Looking at the wiring diagrams, the sensor returns have NOTHING in common. They are all individual circuits, that don't share anything. They run from the various sensors, to the PCM, to separate pins. Nothing is shared. WHY you are seeing five volts on all the return wires is a mystery to me..... and with two PCM's???? That one is a mystery to me.... I don't see any way that could happen.....

In any event, basically every 'sensor' is on the five volt bus. TPS, MAP, ECT, IAT, all the O2 sensors, any speed sensors you have... (2 or 4 wheel ABS?) Cam and Crank sensors.... And I am probably missing one or two....

 

I'm running out of ideas...

Today I had a partial day because of quarterly plant inventory - and will have tomorrow and Saturday off as well.

I got off work thinking I had a game plan... I unplugged all of the sensors related to the C1 plug... MAP, Crank, Cam, IAT, TPS, IAC and O2 sensors... Still 5V on signal wire at MAP.
Not sure if this is right but I also was seeing 12v on two pins at the IAC and nothing at two others. Pinout says they all source from an IAC driver - might need to look into the diagrams to see if that was right or not...

Decided to rip apart the engine wiring harness again... got it fully stripped down to everything I could reach in the engine bay and still couldn't find any exposed, burnt or cut wiring. even made sure to separate everything out.


*I plugged in the sensors I could again to make sure I didn't get anything in the connectors before I started laying all over the engine bay*

I checked every ground I could get to and grounds to the connectors for resistance to ground at the battery and got almost nothing at all of them. anything that showed up was around .5mohms

Pulled the PCM C1 connector and disconnected the sensors again and checked for continuity between the 5V output and all the pins on the connector - OL everywhere. Checked every single connector I'd pulled to see if I found any different results and still, OL everywhere.

Out of paranoia I decided that it was time to pull the dash and check if there was something to what the PO said about maybe messing something up with the wiring when swapping the dash out. Pulled the dash and found no signs of modification outside of the headlight switch and wiring to the heater controls - which I'd already fixed. One thing I noticed when taking the dash apart was that he had smashed/cut the wiring to the OD button. Though I don't think that wouldn't have anything to do with what's going on, would it?

Put the dash back together and called it a day... Called the company I ordered the new PCM and they were "busy with too many calls" so I sent the customer service an email asking about the status of the PCM. I ordered it 12 days ago and when I ordered it, they told me it woulld be 3-5 days to process and program the PCM before sending it out. Seems like it should be here by now...

Looking at the pinouts again, maybe I should try disconnecting the ignition coils as well? Going to look into testing all of the connectors at the PCM and see if something else weird and unrelated is causing a problem in the PCM? Like I said, running out of ideas of things to check/do here...

Oh yeah, and I had the glovebox out before but when tearing the dash apart this time I noticed...


Apparently my truck is named "Diane" Haha!
Also found an original owners manual for a 1999 Dodge Ram - pretty cool!

 

IAC is 12 volts. Two circuits. PCM energizes both circuits to lock the IAC in place.

O/D switch is indeed 5 volts...... That *might* be the problem......

That one is definitely puzzling. By your tests, it is NOT a wiring problem.... but, the same problem with two PCM's?????? hhhhmmmmm........

With all the various sensors unplugged, you still see 5 volts on the signal return wires?????

 

Well, the company I ordered my PCM from got back to me. They had 3 in stock and upon testing found that they were non-repairable...? So my only option is to send mine in for repairs/programming or wait for another to come in.

That raised the question in my mind - is my problem a "non-repairable" issue as well? Theyd asked the codes/issues I'm experiencing so I told them and then asked what the issues with the ones they'd tested were and if they thought mine would be a non-repairable issue as well. That was this morning - so I doubt I'll hear from them again until at least Monday at this point...

I can't find much about my specific issue online that people haven't solved by finding bad wiring... So this can't be that common of an issue, right? But like you said... 2 PCMs doing the same thing!? That's too weird of a coincidence....
Unless it's more common than we think and people just take the trucks to a shop or dealer and they just throw a new PCM in them...?

Sort of makes me want to go part-out hunting to call to the u-pull and see if they've got a 98-99 v10 in the yard with a PCM. Would suck to drive the hour to the lot and not find anything. Well - you always end up finding something - just not always what you need.

Did a few more tests - more like sanity checks on myself. Tried unplugging C2 and C3 at the PCM to see if any of the components on those plugs could be messing with things - still 5v on signal return wires.

Pulled every relay and all but 3-4 fuses (the ones I left in were required to get the PCM power) to see if there was some component doing something weird - still 5v. Wasn't expecting much because I don't have any draws on my battery - which I've checked every time I go out there...

Decided I was fighting a listing battle and started in on some chassis re-wiring. Probably should've done it before but tests had shown they aren't really interfering with anything - besides my taillights and blinkers all worked - somehow. Haha!

The before:



The after:



After I fixed those sections of wire I DID check to see if for some reason that fixed my 5v issue - and no...

Oh! I did find this when I was working on wiring...


Was going to the pumpkin... VSS maybe...? I haven't looked into it yet. Someone had wrapped it all up with duct- tape and electrical tape but when I grabbed the harness it felt hard. I have it a twist and it all crumbled and fell apart. I was sure I'd found my short! Cut the wires back and got them separated!.... nope. Still 5v on return.
These wires were twisted... so I assume when I splice in new ones I'll want to twist them up as well...? Why's this? Are they worried about signal interference?

That was the day...
Giving Diane all my love but she still won't love me back.... 😂

 

Yep. The wheel speed sensor is a signal generator, so, twisting the wires reduces RFI.

The 5 volt supply is from the PCM. All sensors are sourced from the same supply. (but maybe not the same pin.) If there is an internal short, (bad circuit board) yep, that's not exactly repairable. I would be tempted to REMOVE a couple of the signal return wires from the PCM connector, and check those pins with ignition on, and see if the PCM STILL has 5 volts on those pins. If they do, there's the problem..... Can test the other PCM the same way.

 

So do you think the VSS could've fried the PCM?

All of my testing until today has been on the C1 PCM connector and related sensors... So I never saw an issue from the VSS since it's on C2 - if I remember right..?

Would doing what you said be an equivalent to what I did by cutting the MAP return wire close to the PCM? Obviously less intrusive since no cut wires... Would still be measuring the wire straight off the pin from the PCM. Or am I understanding wrong?

​​

 

Pretty much. If the VSS is working properly, it probably isn't a contributor to your issues.

The idea is, to see where the voltage on the signal return wires is coming from. If you still see 5 volts on the PCM side, even with the wire disconnected, it's a pcm problem. If you see 5 volts on the sensor side of the signal return wire, when it is disconnected, problem is elsewhere. If you see five volts on BOTH sides..... I think you are just screwed....

This is gonna be a fun one to chase down, and I am most interested in just what the problem is here. Where are you located??

 

That's the thing - I have no clue if it was working properly...? They had it on a trailer already when I went to pick it up and we brought it to the house. I don't recall ever looking at the speedo when I was pulling it in the shop. I only drove it about 100yards at a crawl - most of which was in reverse - so I was looking backward.

Okay - Well, I think I can achieve what you're asking me to check and fix something I told myself I'd "do the right way" after I figured out the issue. If you recall one of my original posts I said I had cut the 5V Return wire for the MAP sensor a few inches from the PCM connector and found that I still had 5V on the PCM side. I'm pretty sure I checked the harness side of that cut with the MAP unplugged and didn't find 5V but I don't specifically remember what I found... Anyway - I need to cut this connector out and and re-crimp and get some heat shrink tube over it - so I should be able to test both of what you're saying when I'm doing that.

Not that I'm not willing to pull wires out of the connector and plug it back in and check actual pins... But I remember when I took the connector apart when I was trying to check if something has shorting out internally somehow and it was kind of a pain to put back together... Haha!

I'm located in central Iowa. Gonna come down and help me out? Plane ticket might be cheaper than your gas to get here! 😂
My wife was telling me last night that I should try to join some "Iowa Dodge Ram" facebook groups and see if there were any experts locally that would be willing to help out - but I'm not the biggest fan of social media....

Was looking at the inventories of the junkyards - u-pulls and the ones that just sell parts and list their inventories.... No v10's. There's a yard in Minnesota with a PCM that almost exactly matches my PCM VIN - but they want $250 for it. Found maybe 5-6 part-outs for v10's - one being the one I already nabbed the PCM from and the others are in the years surrounding 98-99 - so not sure I'll get lucky enough to grab another one for cheap.... Obviously there's more testing to be done I was just looking around at what I could late at night...
Probably the most frustrating thing about this is that I'm am machine mechanic and work on million dollar machinery with far more complicated wiring and electronics on a daily basis - but THIS is what's stumping me. 😂

 

Spent some time this morning fixing grounds under the battery tray then clipped that 5v return signal wire at the PCM.

These are measured to battery ground...
"New" PCM:
5.2V coming out of the PCM.
0V on the wire on the harness side - MAP unplugged.
.009V on harness side of wire with it plugged in

5.21V at 5V signal on pin at MAP connector
.009V on the ground on pin at MAP connector
​​​​Getting 1ohm of resistance from pin to battery

Original PCM:
5.16V coming out of PCM
​​​​​​0V on harness side of wire
.007V on harness side of return wire when plugged into MAP
​​​​​

5.18V at 5V signal on pin at MAP connector
.005V at ground on pin at the MAP connector
1.5ohms of resistance from pin at MAP plug to battery

Think I got two bad PCMs here....

I don't think there could be anything else feeding in to the PCM that could be effecting the 5V bus, right?
So something wrong with the internal grounding in the PCM as well? Where does the PCM ground to?
I kind of want to splice into the sensor and PCM ground wires and give them a line directly to the battery...

 

Two PCM's with the exact same VERY RARE problem isn't very likely, but, it isn't impossible either. You could try a third one..... and see what happens.

Try unplugging EVERYTHING on the 5 volt bus. Leave one sensor plugged in, (map is already set up for the test) see if you still see five volts on the PCM side of the sensor return circuit.

Buncha years back, I jokingly suggested to another member, that it would be cheaper for him to fly me down to Memphis, put me up for a few days, and I would fix his truck (plenum, and timing set)...... Imagine my surprise when he asked which airport I wanted to fly out of. So, off to memphis I went for a week. It was HOT and humid there, so, I could only work for a few hours in the morning, then it was just too hot. Got it done though, and it ran great.