OK, I finally got my head out of my butt and found the link for testing the instrument cluster (http://dodgeram.org/tech/repair/elec...anel_check.htm). The test response was as stated in the write-up and everything passed. No codes were displayed and the test exited to normal operation - but the check engine light lit up just as it always does. Now I'm really confused! The PCM gives no DTC's and the instrument cluster passes diagnostics. The Check Engine light during the instrument cluster test comes on and goes out according to the writeup. WTF?

 

Ok, according to the FSM, the pcm tells the cluster to turn the lamp on, or off. Furthermore, if the cluster doesn't recieve ANY message from the pcm, it turns the light on by default. I am unable to come up with wiring diagrams though.

 

Thanks for that little tidbit HeyYou. That's definitely nice to know. So, how do you verify communications between the instrument cluster and the PCM? According to one of the responses at this link (http://answers.yahoo.com/question/in...73655AAbk3eM):

"NOTE: If a CCD data bus message is not received by the instrument cluster for twenty seconds, the Malfunction Indicator Lamp (MIL) will flash and a "no bus" message will appear in the VFD odometer display." In my case, I do not get this message and my MIL does not blink.

Seems the only way to track this down is to have a DRBIII scan tool.

 

Wrong, the key trick doesn't work on 98's only for some reason. I'd say you have a pcm problem. But I'm not about to spend your money throwing parts at it.

 

This question has been deleted?



That's not gonna help...... Wonder when that happened. Supposedly, the cluster gets its que from the CTM, Looking at the pinouts for the cluster connectors, (there are only two, right?) CCD bus is just two wires. + and -........ (ethernet in your truck, who'd a thunk it?) So, seems the cluster has to interpret the incoming data, and do it's tricks from there....... which also adds another part to the chain of possible failures, the Central Timer Module........

 

Sorry about that link. Apparently two characters are tacked onto the end of that link that don't belong there - "". Delete those characters at the end of the string and it'll get you there. I wonder if there is a definitive way to test the CCD bus. Seems to me that the instrument cluster test does just that. Per the writeup on the test, there are failure messages associated with the CCD bus (900, 920, 921, 940, 950). Both the ACM and the CAB are both on the CCD bus and those test passed as well. Here's a link to a page where a graphic diagram of the CCD bus can be accessed (bottom of the page): http://mopar.mopar1973man.com/cummin...al/ccd/ccd.htm

Since I did not receive the 900, 920, or 921 messages, I'm inclined to believe that the instrument cluster is talking to the PCM. Also, my odometer is working and updating. I sure wish I had someone else's money to easter-egg this issue. If I can ever zero in on this, I'll be sure to post the root cause here - unless, of course, it proves to be too embarrassing (lol).

 

All of the gauges are driven off the CCD bus.. So, the PCM IS talking to the cluster, question becomes, what is it telling it?

You 'might' be able to plug a decent scanner in, and find out if the PCM thinks the light should be on. If it doesn't, more'n likely a cluster issue, if it does, need to find out why it wants the light on. So far as I know, if won't just turn it on for giggles....... (or, will it?)

 

The instrument cluster test turns the CEL on and off (instrument cluster seems to be OK), so I'd bet that it's getting a message from the PCM to turn it on. I suppose the instrument cluster could be the problem, but it's more probable that the PCM is, in fact, sending it the message to turn on. It is a bit bizarre that I can't get it to turn off - even disconnecting the battery for a bit doesn't do it. One would think that after removing power for 5 minutes that it would take the PCM at least a number of seconds before it determined that the CEL light should come - but it doesn't. It comes on immediately. Hell, the truck runs great now that I've done the plenum fix and replaced the O2 sensor. I think I'll just remove the light bulb and wait until I can find further help or the truck starts acting up (lol). I ain't spending the $$$$ required to just swap those components out.