What is the deal with.......

[Diezel_Cowboy]

the brake/abs light on my 94'? It seems like it comes on everytime we have hot weather, but my brakes always are working fine when it comes on.
I seem to remember something about a sensor on the rear axle that goes bad?
Any good explanations/suggestions?

 

[fx4wannabe]

Its got to be a sensor somewhere if the brakes are fine.

 

[Agnem]

How is your speedometer? That rear axle speed sensor is a known failure item.

 

[TLBREWER]

How is your speedometer? That rear axle speed sensor is a known failure item.

I was going to ask a similar question, but now I'm a little confused. I have an ABS light that stays on all the time in one of my '91's. And now the speedo has quit working. It has a ZF tranny, but I'm pretty sure the speedo is cable operated and plugs into the top of the tailshaft if I'm not mistaken.

So is the sensor in the diff the only thing that affects the ABS light? The ABS is working correctly as near as I can tell. I also have a constant dash brake light on--on the dash. Don't know if that's related. I'll make that a separate post.

I'm thinking about fixing the ABS light by removing the light bulb since it seems to be working fine.

Tom

 

[TLBREWER]

Forgot to mention that the ABS light comes on about 15-30 seconds after first starting the truck, moving or sitting still, and stays on until the truck is shutoff.

Tom

 

[tonkadoctor]

I was going to ask a similar question, but now I'm a little confused. I have an ABS light that stays on all the time in one of my '91's. And now the speedo has quit working. It has a ZF tranny, but I'm pretty sure the speedo is cable operated and plugs into the top of the tailshaft if I'm not mistaken.

Tom

Tom

The speedo in my '90 is cable driven off of the tailshaft housing. IIRC your's should be the same, I don't think the electronic speedometers came out until '92 or '93.

Pull the gear out of the tailshaft housing. It's a 10 mm or 11mm open end wrench that you need to remove the bolt on the retainer. Then you can check the gear on the end of the cable and check the cable.

If the gear on the end of the cable is chewed up check the tailshaft bearing for play and put your finger in the hole to see if there is a metal bearing race from the tailshaft bearing floating around in there. That's how I discovered the bearing failure in my tranny, it still shifted and sounded fine.

 

[SKimballC]

Tom, ABS and speedo are not related on a '91. Try replacing your sensor in the rear diff and see if that cures you light. Which reminds me, maybe tomorrow I'll finally go and try to get my broken sensor out of MY rear diff.

 

[TLBREWER]

Thanks for the clarifications fellas.

Tom

 

[Diezel_Cowboy]

I was going to ask a similar question, but now I'm a little confused. I have an ABS light that stays on all the time in one of my '91's. And now the speedo has quit working. It has a ZF tranny, but I'm pretty sure the speedo is cable operated and plugs into the top of the tailshaft if I'm not mistaken.

So is the sensor in the diff the only thing that affects the ABS light? The ABS is working correctly as near as I can tell. I also have a constant dash brake light on--on the dash. Don't know if that's related. I'll make that a separate post.

I'm thinking about fixing the ABS light by removing the light bulb since it seems to be working fine.

Tom

Those are the same symptoms my truck has! Brake and ABS light both come on after the truck reaches operating temp and it is hot outside.
The speedometer functions normally.

 

[Agnem]

The 87 to 91 trucks have an ABS test that you can perform. There is a test wire hanging under the dash (mine was near my parking brake cable) which you short to ground with the key on, and the ABS light will flash out a code that will tell you what the problem is. I had an ABS problem which turned out to be a bad ground. You'll need to check the service manual for the code chart.

 

[SKimballC]

Very interesting. Anybody have the code chart?

 

[TLBREWER]

Tom

 

[TLBREWER]

Shane

While I was working on this problem over the weekend, I found the code chart in my Haynes manual. Didn't even know it was there. I did not get to the wire code test that Mel mentioned. When I pulled the sensor out of the diff, it came out in 2 pieces . Well, actually the top electrical part came out, then it took me two freakin hours to get the sensor part out! About $25 at Napa for a new one, and, whala both the "ABS" and "Brake" lights are now out.

One more thing off the list.

Tom

 

[SKimballC]

Tom, you jerk... MY sensor came apart in 2 pieces, too... one of which is still in the diff. I haven't had the time to mess with it, but I really need to in the next few days. I did drain the diff one time a while back, and I looked at it, realized I didn't want to deal with it, refilled the diff , tried to forget about it, and now realized I should have just tackled it then.

How'd you end up liberating the broken-off piece?

 

[TLBREWER]

Tom, you jerk... MY sensor came apart in 2 pieces, too... one of which is still in the diff. I haven't had the time to mess with it, but I really need to in the next few days. I did drain the diff one time a while back, and I looked at it, realized I didn't want to deal with it, refilled the diff , tried to forget about it, and now realized I should have just tackled it then.

How'd you end up liberating the broken-off piece?

Mine broke off in such a way that I could get needle nose on it...just enough to pull it up and drop it again. I dug around the outside of it with a scribe, pulling out plastic pieces to get a better bite. Then I just grabbed it one time and it slid right out. I was very close to draining the diff and going at it from the inside at that point. My neck and shoulders were killing me. Good luck.

I'm adding it to my list of things I never want to do again.

Tom