I thought about failure modes a bunch last night, going over all the symptoms I've had in this debacle, the chattering from the fuel pump relay, injectors dumping fuel randomly, loss of idle control and finally limp mode with the trans and finally had to get up and get out my schematic book and look through it to see if there was one component somehow connected to these systems at least in a peripheral way.
And after some consideration I came up with the oil pressure switch.
On the motorhome and perhaps some other rigs with the 7.4 the fuel pump power runs through a switch in the O/P sender and returns a signal to the PCM, so if the sender failed it could possibly pull the F/P circuit low and pull who knows how much current off of other PCU circuits if a contact ring in the sender was making contact with the metal case wall.
Anyway, that was my theory and I was able to get one at the parts store this morning so with high hopes I put it in along with a new MAP sensor.
There was no change it how it ran though. So I hunted & pecked, rechecked power supply to everything I could think of finding no problems, and then put my O-scope on the IAC to see if I could catch any wave form there.
Saw nothing on the scope, checked that I was in the right ranges and all I could see was a little static.
The only other thing I found that was suspect was a spark plug wire with a slightly melted plug boot so I changed that.
Took a sit and a think for awhile and decided I had nothing to lose popping the old PCM back in to check, the one I was pretty sure I'd killed.
So I did. It gave me a double blink with the MIL in KOEO - something I've taken as a sign of life, and it lit right off and ran beautiful with no MIL when I turned the key.
Huh?
If you feel confused imagine my disposition seeing that.
Went for a test drive, it died at the first three stops, but stayed running if I used neutral when stopping. Restarted immediately each time though and wasn't setting a MIL.
I found a place to pull over and relearn the IAC, plus made a small adjustment to the TPS which was reading .6 of a volt instead of the 5.8~5.9 it's supposed to have.
The rest of the test drive was perfect, didn't die at stops, got to full temp and stayed running smooth.
So I'm wondering something on this, is there a PCU failure mode where the only way to clear the gremlins is to power it down and restart from scratch?
If so this is the first time I've run into it, but considering I may have been running leaky voltage all over the harness with a bad oil pressure sender I wouldn't blame the thing for going turtle on me.
Anyway, it seems to be doing Ok right now, I'm still going to be nervous driving it at least for a while - but if it still runs in the morning I'm going to set out and see how far I get.
I've got 1200 miles to go and that seems like a big ask of this old rig, but I'm not finding anything else wrong so here goes...
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