ABS Problem (maybe)?

Shteets · #1 06-06-2008, 01:24 AM

1998 Silverado 1500 4x4: This one is a good one. When I am approaching a stop sign moving slowly, I can sometimes feel a little bump (like it will pop up and let down quickly) in the brake pedal, then a mild ERRRRRRRRRRRRR sound that is coming from the engine compartment (I assume it is the ABS unit). That has been happening occasionaly, then today was the topper. While leaving a stop sign, I accelerated to about 40 miles per hour and noticed the speedometer jerked back down to zero and back up again rapidly about three or four times. While the speedometer dial was doing this the transmission slipped like it was popping in and out of gear. While this is all happening the ABS light comes on and stays on. I can turn the truck off, restart it and its good for a while till it happens again. Anyone have any good ideas what the heck could be causing this? Front pads are new, rear shoes are at 50 percent. Can the ABS effect the speedometer and transmission? Hard one for me to swallow, I am old school and speedo cables either are broken or not.

Sonny01 · #2 06-06-2008, 03:47 PM

When I am approaching a stop sign moving slowly, I can sometimes feel a little bump (like it will pop up and let down quickly) in the brake pedal, then a mild ERRRRRRRRRRRRR sound that is coming from the engine compartment (I assume it is the ABS unit).
You have a mis-match between ABS sensor outputs...the sensors are located on the wheel hubs. Do a search on the forum on ABS and you will find how to test them. As far as the other things going on...fix the sensor problem first and I bet the rest go away.

j cAT · #3 06-06-2008, 05:07 PM

Quote:

Originally Posted by Shteets

1998 Silverado 1500 4x4: This one is a good one. When I am approaching a stop sign moving slowly, I can sometimes feel a little bump (like it will pop up and let down quickly) in the brake pedal, then a mild ERRRRRRRRRRRRR sound that is coming from the engine compartment (I assume it is the ABS unit). That has been happening occasionaly, then today was the topper. While leaving a stop sign, I accelerated to about 40 miles per hour and noticed the speedometer jerked back down to zero and back up again rapidly about three or four times. While the speedometer dial was doing this the transmission slipped like it was popping in and out of gear. While this is all happening the ABS light comes on and stays on. I can turn the truck off, restart it and its good for a while till it happens again. Anyone have any good ideas what the heck could be causing this? Front pads are new, rear shoes are at 50 percent. Can the ABS effect the speedometer and transmission? Hard one for me to swallow, I am old school and speedo cables either are broken or not.

the transmission sensor monitors the rear wheels, the front left/right sensors monitor the front wheels....the transmission sensor goes to abs and speedometer...if this is bad/dirty/wiring defective,,,, abs and speedo/computer will get into slip/slide mode....your symptoms..

Shteets · #4 06-07-2008, 12:10 AM

Front sensors are good, now I'm going to find a way to test the speed sensor. Thanks, and I'll post how this all finishes out.

Sonny01 · #5 06-07-2008, 08:16 AM

How did you test the front sensors?

j cAT · #6 06-08-2008, 01:13 AM

Quote:

Originally Posted by Shteets

Front sensors are good, now I'm going to find a way to test the speed sensor. Thanks, and I'll post how this all finishes out.

THE FRONT SENSORS DO NOT GO TO THE SPEEDOMETER !!!!!!

The transmission speed sensor/wiring/connector has issues most likely...this is the signal that the speedo is using....as well as the ABS.........

Sonny01 · #7 06-08-2008, 08:41 PM

THE FRONT SENSORS DO NOT GO TO THE SPEEDOMETER !!!!!!
Why are you yelling?

j cAT · #8 06-09-2008, 01:55 PM

Quote:

Originally Posted by j cAT

THE FRONT SENSORS DO NOT GO TO THE SPEEDOMETER !!!!!!

The transmission speed sensor/wiring/connector has issues most likely...this is the signal that the speedo is using....as well as the ABS.........

1st off this message was not directed to you......2nd I previously stated this fact and i felt it was somehow not clear....as he checked the front not the transmission speed sensor

Sonny01 · #9 06-09-2008, 03:30 PM

I see...I'm still waiting to hear how he tested the front sensors.

Shteets · #10 06-11-2008, 09:17 PM

I used a voltmeter and checked the resistance of the sensor. once I connected the leads, I spun the wheel to make sure the resistance changed. They both did. I also moved the wires around to see if the resistance changed. If it changed from moving the wires I would have suspected the wires. You know, after I disconnected both front sensors and checked them it stopped having the problem for about three days. Just started again so I am going to mess with the speed sensor. Is there a good way to test the speed sensor??

Thanks

Sonny01 · #11 06-12-2008, 01:16 PM

You need to check the sensors the same way except set the volt/ohm meter to AC milliamps. 350ma is the minimum per side but the one with the largest drop is the bad one.

j cAT · #12 06-12-2008, 04:42 PM

Quote:

Originally Posted by Shteets

I used a voltmeter and checked the resistance of the sensor. once I connected the leads, I spun the wheel to make sure the resistance changed. They both did. I also moved the wires around to see if the resistance changed. If it changed from moving the wires I would have suspected the wires. You know, after I disconnected both front sensors and checked them it stopped having the problem for about three days. Just started again so I am going to mess with the speed sensor. Is there a good way to test the speed sensor??

Thanks

in addition to shorted ,open, connector damage etc......remember if the sensor pick up head is containinated with metal debris the sensor out put will drop....low a/c milli volts and this will cause problems such as yours..

removal and cleaning may be necessary.....if any of the 3 sensors have metal on them this abs/speedometer problem may be the smaller problem as this should not occur..

maxwedge · #13 06-12-2008, 08:42 PM

The simple solution to this is a scan with an abs function scanner which will allow someone to see the speed sensor inputs and compare which one drops out at low speed, causing a false activation of the abs. This happens so quickly a dvom will not pick it up, only a scope or the scanner will. A common cause of this was rust build up under the ft speed sensor mounting flange causing the sensor to back away from the tone ring, causing a weak signal at low speeds, GM had a recall for this in the " salt" states.

Sonny01 · #14 06-13-2008, 12:06 AM

This happens so quickly a dvom will not pick it up, only a scope or the scanner will
Wrong...if you jack up the front end of the truck and spin the tire with a VOM attached you will get a reading...I have done it and the info on how to do it came from a GM tech bulletin.

Shteets · #15 06-13-2008, 01:09 AM

Well, I tried to clean the connector on the speed sensor now the truck is undrivable. It's stuck in first gear and speedo goes berserk. I used TV channel cleaner to clean the connector and sensor contacts up (didn't look dirty or corroded). It didn't like that at all. So it looks like it is either the sensor, a bad connector, or possibly (and I hope not) some kind of short in the wirring from the sensor to where ever the heck it goes. I think I'm gonna say uncle and bring it to someone with a reader and pull a code out of the stupid thing. I should go buy something with a speedometer cable and points

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