Speedo doesn't work unless it's cold
 

I bought the truck (2002 Dodge Dakota Sport 4x4, 5.9L) as a fixer-upper, but I've run into a problem that I can't solve: the speedo doesn't work...most of the time. In fact, it only works when it's cold outside and the truck is cold. So, if I start it up on a morning like today (20 degrees out), it will work until I turn the truck off. Then, regardless of the outside temperature, it will not work until the truck cools all the way down again.

I started doing research when I first bought the truck back in May. At that point, being May, it was never cold enough outside for me to experience the "it only works while cold" phenomenon. At that point, I was just doing research based on:
- speedometer not functioning
- CEL for an O2 sensor (not sure which one right this minute, but can find out)
- ABS and brake warning lights on
- Airbag light on

Also, the guy I bought it from said he took it to a mechanic that said it needed a new "engine computer." This is a guy that is not mechanically inclined whatsoever, so I wasn't sure if he meant the PCM or BCM. So, starting with the easiest, I replaced:

- Speed sensor on the rear diff
- Speed sensor on the tranny
- PCM

Still no joy. Around that time, my wife and I sold our house and bought a new one, so I put this project on the back burner for the time being. I noticed that the speedo was working in the cold when I had to drive the truck one morning this fall. Other than that, I haven't been able to find anything to lead me in any kind of constructive direction as to what to do next. That's where I hope you guys can help. Thanks for reading my wall o' text.

The cluster (and speedometer) is a computer. The next time it fails try this:

With the truck OFF, depress and HOLD the trip reset button on the cluster. While holding it, turn the key ON (don't start it). Continue holding it until CHEC appears in the mileage display.

It will test all the lights and sweep the gauges.

Write down any numbers you see. The test is finished when the mileage is displayed.

Post back the other codes when you can.

Thanks! I did what you said, and got 950 and 999. The CEL flashed and then stayed on, as did the airbag light.

Thanks again!

The speedo swept correctly then even though it wasn't working before?

The code you got is for the ABS: The cluster is not receiving an ABS lamp-on message from the CAB (Controller, Anti-Lock Brake). Check the fuses. There should be a 40 amp fuse in the PDC (under the hood) and a 10 amp fuse in the JB (side of dash).

The speedometer issue may be related. The CAB supplies a signal for the speedo.

Can you post the codes you got for the PCM? You can also test them yourself. With the truck not running turn the key to ON (don't start it), back to OFF, then ON, back to OFF again, and finally leave it in ON. Codes will display in the odometer window. It will show P DONE when finished.

Do you know if you have rear wheel or four wheel ABS?

I checked again to make sure. When I tried it, it swept the gauges, but I don't know whether it did it correctly or not. It swept them in stages; the speedo went to about 20 mph, then 30 or 40, then 50, then 70, then back to zero.

Haven't checked yet b/c it's raining here, but thanks. I'll let you know what I find.

Ok, I followed this procedure as well and got P 0037, P 0500, and P 0138. I looked them up, and it seems like the 0037 and 0138 are O2 sensor-related, right? 0500 is the speed signal code, right? Well, like I said, I replaced the speed sensor on the tranny bell housing and also on the rear diff. I've read elsewhere that sometimes the Autozone versions of these parts don't work and you have to get the Mopar part from a dealer. Is there anything to that? Any other ideas?

Thanks so much for your help, man. I really appreciate it.

P0037 O2 sensor 1/2 heater circuit low. That's the drivers side downstream.
P0138 1/2 O2 sensor shorted to voltage. same as above.
P0500 No vehicle speed sensor signal, yes, the sensor on the pumpkin.

Lots of people are having problems with aftermarket sensors lately.

Do NOT use Bosch O2 sensors, our truck HATE them. Use NTK, they are the OEM.

There are calibration marks around the speedo but it seems to be working correctly.

If you have changed both magnetic pickup sensors and your dash is giving a CAB error, I think it might be time to chase fuses and wires. If the RWAL CAB is unplugged (the litte 4 pin connector), your <brake> and ABS lights should both be on solid but speedo works (ask how I know). I don't know the error modes for the 4WAL CAB.

Okay, thanks. I have no idea what brand the replacement sensor I installed is.

I think I'm going to check the fuses that magnethead suggested, and then buy an NTK sensor and install it.

As for the O2 sensor, I bought a replacement but haven't installed it yet. I'll probably try to take care of that at the same time.

Does it tell you anything that the speedo works when it's cold, though? That's the weirdest part for me. For instance, it was 15 degrees here yesterday, and the speedo worked fine when I drove it for the first time. Parked it at the grocery store, did some shopping, and came back...no speedo. WTF? I mean, I'll check the wiring and fuses because why not, but if the fuse was blown, it wouldn't work at all, right?

I think I read somewhere else that changing the diff fluid can help with this...is there anything to that, or just someone talking out of their ass?

Usually solder joints do the opposite- come apart in the cold and re-melt in the heat. I'm really not sure where else to look.