Symptoms of a bad/failing ECU?

[AuroraGirl]

System grounds are good on both harnesses. So sounds like an ECU fault yea?

Now you need to determine if your PCM is sending ground on the pin that we have discussed already...

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the power is piggy backed off the brake light. that must work? since you had ti working with test light.

Then the 419 BRN WHT wire to the ecm is the ground wire. follow this to the ECM connector, I would personally either probe the wire to see if it has ground (use a sewing push pin, pierce, then alligator clip and then go to your make shift bulb or SES. Then provide it power or turn the key. Work? Then its the wiring from that point until the cluster

if nothing, then you need to inspect the connector, terminal, ECM connector pin etc which is why I broke the recommendation for what to do up for the easy peasy and the "get involved more"

KISS

 

[AuroraGirl]

System grounds are good on both harnesses. So sounds like an ECU fault yea?

No you dont seem to understand what we are saying. The wire from the PCM does NOT power the Bulb. The Bulb is "powered" by a fuse in the fuse box. The bulb has no ground except when the ECM chooses it to be
So the ground from the pcm is the wire that connects it to the PCM. which provides it ground. the ground then goes through the ECM ground on the intake or whatever.

 

[SquareRoot]

She's back !!!

 

[nmyron3983]

My brake lights do work, yes. But I feel like we're saying the same things, and that maybe I am just relating it poorly.

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I followed the flow chart above for "Does engine start" = yes. I pulled the battery leads, I disconnected both harnesses from the side of the ECM, reconnected the battery leads, turned the key to on, inserted a t-pin into A5 on the ECM harness, which is the brn/white wire for Circuit 419 shown in the second image, and grounded that using a 12v unpowered test lamp connected to chassis ground, and the SES light illuminates as long as I probe it. That would say to me that power is flowing up from the fuse box, through the cluster, and my 12v unpowered test lamp can successfully complete the circuit by taking C419 to ground right? I didn't feed power into it using a powered lamp or anything, I just completed 419's ground path using my unpowered test lamp connected to ground as the flowchart suggests.

To ensure the ECM can actually ground anything, I left the key on, put a t-pin in b1, a 12V feed on the A/B harness and probed A12 (edit: this was supposed to be A12, system ground, I am a bad thumb typist), the system ground on the A/B harness, as well as C16 (12V) to D1 (system ground) on the C/D harness to confirm I could send each 12v feed on each harness to the system grounds. Both illuminated my unpowered 12V test lamp. So both of the ECM's ground paths are good. That would say that if the ECM were doing the diagnostic portion of it's startup, it has all the necessary paths to ground C419 through to either of it's harness grounds.

Edit: doesn't probably help that I keep using ECM, ECU and PCM interchangeably. LOL. I changed them all to ECM

 

[nmyron3983]

Also, I determined using the pinout guide you provided that my first tests performed the night before my initial reply weren't actually valid, the ones I described in the linked post below, as I had backprobed the wrong wires on the ECU harness. I think thats also part of the issue, as I hadn't actually tested the brn/white wire at all at the ECM harness, which runs to pin A5 on the A/B (I was actually in a wire on the C/D harness during those first tests)

Post in thread 'Symptoms of a bad/failing ECU?' https://www.gmsquarebody.com/threads/symptoms-of-a-bad-failing-ecu.18902/post-771890

So I think that's adding a level of confusion here.

 

[AuroraGirl]

She's back !!!

 

[nmyron3983]

So, just wanted to provide an update and closure. After the diagnostics last weekend proved the ground path to the ECM was solid, and that with key on grounding C419/pin A5 to the chassis allowed the light to kick on, I debated, then I opted to order a remanned ECM from ACDelco.

Just got it in this AM, swapped a minute ago after transferring the PROMs over from the old unit. Lamp is now on at key on, and disappears while running.

Once I get the dash back together (I took the pad off, reaching through the AC vent cavity let me get more light in there) I'll take it for a drive cycle then check for codes, but, we're at least solid on a solution here.

Now I just need to troubleshoot that 4WD lamp. I'm pretty sure the issue is chafed wires because it's laying under the vinyl in the driver footwell. Prob need to roll the vinyl back and see what's happening under there.

 

[AuroraGirl]

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do you have 67?

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[nmyron3983]

I'll get up under there tonight and take a look

 

[nmyron3983]

Hooowee. I have an NP208 case. And if thats the right diagram, it's missing. I found a two pin black/white harness dangling by the front output shaft.

This P.O. is a bit of a arsehole it seems. Gonna take this diagram under the truck and see if I can sort out where it would thread in and go from there.

Probably aught to drain and fill it now, and spray it down with some degreaser. There's gear oil all over down here.

 

[AuroraGirl]

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my np208 came out of a 85-86 most likely, and here appears to be the switch which I literally did not notice ever before until reviewing this pic. huh.

 

[nmyron3983]

It wasn't missing, for whatever reason the P.O. (prior owner) had disconnected it? I have no idea why.

Oil pressure gauge is next, it pegs at engine start and stays there, but if I kill the motor and turn the lights on it drops to 30-ish PSI. Guessing that's likely a ground issue, looking up data on it now.

 

[nmyron3983]

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my np208 came out of a 85-86 most likely, and here appears to be the switch which I literally did not notice ever before until reviewing this pic. huh.

This is where I found mine.

I checked the RPO's and the 208 matches, so it's not like it's a swap (maybe it is, maybe the prior owner trashed the t.c. and swapped this in and left it off?) But the position sensor was present, just buried way back in the tunnel on the top of the housing. Case is covered in grease. Probably need some new input and output seals.

Light for 4wd is working now that it's reconnected. LOL!!

Man, if this is what I have to look forward to, it's going to be a fun journey.