakes when there is loss of traction. Good Luck
 
  #3  
Old 12-14-09, 06:17 PM
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Sensor gap? It appears that it is either screwed down or it isn't unless I missed something.

I did look down inside but I didn't check all of the teeth, when I saw that it looked like new in here I didn't check it all, I probably should have.
 
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Old 12-15-09, 01:28 AM
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Ok forget the gap, several systems are just push in and gap can be incorrect, your gap is fixed. Also I might add that just because a part is new does not mean that it is good. Take a ohm reading of the other sides sensor (using a digital volt ohm meter) and compare the readings, normally a bad sensor will be open or have way to much resistance. Volt/Ohm meters are not that expensive, get a cheap one if you don't already have one, they can be used for many things.
 
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Old 12-18-09, 05:51 AM
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I hope to get back under the fenderwell this weekend. I have a volt meter and I'll check it out. It cant be completely open since that would cause the ABS to fire as soon as I hit the brake.
 
  #6  
Old 12-21-09, 05:35 PM
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The saga continues....

I replaced the drivers side wheel sensor with used one, no change
I put that original drivers side sensor on the passenger side, now the problem is different! While driving the ABS light randomly comes on and the brakes work sometimes and sometimes not. I'm wondering if both wheel sensors are bad??

I found that the connector under the passenger side is in pretty bad shape, I had to tape the 2 halves together because the clip broke. I found that the mounting surface for the passenger side sensor was really rusty, I cleaned it up and flatened it out. I checked the reluctor wheel down inside of both hubs and it looks like new. I checked for play in each wheel bearing and they are good plus I have no growl.

Why did my passenger side wheel sensor have a stainless spacer under it and the drivers side did not? I removed the spacer since I'm thinking that it is a leftover from a past replacement???

If it were up to me I'd put 2 new hubs on this thing but it's my son's truck. College budget dictates what we do to it, plus I have a friend with a recycler yard so I get free used parts.
 
  #7  
Old 12-23-09, 06:10 PM
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ABS problems are difficult, at best, to diagnose with out a scanner to watch operation. You may have, originally have had one sensor (speed signal) falling to zero before the others, this would have accounted for the ABS "event" during slow stops. Sometimes, however the difference in speed readings between "old" and "new" sensors may cause the system to still "see" a wheel lock-up {so to speak}

The best way to address would be to rescan the system to see which sensor is bad or loosing speed signal too quick quickly. I don't think replacing both with out testing is a good idea, mostly because there may be a rear sensor issue as well-this would make any "shotgun" repair dubious at best.
 

Last edited by dimples82; 12-23-09 at 06:12 PM. Reason: missing information
  #8  
Old 12-23-09, 08:52 PM
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I did have it scanned and I was told that the drivers front sensor was bad, therefore I changed it first. When I did that and it made absolutely no difference I guessed that maybe he meant the passenger side. I then installed the original drivers side sensor onto the passenger assuming that since it made no difference to the problem that it was probably good. Now that I did that, I get an intermittant ABS light which I never had before and the system seems to work sometimes and malfunction other times. It cycles thru working and not working thru the course of 1/2 mile intermittantly.

Do you have any idea why the passenger side had a little stainless thin spacer under the sensor and the drivers side did not? I also notice that the hub had been changed at one time on the drivers side and the passenger side may be original.
 
  #9  
Old 12-25-09, 10:11 PM
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The spacer is used to provide a proper air gap between the Reluctor wheel and sensor if you still have it reinstall and see if that solves the problem you are currently haveing. If that one goes away and you still have the orignal problem this is GM' s TSB on how to fix. Brakes - Low Speed (Below 5 MPH) ABS Activation
Notes

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

Supercede:

This bulletin is being revised to update the correction and warranty information. Please discard Corporate Bulletin Number 02-05-25-006A (Section 05 - Brakes).

Condition

Some customers may comment on ABS activation at low speeds, usually below 8 km/h (5 mph). Upon investigation, the technician will find no DTCs set.

Cause

The cause of this condition may be an increased air gap between the wheel speed sensor and the hub reluctor ring due to rust and debris built up on the sensor mounting surface.

Correction

Measure AC voltage and clean wheel speed sensor mounting surfaces.

1. Raise the vehicle on a hoist.

2. Disconnect both the front wheel speed sensor harness connectors.

3. Place a DVM across the terminals of each sensor connector.

4. Rotate the wheel with hand speed and measure the ACmV's. The reading should be at least 350 ACmV's.

5. If the reading is between 200 and 350 ACmV's, remove the wheel, caliper and rotor in order to gain access to the speed sensor.

6. Remove the wheel speed sensor and plug the hole to prevent debris from falling into the hub during service.

7. Clean the wheel speed sensor mounting surface using a wire brush, sand paper, emery cloth, ScotchBrite(TM) or other suitable material. Be sure to thoroughly clean the wheel speed sensor surface. There should be no rust or corrosion.

8. Check the sensor head to determine if it has been warped/distorted due to the corrosion build up or other causes. Check the mounting surface on the sensor head for flatness by placing it on the edge of a metal machinists scale or other suitable straight edge to measure the flatness. Check the sensor for flatness in multiple (minimum 3) positions/directions. If the sensor head is distorted, replace the sensor.

9. Apply (spray) two thin coats of the specified rust penetrating lubricant (corrosion inhibitor) to the complete sensor mounting surface on the bearing hub. Allow to dry for 3-5 minutes between coats. Use ONLY Rust Penetrating Lubricant, P/N 89022217 (Canadian P/N 89022218).

10. When the corrosion inhibitor is dry to the touch (about 10 minutes), apply a thin layer of bearing grease to the hub surface and sensor 0-ring prior to sensor installation. Use ONLY Wheel Bearing Lubricant, P/N 01051344 (Canadian P/N 993037).

11. Install either the original sensor or a new one in the hub and secure the sensor. Ensure that the sensor is seated flush against the hub.

12. Install the rotor, the caliper and the wheel.

13. Place the DVM across the sensor terminals and recheck the voltage while rotating the wheel by hand. The voltage should now read at least 350 ACmV's.
 
  #10  
Old 12-26-09, 08:20 AM
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ManiacMechanic.... that is some great information! I am going to take the connectors off and give it a try. I did save the spacer and that is the wheel that was really scaled and rusty. Now that I have a preceedure to testing it, it should make it real easy to diagnose.
 
  #11  
Old 12-26-09, 02:53 PM
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No problem Hotrod glad to help
 
  #12  
Old 12-27-09, 07:33 AM
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MM, do you know if GM sells a shim kit for these?
 
  #13  
Old 12-27-09, 12:43 PM
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They should still be around you will have to deal with the dealer parts dept make sure you have the last 8 of your vin written down when you call. (GM parts guys look up parts based on that)
 
  #14  
Old 12-27-09, 03:20 PM
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I understand the TSB, but I guess I'm still confused on the spacer. The way I understand it, if the hub casting has corroded the sensor is pushed away from the reluctor wheel therefore lowering the mv output. I would then think that if the spacer was installed the sensor would then be further away from the reluctor and would drop the mv output??

So is the spacer used to calibrate the output or is it used just because it is stainless and may allow the gap to remain more constant?

I plan to jack up the front and roll the hub and see what I get. Maybe I can move the sensor around to get the voltage that I need, there was a little play in the mounting once you loosen the screw. The drivers side hub looked to have been replaced before and the sensor mount looked perfect. The passenger side looked pretty bad so I plugged the hole and flat filed it, it looked to me like it was good as new when I was done.
 
  #15  
Old 12-27-09, 08:31 PM
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Stainless will act as a barrier for the extra corrosion also and to help prevent warpage.Kinda like the dissimlar metals creating a battery witch can confuse the PCU into thinking the wheel is moving faster or slower than it really is 350 acmv is just a little over 3 volts you throw in an extra 3/4 volt from electrolisis and you are out of spec real fast and confuse the comp so it throws a light at ya or erratic operation. The air gap is also part of the equation and the added space could have been fact installed due to a casting error on that part at the time of production.Thats a big part of why GM parts people want the last 8 of the vin.
 
  #16  
Old 12-29-09, 11:03 AM
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I thought that it couldn't get worse.... I was wrong!

MM, your input was key, I think that I am on the right track now. I removed the drivers side wheel and rotor and turned the hub, I got about 125mv at the plug. I removed everything and cleaned it again and reinstalled, I got about 800mv! I had to remove the rivet that holds the stainless spacer on, flat file the surfaces, polish and flatten the stainless spacer, and reassemble everything witha thin coat of waterproof wheel bearing grease. Honestly, I think that this was my one and only original problem. At this point I'm thinking that I'm golden.

So great... I'm on the right track, right??? I removed the passenger side and checked it... 0 mv! This is the side that the stainless spacer fell out of and I left it out thinking that I would get closer to the hub and generate a higher mv. Well, the thought may have been somewhat valid but the execution was a complete failure!!! Removing that stainless spacer, which BTW they are riveted in but my rivet rusted off, proved to be too little space and the sensor hit the reluctor wheel breaking the sensor and chipping a tooth on the reluctor! Who would have thought that a steel reluctor wheel could be broken by a plastic sensor, well guess what, it can be!

I reinstalled the old speed sensor and found that I got 300+ mv. I reinstalled everything and went for a test drive. Well guess what.... 1 missing tooth on the reluctor wheel is enough to fire the ABS on every wheel revolution!

I ordered a new Timken hub/ABS assembly for the passenger side. I learned thru word of mouth that the Chineese hub assy isn't worth the 1 year warranty that it comes with. Thru Ebay I can get a Timken assembly with free shipping for $118 as compared to a Chinese unit locally for $99.99. The only bad thing is the 4-9 day wait.
 
  #17  
Old 12-29-09, 11:37 AM
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But just look at all you are learning about '98 GMC Sonoma ZR2 brake systems! Pretty soon everyone will be bring their '98 GMC Sonoma ZR2s to you because they don't want to pay $700 to a dealer...you'll be rich!!!

lol

Actually...hopefully your son is right there learning all this...it IS his truck!
 
  #18  
Old 12-29-09, 06:33 PM
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Strangely enough my older son would have been doing the left side while I was doing the right side, he also has a ZR2. He rebuilds his own dirtbike motors, and has installed motors in friends vehicles. The youngest son had better get a good job to pay someone to do his work for him, dad can't always do it.

Actually my wife hounded me to get him to help but I couldn't wait. I had the time and the drive to do it then and not wait until he was ready, I had to get started.

Am I getting an education of ABS, you better believe it!
 
  #19  
Old 12-29-09, 09:19 PM
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Glad to hear your in the home strech sorry to hear about the RH side though but you got her going the right way.Computers is autos make our lves easier right LOL.I spend aprox 10k a year in classes for my 3 guys I have working for me and another 150 a month to get info from manufactures just to keep up.Glad to help you out.
 
  #20  
Old 01-01-10, 01:55 PM
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*********** IT'S FIXED!!!!!!!!!! **************

Thanks all who provided information, Maniac Mechanic, I can't thank you enough for the TSB and test info. I must say that I learned a lot on this project.

Final analasys: a little rust under the stainless spacer on the drivers side caused increased gap of the speed sensor resulting in a low speed sensor voltage.

After 4 times tearing the wheels off, replacing 2 wheel speed sensors (used), and causing one sensor to break by leaving out the stainless shim resulting in $118 worth of hub parts, I find that a smooth file could have fixed the problem for free! Live and learn!

Thanks all!!!Beer 4U2
 
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