1992 Chevy P30

[boba]

I'll pull the harness out one more time and look at splices over by the DRAC module and steering column, resistance check on the speed sensor pair look ok, not melted together, broken or going to ground as far as I can tell.
It seems like splicing in grounds to the ECU might have done the trick, I'll know more when I get it back together and see if I can actually drive it anywhere. :-)
[circuit grounds cause all kinds of weird symptoms when voltage gets back through the circuits] < that's the part that worries me, I don't know why or where it failed exactly, but it seems like that's what I'm up against on this one.
Thanks for fielding my question here Jeff, I'll post back when I know more...

[MO LS Noobie]

Stray voltage can happen when you have high resistance at ground connections. Instead of going to ground to complete the circuit, it backfeeds into other circuits causing havoc.

[boba]

I put everything back together this morning thinking I'd repaired the bad ground and I was ok. It started and ran fine last night.
It didn't get warmed up to go down to the tank dump this morning before it was acting up again. This has become the consistent pattern.
Got back in and checked the ground on the thermo-housing, cut the ring terminal off and saw that the wire was discolored back under the insulation, got aggressive and cut 4" back off the wire till it was well and clean and crimped on a new extension tail and new ring. Fastened it to the alternator bracket. Seemed ok for a hot minute, but it didn't maintain it's manners for the 1/4 mile to the dump station. And it flipped the speedometer needle all the way over also which I've never seen before.
I ordered a replacement 7060 ECU off of Ebay, I gotta get this thing driveable soon and I can't think of anything I've left un-checked.
Re-checked the TPS voltage (it was a little high) Set the throttle screw to adjust, and then re-did the IAC set procedure too just for grins. I didn't need to adjust for base idle - it was good, but it won't hold target RPM (should be 750, idles at 800).
Did those settings with the scanner so that's what the ECU is seeing.
Anyway, that's where it is for right now. When it went bad it was all of a sudden, and I haven't found a bad sensor anywhere in the system...so with crossed fingers and low hopes...it gets another ECU.
Past that I got nothing.
If I can't get it going it'll be out in the California desert with a signed title in the tray over the drivers seat.

[boba]

Tested the DRAC module, first test was to plug into a breadboard and measure current draw on 12V power.
Failed. Module draws no current. Complete open circuit.
I'm using the pinout from the factory service manual, pretty sure I'm on the right pins.
I have some discrete electronics with me, can this thing be repaired?
Anyone have s schematic?

[MO LS Noobie]

I have some discrete electronics with me, can this thing be repaired?
Anyone have s schematic?

I have no experience with DRAC modules, but a quick Google search turned up:

https://www.udmodule.com/

https://atracom.blob.core.windows.net › gears › 2005 › 2005-04 › 2005_04_04.pdf

https://harristuning.com/Tbi/vssb-drac-calibration/

I hope they help.

Jeff

[boba]

I was pretty much just chasing down hunches and trying to verify the condition of things that may have been causing a problem, I don't know that the DRAC should draw any current with power connected or not, the output speed sensor will generate a AC voltage as it runs anyway - so that may be what really powers it up.
I'd found the UD and Harris tuning links the other day when looking, can't get the atracom link to work though. It would be nice to get an explanation of how the DRAC does it's job so I'd know what to look for.
I did use the Harris link to set jumpers on the DRAC I do have, the speedo has always read a little fast so it was worth doing and I'm curious to see if it makes a difference.
I honestly think this is going to end up being a 7060 with bad output drivers.
Nothing else makes sense to me after all I've looked over, the replacement ECU will be here early next week...and if it's the correct one I'll know more then.

[boba]

Replacement ECU showed up today, engine started right up & ran fine. Now for a road test & see if the trans will shift.
Edit - mixed success. Didn't work so well once I drove it, trans did shift which was a worry with an unknown PCM, but it still gave me an MIL after just a short time and idle rpm changed.
Looked through the driveability manual and found a reference to check the neutral switch, I did that and it was out of adjustment (how I don't know) but I reset it and things seemed to smooth out after that.
Took it out and ran errands in town all afternoon and it was fine in mixed driving, highway and in traffic. I was thinking once again it was good to go...and then it gave me another MIL and a high idle. (I've not been able to get a code out of this thing the whole time, when it goes bad I also get no-com with the scan tool.)
On the way out of town it went to neutral again but I was ready for it this time and got off the gas, the trans dropped back into gear without me having to stop this time and I got it back to camp Ok.
But I'm wondering if I've got a bad DRAC module and might be losing the VSS signal? To my mind that could do weird things to trans operation.
I can put a scope on it tomorrow and check a little further.
Pretty well frustrated with it for right now, but I'm not quite ready to give up on it just yet.