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" No Bus" how to confirm this is the pcm and not a short?

4K views 21 replies 3 participants last post by  ScanMan79 
#1 ·
I have a 2002 dakota 4x4 4.7l manual transmission. the other day it left me dead at mcdonalds when the guy in the window said "Have a great day." after pushing my truck from the window with help from the two dodge owners behind me, i am on my way to ? not sure, but hoping to confirm before tossing money into this recently purchased high milage truck.
Ok, cluster test gave 920,921,999 codes. all fuses are good and i have tossed in a new crank shaft sensor. renting an obd2 reader for a deposit of 140 bucks at auto zone has left with waiting 3 to 5 business days waiting for my deposit to come back and nothing else. I am seeking away to confirm that the pcm is toasted before dropping the money on one as i am done 140 bucks thanks to auto zone. Anyone have suggestions or the path i need to get my answer?
 
#3 ·
ScanMan, here are some checks you can make.
This is not from the FSM but from Haynes.

To paraphrase:
"test for battery voltage to the coil by disconnecting the connector from the coil & using a MM to check for volts at the dark green/orange wire with the ignition ON.

No voltage=bad ASD relay.
If volts: use the ohm-meter function of your MM & check primary & 2condary resistance of the coil. My Haynes lists 0.95 to 1.20 ohms & 11,300 to 13,300 ohms respectively, but I believe Haynes is giving me inaccurate specs on that.

If coil ohms out good: check the trigger signal from the PCM
. Using an LED test light (inexpensive from Advance or the like) back probe the coil driver terminal (black/gray wire) & check for a flashing light as an assistant cranks the engine.

If no flash: then check the operation of the cam position sensor & the crank position sensor.

If those sensors test good: send the PCM out to be diagnosed."
 
#9 · (Edited)
. . . I just edited my last post to this,
but,
let me also throw this at you.

From another handwritten note (I am not absolutely 100% positive, but I think I took this from a post that Old Marine made, & he was quoting the FSM)
I wrote that: "if the PCM senses no signal from the crank or camshaft position sensor(s) it will deenergize the ASD and the fuel pump relay."

Meaning to me: that if either your crank or cam sensors were bad, you would not read voltage at the green/orange wire at the coil.
 
#10 ·
. . . I just edited my last post to this,
but,
let me also throw this at you.

From another handwritten note (I am not absolutely 100% positive, but I think I took this from a post that Old Marine made, & he was quoting the FSM)
I wrote that: "if the PMC senses no signal from the crank or camshaft position sensor(s) it will deenergize the ASD and the fuel pump relay."

Meaning to me: that if either your crank or cam sensors were bad, you would not read voltage at the green/orange wire at the coil.
Thank you for the information. Yes, the truck is the same. 2002 4.7 RWD, with the exception of mine is an automatic. I will do some more in depth testing on this circuit before moving on to something else. I have a pdf of the FSM but trying to follow there wiring diagrams has a bit challenging. I have a Chilton manual as well for this truck, but my old man security system got activated the last time I used it and I put it "somewhere I'll know where it's at" and have yet to relocate that great place again! If I come up empty handed on further diagnosing, then I'll start a new thread with all the current details of where I've been so far. Thanks again for your input.
 
#12 ·
Check this out!
(This is a quote from Old Marine related to the ASD relay to a thread by someone who couldn't get a trigger signal from the PCM to the coil. Old Marine usually gets his stuff straight from his FSM.)

"The coil and the fuel injectors are powered by the ASD (automatic shutdown) relay. The ASD relay is controlled by the PCM and will cut power if (1) the ignition key is left in the Run (not Crank) position without the engine running for approximately 2 seconds, or (2) the PCM does not receive a valid signal from the crankshaft position sensor (CKS). So you could check that the ASD relay is operating properly and you could check that the CKS is good and the wiring to the CKS is good."

Okay, this would explain my first handwritten note about getting 12V at the green/orage wire to coil when initially turning the key 'ON' & then not getting it again until I cranked it over. So all I am getting at here is it might be worth double checking & making sure that you are truly not getting 12v there. If you had the key 'ON' & then had to screw around a bit before you got the probe of your MM on the wire, the PCM would have already de-engerized the ASD relay.
 
#14 ·
Well after a lot more testing and tracing down wires, I think I have narrowed it down to being the PCM. Everything else checks out so unless I'm still missing something somewhere that's the only thing left that I can't definitively test with the tools I have available. I've had the feeling it was it all along but just don't understand how it can work fine one day, be parked for a few days and then be dead when I go to start it again. I guess I'm just glad it didn't dump me along the road somewhere.
 
#15 ·
That's the way mine was, except I was lucky in that when it decided to shoot craps & then crank but not start it was always in my driveway. I find intermittent stuff hard to troubleshoot but when it devolved to terminal with volts at the coil but no trigger signal to the coil I moved towards PCM. The place I sent it to diagnosed a loose pin that they resoldered (which, I suspect is what made it initially intermittent), and, although my memory has faded a bit, they said they replaced a part, I think they said it was the coil transistor. And if so, that may have been because I described the the findings I got when back probing my coil wires.

Here is a thread that started as a result of someone having similar issues.

 
#17 · (Edited)
I wish you well on this, ScanMan. It sucks to be driving a vehicle that you don't trust.
I just reread this entire thread & I had mistakenly forgotten that you were not the OP; among his symptoms were 3 codes related to his transmission (weird, huh?) (on edit: especially since those appear to be codes for an AT & he says he has a manual!) &, unlike you & I, he was also the one who did break down on the side of the road.

Anyway, relooking at the notes that I had provided related to the crankshaft position indicator & your first post on your scan findings on the crank pos indicator, did you, in fact, find a way to definitively eliminate that as a possible suspect? (When I was working on mine in the intermittent mode, I felt that I had eliminated it by ordering one from Rockauto & replacing it.) Just curious, is all.
 
#18 ·
Thanks. Yeah... the crank sensor... Man, what I nightmare that turned into. I was trying to ohm it out and check functionality and being in such a tight spot I couldn't for the life of me get a good reading. So I attempted to remove it to bench test it and it was so corroded in the hole that between trying to wiggle it out and my keen gorilla prowess, the old plastic gave up the ghost on me and I broke it off in the engine. Several hours of work and days later I was able to drill it and finally get the other part out. Put in a new crank sensor and did the cam sensor while I was at it for good measure and, nothing. Didn't change a thing. All the headache created was in vain.

After reading your post about possible timeout on the voltage for the coils, I got it setup with the meter on the windshield so I could see it as I turned the key and sure enough, there it was, battery voltage. So my first results were flawed because I had already had the key on and then was probing around looking for voltage that had apparently already been turned back off.

So with all the following checking out; all sensors, 5 volt reference good at all sensors, grounds all good, getting good results testing the connector pins at the PCM being good (unable to check PCM coms), fuses, and wiring just leaves the PCM if I'm not mistaking.

This is the first issue in a long time that I've really struggled with getting the issue diagnosed and repaired. I'm hoping that I can find somewhere local that can at least test the PCM to confirm before I shell out the money for a replacement one. Knowing my luck, I'll buy a replacement PCM and it still wont work only to find out that I only needed to replace the burnt out license plate light. (kidding of course). :)
 
#19 · (Edited)
Wow and yuk!
Not all in vain -- look at the positive side, now you KNOW you have a fresh crank & camshaft position sensor installed.
Yeah, if I was in your position I would be thinking PCM.
I am not putting a plug in for the place that did my PCM (SIA Electronics), but they advertise (at least they used to) that they will bench-check your PCM & if it tests good on their bench that they will send it back to you for only the bench-check fee plus shipping.

Just something to keep in mind is all, as all that I know about these guys is my one previous experience with them.
 
#20 ·
Well, for a final update. After waiting several weeks for SIA to return the PCM, it started right up and everything is back to normal again. They said they found pins on the E-Prom that had came unsoldered and they reflowed them fixing the issues I was having. So in this case anyway, it was indeed the PCM at fault. Thanks again for all your help with this. It's nice to have it back up and running and I now know way more about that system than I ever cared to! :p Take care!
 
#21 ·
It's good to read your followup!

Back in '18 when I sent my PCM to SIA, they were advertising a pretty quick turn time that they came no where close to meeting (but it didn't take multiple weeks). (I am sure COVID has slowed stuff down everywhere, but still . . . .)

You might find this interesting, it is a quote that I pasted from either this bopard or another board. Keep in mind I am quoting, & I have no idea how credible the post is or the person who made the post:
"I have been studying the ECM's that come with these vehicles. I have found out that it is very common to have a couple of circuits fail in them. The coil driver, voltage regulator, IAC motor IC, step up transformer that gives you the "NO BUS error" and the EPROM. All of these issues are caused by broken solder contacts on the board. If you are good with a soldering iron you can fix the ECM by re flowing the solder contacts."

In the time since I last posted here I learned a few things related to the 4.7 that might have affected the applicability of some of the troubleshooting I typed for you,
but the main thing is that you are back up & running!
 
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