2nd Gen Non-Engine/Transmission ABS light and Speedo

[DSeamans]

My buddies 98 12ver had the ABS light come on and the speedo only reads roughly 50%. At 60 it bounces around 30. He replaced the diff speed sensor right away with no change. We cleaned all connectors and ohmed the wires to the Cad. Spinning the tires and we had voltage at the CAD connector. A prior thread said it should be in the millivolt range. It seemed to be right according to that thread. Then he got a used Cad and nothing has changed. The place he got the cad from said it was working in the old truck when they pulled it. Now what? He did just replace the card and not the pressure controller the brake lines go through. Would that affect it? What do we check next?



Thanks,

Dave

 

[Road Dog]

The ABS sensor on the rear diff is a hall effect transistor that generates pulses as the ring turns. You can't get any meaningful information with a multimeter (especially a digital one).

If you unplug the diff sensor does the speedo still work? Earlier RAMs (including maybe your friend's) have a second vehicle speed sensor on the transmission or transfer case.

If the tone ring is damaged then you'll get erratic speed readings. (The tone ring is attached to the carrier and looks like a gear. The ABS sensor detects the teeth as they go by. ) Your friend will have to remove the diff cover and if the teeth are broken off then you friend will have other problems too.

 

[fest3er]

... He replaced the diff speed sensor right away with no change. We cleaned all connectors and ohmed the wires to the Cad. Spinning the tires and we had voltage at the CAD connector. A prior thread said it should be in the millivolt range. ...

This seems electrical. You have a connection somewhere that is flaky. But you checked all connections. If the sensor wires were ever cut to splice in a speedo correction box, those splices need to be checked, too.



At around 3MPH, the voltage from the sensor should be around 500 mV. The voltage should steadily increase with speed to about 2. 5V at around 25 MPH. (This is from 8 y. o. memory, so the values are 'guesses'. )



Did you inspect the tone ring in the rear diff? If it or its teeth are damaged, I would expect the speedo to behave in such a fashion.



My CAD failed about 6 years ago. I bought a new one from Dodge based on the VIN. It has never been able to be 'programmed' with the right tire diameter; it complains about something being wrong. But Dodge dealer wouldn't make good on it. So the yellow ABS light has been flashing ever since. I no longer see it. I used a speedo correction box to dial in the speedo. And the ABS works anyway. And I changed only the CAD, not the hydraulic unit.

 

[HHhuntitall]

Borg Warner ABS101, part number from Orielly's, is the field generator in the top of the rear end. I just fixed a '98 24v doing the same thing, and one of mine did it about a year ago. They also turn on the ABS light in the 12valves, even when the speedo works, when they go bad. I know you said you ohm tested the wire loom, but somewhere, there is a TSB on the wire loom behind the driver front tire, stating how to fix the wire connections for the ABS and bypass the connector. I don't remember what years it entailed, but I believe one of my '96s or 97s had it done. I believe water got into the wire connection and corroded the connectors.



Usually, the sending unit(field generator) seems to get water in them, and then freeze and bust, letting condensate get in them and rust the magnet for the generator, destroying them from the inside out. Pull it out and look to see if rust is formed on the end. If it is, it's probably faulty.

 

[firefighter_2000]

Fully seated

First make sure the sensor is FULLY seated in the opening in the rear end. I didnt get mine all the way in the first time and it gave the same wierd readings. If that still doesn't solve it, follow the wires towards the front of the truck checking for chaffing. I found my chaffe right in the wheel well where it passes through a metal loop. Respliced the wires and put a good healthy dose of electrical tape around it and no problems since.

Hope that helps.

Craig

 

[DSeamans]

Speedo

He replaced the sensor and said the tone wheel felt fine. Its only a year old - I replaced it when I rebuilt the diff for him. The connector behind the front left wheel looked like new. For giggles I did clean it and used dielectric grease when reassembled. When ohming the loom, we went from diff to that connector and from there to the cad. Put the connector back together and ran the whole thing. No jumper and no shorts between wires or to ground. Jumper in and it read great. Physically followed the wires from diff to cad and they were fine - no chaffing or signs of rubs or breaks. Wish I still had mt old Simpson 260 to check for pulses!

 

[HHhuntitall]

Is there another sender on the t-case(4x4) or the transmission tail housing? Some 12 valves still recieve a signal from the field generator in the transmission.....

 

[fest3er]

Is there another sender on the t-case(4x4) or the transmission tail housing? Some 12 valves still recieve a signal from the field generator in the transmission.....

The '98 uses the rear-diff sensor only. Earlier years based the speed on the trans output.

 

[DSeamans]

Sensor

It does have a sensor in the transmission. It was replaced this summer when Dave Goerend put a new transmission in. The cummins shop nearby said only the diff sensor affected it. I'm having him bring it by some night this week and I'll pull the diff cover to get a good look at the tone wheel and check the clearance to the sensor.



Dave

 

[HHhuntitall]

Yes, BUT, if the Diff sensor is the only thing affecting is, why is it there? I've got a cutomer's '98 that I put a standard in, and it does something similar. The speedo works pretty much all the time, but it bounces every now and then. Enough the cruise won't work..... so what's with the sensor in the transmission, and what's it's purpose, then!?!#@$%!

 

[DSeamans]

Sensor

Don't have a clue! transmission control?

 

[DSeamans]

Found it

Gee, I think we found the problem. Right away I had told him to check the tone wheel and he said he thought it was fine. Finally had him pull the diff cover and he found 3 chunks of the pinion gear in the case! I replaced the gears for him about 100k ago when he wanted to switch from 4:11 to 3:54. The tone wheel still had teeth, they were just a wee bit short. The weird part is that it doesn't make any noise or vibrations. When we checked the wires I spun it by hand and didn't hear/feel anything. I guess now I get to do a complete rebuild.



Dave