95 LT1 Erratic voltages EEhack

**steveo** · 04-02-2023

oh another thing to check is the fuseable links in the engine bay, i have seen them corrode. you can run a jumper from positive battery terminal to the red junction box to bypass them and see if that helps.

**kur4o** · 04-02-2023

It is also possible that the pcm have some inside problems[water infiltration]. Does the 96-97pcm acted like that. You can bench test the pcm.
5v regulator inside pcm may went south. Also measure the voltages and confirm they act like that. There could be some glitches in data communication too.

I`ve been having voltage drop at pcm, and nothing completely cured it. Adding extra thick wires to fusebox, separate wire to fuel pump and so on. Voltage reading at pcm never match the voltage at battery. At the end I had to add a separate wire to a spare a/d channel to read voltage at injectors, so offset being used are as accurate as possible.

The one wire that goes from fuse block to pcm is really weak. You can also check spark wires for arching, running engine in a complete darkness.

**Opti** · 04-02-2023

Alright alright well that gives me a place to start and some things to look at. I'll start chasing it tomorrow. If I find anything I'll post back.

**In-Tech** · 04-03-2023

kur4o is correct, it could be inside the ecu. The suck is chasing shite you can't see, lol
I built a cap bank with switches so I could figure out what it needed, then soldered in the capacitance. Figured it out once the harness was out to replace ecu with a holley ecu to talk with the holley dash.
Turned out to be the noise from the o2 circuit had chafed a bit

**x11 nut** · 04-03-2023

Been chasing bouncing TPS voltage for months, a buddy that is an old timer GM Master tech told me to replace the temp sensor and it fixed it. Old temp sensor was still reading in the logs but it shares ground with TPS. Not sure it is the same as your issue as you have it going with more sensors.

**In-Tech** · 04-03-2023

Originally Posted by x11 nut

Been chasing bouncing TPS voltage for months, a buddy that is an old timer GM Master tech told me to replace the temp sensor and it fixed it. Old temp sensor was still reading in the logs but it shares ground with TPS. Not sure it is the same as your issue as you have it going with more sensors.

Yes, the analog ground chasing can be a biatch :( My little cap bank can work on either, but mainly used on the 12v ign sw side.

**steveo** · 04-04-2023

you could unplug every sensor to see if your input voltage stabilizes

**kur4o** · 04-04-2023

All these sensors use 5volts reference from pcm, If a sensor is bad, it can screw or fluctuate bad enough the 5 volt source. Really good idea to start unplugging stuff.

**Opti** · 04-05-2023

I unplugged everything I could get to. About the only thing fluctuating I didnt unplug is the evap temp sensor and o2 sensors. I dont know what the evap temp sensor looks like and I just forgot about the o2s.

I only had a couple minutes so I couldnt get much done but here is what I did.

I tossed the obd2 computer back in and it fires up and runs fine. I do have a super high idle on the obd1 pcm that i assumed was unrelated to swapping the computer and I noticed the erratic readings when I tried to diagnos the high idle.

I check voltage to the obd1 PCM right at the connector (both orange wires) and its steady at 12.06V, it will move to 12.07 every couple seconds but not the frequency like the readings im getting on eehack
I checked voltage out on the MAP sensor (a reading that is fluctuating) and it is as steady as the voltage into PCM.

I did open the obd1 pcm to look for obvious signs of water intrusion or damage. Didnt see anything.

Does this mean I likely have a bad OBD1 PCM1?

**NomakeWan** · 04-05-2023

Your multimeter might not be fast enough to detect (or, more likely, display) the fluctuations in voltage. Problems like this are where an oscilloscope come in handy.

That said, if you do confirm that the voltages external to the PCM are stable, including the ones that the PCM itself are generating (such as the 5V for the sensors), then process of elimination suggests the PCM is indeed bad. Maybe an ADC circuit, or one of the ADC's reference inputs. It's still able to communicate over the bus, so the whole PCM is not bad, and it's able to generate valid serial data. But clearly something related to the ADC loop is failing.

**kur4o** · 04-05-2023

You can check the ground strap between 2 boards inside pcm, I read somewhere they can pose such problems. Or pcm can`t filter noise from engine it hits the ADC.
Tom H might give some clues what may be the hardware cause.

**Opti** · 04-05-2023

I should have a little more time tomorrow for testing. I'm going to go back and unplug everything and see if it still fluctuates.

Also will check the a voltage output from pcm.
I think I've got a buddy who has a 94-95 pcm laying around I could swap in and check.