dan239

I had the abs codes checked today and there was no codes set.

Any further suggestions will be appreciated.

Daniel

SIXSLUG

I am wondering if the resistance in the sensor is causing the issue. If soldering the wires up without the interrupting of the sensor is causing a stronger return to the ecm, maybe signalling the sensor is signalling a fault.

Sorry, kinda tired, but do you get what I am gettin at?

dan239

I think if that were the problem, the lights would stay on at startup.

Daniel

j-fox

The speed sensors have a positive and negative wire. Are you sure you matched them? My truck is gone so I can't go look at it.

dan239

Yes, I have rechecked them several times to make sure they are matched.

Daniel

rickf

So you are saying the light comes on after 500 feet, that would indicate the computer seeing a difference in wheel speed between wheels but you are not getting any codes? Are you checking for codes while the light is on? Yes the battery disconnect will eliminate the codes, will have to be disconnected for an hour or so. I usually turn on lights to make sure system is drained. If the light is out when you first start then there is no code to start with, the code is set when the light comes on. Got me on this one, I am used to diagnosing complete systems so this is a little out of my domain.

Rick.

dan239

If a code is being set it is going away when I turn the key off. Turning the key on and the lights go out indicating no code. Start the truck and no lights until I move about 500 feet, then the abs and brake lights will come on. The only way to check for a code when the lights are on would be to put the scanner on it while it is running.

Daniel

rickf

Exactly right on all counts, now to figure it out. If you unplug the rear sensor will the light come on as soon as you start? I am going to assume yes. The sensor is noting more than a magnetic pick up that translates magnetic pulses to electrical pulses, square wave. The faster you go the shorter the square, pulse width. Front and rear pulse width should match for the computer to read that all tires are rotating the same speed, if one is skidding then the pulse width will not match and anti-lock is activated.
I am thinking out loud here to try to come up with a solution, anyone jump in with ideas.
If the two front sensors are tied into the rear sensor then all sensor inputs will be identical hence no activation.
I was thinking that maybe because the computer saw no differentiation in the wheel speeds at all it might see that as a problem but this is too early of a generation for that plus it would have to travel much, much further to trigger that type of a code.
This may sound strange but are you sure the rear speed sensor is good?

dan239

I had the rear sensor go bad several years ago and it set a code. I am sure the rear speed sensor is good or I would not have my speedometer. If I should disconnect the rear sensor, the system would sense the open circuit and the abs and brake lights would be on from the startup. The front sensors are disconnected and the rear sensor is feeding the rear sensor inputs on the CAB [Control Anti-lock Brake] and is also feeding the front right and left inputs on the CAB.

rickf

The speedometer is fed from a sensor on the transmission. There are two on the transmission, one for the transmission shifting and one for the speedo. Then there are three more for the ABS, the two front wheel sensors and the one mounted on the rear differential. That is the one in question.

SIXSLUG

Maybe a dumb question, but are all your tires the same size?

Humblecowboy

The abs on my 99 3500 haven't worked for the past ten years. Something 'Popped" under the hood and the abs quit and the cruise control. When it worked, the computer put me in the middle of way too many intersections and some with a cop watching.

j-fox

Try this to see if it changes things.
Reconnect the rear to the original wires.
Leave the front two paired up.

The computer can set a cade and not turn on the check engine light.
some code readers don't see all codes either.
My Smarty has found codes that a small shop couldn't find with thier expensive reader.

dan239

I wll have to disagree on this. On my truck the speedometer is fed from the rear abs sensor. That is why if you pull the abs fuse you will lose the speedometer. I also checked my service manual and it clearly says the same thing.

Daniel

dan239

All tires are the same size and almost new.

Daniel