ABS Brakes on 98 Tahoe??
 

Any experts on ABS brakes? My friend has had a problem for sometime now.. it seems that the ABS motor will actuate when the wheels are turned slightly.. you dont even need to apply the brakes.
He had it to a service shop last summer and the code in the computer said bad wheel sensor.. they replaced it and it continued, then they told him that you need to replace the sensors in pairs. But it seems GM will sell them seperately, or at least the shop, non GM replace one sensor..

Any help is appreciated.. this vehicle will be our tow vehicle to LOTO for the Shootout..

Thanks

Dick

Dick;

Do you mean that the ABS turns on when you steer the wheels, or when you roll the wheels?? I have never heard of this problem while steering the front wheels. If you are referring to rolling the wheels, I have not heard of this problem with G.M. vehicles. I have seen it happen to some Ford models though. I have found that it is caused by a sensor failing. Most times it wont even store a code. But, you said that this truck had a code stored. Can you send me the code #?? Is the ABS light on now? Can you scan for any current codes? Remember, a 1998 truck is OBD II, the codes can not be cleared by simply disconnecting the battery as in older cars and trucks. A scanner must be connected to read and clear codes. Give me as much information as you can, I can try to help you out on this one.

Also, whoever told you that wheel speed sensors have to be replaced in pairs does not know what the hell he is talking about..

Two possible problems-I have seen both.

The wheel bearings have a slight amount of play and during braking the sensor is losing contact due to too much air gap. But most of these have the sensor as part of the bearing so you may have already replaced the bearing.

If the bearings are tight or new then it is the brain box. If you monitor the wheel speeds on a scanner as you come to a stop you will see the frt wheels drop to 0 when the rear wheel speed is still 6-7 mph-this is why the ABS activates. If the bearings are good it needs a brain

checkmate454mag,

Thanks for the reply!! It is my buddies truck. Last year we drove to Loto with it and when you applied the brakes the ABS motor would run. It was intermittent. Well I talked to him about using his truck this year and he tells me when the truck is at slow speed and you turn the steering wheel with no brakes applied the ABS motor runs.. I said "What??"
Last year he took it to some garage. They read the code and it told them "bad sensor".. so $600 later they bring the truck back and he drives it.. same deal.. ABS motor runs when you apply the brakes.. not sure if the steering wheel had to be turned or not. So the guy says let me keep it here and check.. then he comes up with the change in pairs story.. I figured if that was true they would sell them in pairs.. go figure. I figured they didnt know what they were talking about.
I am thinking that by turning the steering wheel a wire is shorting out and turning on the ABS motor. I have not looked at it so I cant tell. Any clues as why that motor would run without the brakes being applied?

I sure appreciate the help..he has been pulling his hair out. and driving with some dangerous brakes..

Dick

I have been through the WHOLE THING !!!!!!!!!!! Trust me when I say this,,, disconnect the ABS and pull the dash bulb !!!! It's not worth the hassle !!!!!!!!!

We have driven without ABS for years, you wont miss it !!!!!!!!
Disconnecting it reverts the truck back to regular brakes, and wont cause anyproblems.

insptech,
Thanks for the reply!! He has had one side replaced and you are correct the sensor is in the bearing. But this thing goes off without the foot on the brake.. just by turning the steering wheel a little bit?? Does that ring a clue. I told him we need to go to Auto Zone and use there scan tool. He wants to fix it right, but is afraid people will just change parts till the whole truck is new again.. you know the drill ,, diagnose by replacement..
I will have him check your suggestion out.. any knowledge he has will help him when talking to a mechanic..

Thanks.

Dick

obnoxus,
Thats what I told him to do. But he would like to fix it right.. but if there is a problem with this year and make of vehicle.. then I can see the disconnect theory, but I havent heard of that many problems, with them. But then I am driving a new 1989 Blazer that I just built..the anti locks work pretty good on that one.. pump pump pump..

Thanks

Dick

Dick;

I just looked to see if there are any technical service bulletins on this one, and of course none came up. I have never seen a problem like this one. Turning the steering wheel makes the ABS pump activate!!! THATS BIZZARE!! There are no wires in the column that are in the ABS circuit, with the exception of the brake light switch, which is not actually in the column but it is pretty close to it. I don't think that can cause the problem you have. I have the wiring diagram for the circuit. I tried to attach it here on the board, but it came out to small to read. I will send it to you directly. Try to get me any stored codes. I can get any information that you need on this. Good Luck.

If it is doing it with the brakes off I wonder if the brake pedal switch for the ABS is stuck on?? Thsi would tell the unit that the brakes were applied and when you turn the wheel speeds are different-voila ABS unit activates. You need to check it with a scanner while operating.

P.S. It may not set codes-the system may think it's fine.

i work at a chevrolet dealership, and found when that happens it is usually the front hub on the right side that causes the problem. now dont get me wrong i have seen the other side bad but usually the right hub bearing assembly

Doctor Dave,

I believe that is the side that was replaced, right side wheel bearing. And it still would do it. Then the guy came up with the story of "replacing them in pairs".

I think insptech has a good point, "You need to check it with a scanner while operating."
I am hoping that will give a clue as to what the computer is seeing or it's response to the inputs.

My buddy emailed me with some other info.. I will post it here to see if it helps..

1. During all this, the ABS light on the dash has never come on.
2. The pump motor will start running when the truck is moving forward at 5 to 10 mph. Steering wheel can be straight or slightly turned left or
right. With the wheel turned, it will normally start the pump sooner. No foot on brake pedal required!
3. With no foot on brakes, the pump will run for an average of 5 to 12 seconds before shutting off. If you tap the brake pedal while you hear it
running, it will normally shut off within 1 or 2 more seconds.
4. I have not noticed any severe braking problems at speeds over 35 mph. The pump normallly activates on its own at speeds of 3 to 15 mph.
Slow speed smooth stops are the main problem.

My buddy is going to hit Auto Zone to find out if they can monitor or check for codes in the computer. I will let you know what he finds out..

Thanks again for all your help guys!!! :D :D
This board it great!!! I know it isnt boat related, but the truck is used for towing, so it related to some respect..

I will keep you posted.

Thanks Dick

Are we talking the low hum of the pump or are we talking the loud rattle of the ABS system activating?? Does it vibrate the pedal when you are applying the brakes??

insptech,
From what he told me, when you hear the noise (ABS motor running) if you apply the brakes, they pulse and get mushy like they should when it is working properly.. At least when I drove the truck last year, you could feel the ABS working on slow speed braking. I never did hear the motor actuate without the brakes applied. But every time the motor sounds occures the brakes will ABS so to speak.. I hope that makes sense.
Is there an actual motor that does the pulsing on this vehicle? I had a Chevy Van that had a motor mounted underneath and all the brake lines went to it. When it actuated, like trying to stop in at an intersection that was real bumpy.. it would start running and echo thru the truck body .. and of course it would ABS as I slid out into the intersection.. Got to love that ABS when it is slippery.. but any other time.. whewwww..

My feeling is that a sensor is still loose so when he moves the steering it sends info to the computer that wheel speed is different, but I would think the computer would need to see that the brake was applied before it fired up the pulsing motor?? Just a guess on my part.

checkmate454mag,
If you get a chance to send that print I would appreciate it.. did you get my email address??

Thanks again Guys.. big help

Dick

Dick;

I sent you the diagram, let me know when you get it.

There is a pump motor mounted by the fender well under the hood. It should not activate itself if the ecu sees a difference in wheel speeds and the brake is pressed. This condition that was descibed should only set a wheel speed error code. Maybe Insptech is right about the brake light switch being stuck on. You really need to scan for codes and scan all the sensors on this one. Guessing on todays cars just doesn't work. Send me any codes and sensor values you can get. Also, I don't know what kind of scanner you can get at an auto part store. You really need a quality scanner such as a GM Tech 2 or at least a Snap-On.

Vinny,
Thanks!!! I got the print. Still trying to get it to print out on one page.. But I get the jest of it. I am not sure how far my buddy will go with this on his own.. But with all this info he will be educated if he decides to take it some where.

Auto Zone in our area is offering to read computer codes for free. Not sure if they can do what we need here, but it is a start. If not I will hunt up someplace for him to get it scanned. At least he can seperate the BS from the truth..

Thanks again, I will let you know how we make out..

Thanks to everyone that chipped in!!! All suggestions are a big help..

Dick

Dick;

Anytime. Let me know if you get any more information such as codes, etc...

Vinny

Ok Vinny will do. Thanks again.

Dick

abs
 

MR GADGETS I read that in our area we've had alot of problem with rust lifting the sensor off the spindle .He might want to try cleaning the mateing surface and the grease off the sensor. If he decides to give up I'd be glad to look at it at my shop for him, good luck.

As said before, the only accurate way to diag is to drive with a scan tool and watch the vehicle speed signals. I have seen on earlier truck 93-94 where the rear wheel speed signal go's out. Has there been any transmission work done lately. I had one last week where the reluctor inside the trans would move on accel, the computer would loose the rear wheel speed signal and then all hell brakes loose. I am also wondering if somthing is backfeeding into the ABS relay. I will look at a schematic at work and see what I can find.

Ok, Here is the test results so far.

1.) Scan tool was connected and looked for error codes:
None found. As suspected with no ABS light "on".

2.) with scan tool looking at wheel speed we could see that the wheel speeds did not match at the lower speeds where the problem occures.

At 3mph all wheels read the same.
At 4mph rf and rear read 4mph and lf reads 3mph
At 5mph rf and rear read 5mph and lf reads 3mph
It does this up to 7mph and then lf will catchup or equalize and read 7mph.. 8mph.. along with the other sensors.
If the wheel speed difference stays for any length of time, the ABS motor opperates and if the brakes are applied you get ABS action.

We subsituted a sensor on the left front. And yes you can get the senors seperate from the wheel bearing from after market sources.
But the exact same results occurred. Lf would read 3mph while others were gaining speed.
The left front would track at times and at other times not. So I take it this problem is somewhat intermittent. Seeing that the sensors go directly to the ABS brain. I am thinking the problem lies there.

What do you guys think?? Any one seen this type of reaction?

You guys have been a great source of knowledge and my buddy and I appreciate it.

Thanks

Dick

On the newer trucks the ABS will read 3mph when the truck is stopped. All 4 should say 3mph then going up from there. I am wondering if the LF has an interminant open in the wiring between the sensor and the computer. Have you ohm checked the wires and looked to see if the terminals are tight? If you have swapped out sensors then I would look closely at the wiring.

I have seen a truck that exhibited the same symptoms with new sensors installed and the fix was a new ABS module. Looks like that's where you need to go.:(

I also saw this problem after a brake job that I did not do they hung the brake calper by the abs wiring and had a wire haning by a thread it would drop the abs siginal at slow speeds check the wiring from the sensor plug to the module with a meter and the wheel bearing must be tight or it can do it also. Dont ask me why it turns the abs on I think it only does this under 10 mph or so call me if you want and I will walk you through it Dennis 573-808-0598

Heres your fix, guaranteed!
 

Mr Gadgets...
This is my 1st post, and I hope the attached images work, as they are worth 1000 words... I am an advanced automotive electrical tech, and travel all over the country to help dealerships and independant shops solve complex computer engine and body control problems. This on is on the money, and usually I can get it done without travel about 98.9% of the time... Sure wish they were all like this...lol Now, on to your problem, and how you will solve it!

A VR (Variable Reluctance) ABS sensor uses a rotating toothed ring to change the magnetic flux within the sensor. The sensor can generate an AC signal to be utilized by the EBCM as an indication of wheel speed. The EBCM looks at the frequency of the sensors signal and can determine how fast the wheel is rotating. If the wheel isn't rotating, or isn't rotating at the same speed relative to the other wheels and the brake switch is on indicating deceleration, the EBCM will go to work to try and release the wheel it feels is approaching lockup. What you need to understand is, frequency is not the only factor the EBCM looks at when deciding to engage the ABS. The EBCM also takes into consideration the amplitude of the signal generated by the VR sensor during its wheel speed calculations, and it also needs to see a minimum amplitude before it considers the signal valid. Oddly enough, if you look at your scan tools wheel speed PIDs when the truck is at rest, many will read 3 mph! There are a few different things that might affect a VR sensors ability to produce a valid signal. The number of wire turns in the sensor, the strength of the magnet, the relative speed of the toothed ring, and the sensors distance from the toothed ring to name a few. Now, the 2 things you said that are important to me are, 1... you do not
have an ABS problem during high speed braking, and 2... the problem only appears at speeds lower than 10 mph. What happens is with the increased air gap, the stability or strength of a VR signal is lower at slow rotational wheel speeds not meeting the EBCMs minimum threshold, and the result is the 3 mph difference you said was seen between sensors right before stopping. Other causes of low signal amplitude are, a bad sensor, different tire sizes from side to side, a difference in tire pressures from side to side, or a cracked tone ring inside the brg and hub assy, but these are more extreme cases and what I am about to tell you fixes 98.9% of then...THE CAUSE ?????... Apparently, corrosion under the front wheel speed sensors which upon a normal diagnostic inspection would seem irrelevant. It seems that on these vehicles, rust builds up between the sensor and the hub where the wheel sensor mounts, and the rust actually
lifts the sensor away from the toothed ring as it grows (remember... rust never sleeps).

Click here to see a the problem

Click here to see a cleaned useable hub

Click here to see a cleaned useable sensor

With increased air gap comes decreased signal amplitude. The EBCM can't see the signal even though the wheel is not approaching lockup. The EBCM will engage the ABS to try and unlock a wheel that really isn't locked. The signal is still present, but it's below the EBCMs minimum amplitude threshold.

Click here to see aweak amplitude waveform

Click here to see a good amplitude waveform

All along The EBCM doesn't need to see just a change in frequency from side to side, it will also cycle the ABS if it can't see the signal in terms of amplitude, and the computer sees wheel lockup which is not actually there.

Click here to see 1 sensor @ 3 mph stopped waveform
Notice that 1 is 5 mph and the other 3 mph? This is an actual snapshot of the problem occurring, confirming the problem.

Click here to see both sensors showing good 0 mph waveform
I would also recommend this be done during any normal brake rotor removal because of the labor involved - you (or someone) are already there and you'll be eliminating the same problem at a later date in 5 or 6 months because of the environment the sensors live in. If you only cleaned one side and then looked at the lab scope, the amplitude of the other sensor will actually increase, meaning another problem is soon to follow. One BIGGE I forgot to mention is, if there is ANY play in the front wheel bearings, looseness or side to side play or wobble, it will change the distance from the sensor, and create the same problem. I have seen several that had to have the brg and hub assy replaced before the problem was solved, but most of them that were worn somehow, or had a cracked ring (don't bother looking), were on the right side, and the ones with no visible damage, looseness, or traceable problems are always on the left side, and you can take that to the bank even though I can't explain why. Below are what cleaned sensors and hubs should look like. DO NOT forget to plug the hole the sensor goes in when cleaning, or the rusty crud will fall into your wheel brg grease... and then we know what happens... Good luck my friend, and if you need any more info shoot me a message, if you can "FindMe"...

FindMe,

It all falls into place after your explanation!! The pics are great.. We will tear back into it and check it out.. I am sure that will be the cause. Sensitive little bugger that needs such a close tolerance.. but I can see the resultant low amplitute waveform is proof of the problem..

Sounds like a real neat job.. any openings?? I love to solve problems like that. We have just been strapped for time, and my buddy lives an hour away, other wise I would have hooked up a scope and looked at it.

Thanks again for the help. I will let you all know what we come up with!!

Dick

Good post findme, welcome to oso :)

I'm glad to see this :D .I have a 98 Tahoe with intermittent ABS problems myself:(