2G Low voltage in ECMLink but perfect at alt and batt???

[BrandonGST]

So I'm struggling to figure out this issue and can't quite find a thread that describes exactly whats going on with mine. I got my new motor built and dropped in. Now on first startup it was running weird and rough(very rich 10.4) so I checked on ECMLink. In link it was showing 6.4v for the battery! So I multimetered the battery with the car off, 12.6v. Car on the battery is at 14.7v, then I tested the alternator while idling around 1000-1300rpm and got 14.8v. I have double and triple checked all grounds, even added a ground from the alternator through bolt to the chassis. If I leave the car running to full operating temp, the battery voltage in link never gets above 8.2v, even with the alt putting out 14.8 and the battery sitting at 14.7. Does anyone have any ideas on what could cause this? Thanks in advance, this ones got me stumped

 

[DSMPT]

I don't know if this is your case but I had a similar case before with a customer's 2G. In ECMLink, it was reading the voltage way lower than the actual voltage at battery. It finally went 0.1v in ECMLink and got hard to start the engine. It was a bad ECU. Repaired the ECU and the issue was solved.

 

[dustyboner]

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[BrandonGST]

Thank you for the quick responses. DSMPT, That does sound pretty much like my issue, I have always had a lingering voltage issue with the car, but before this motor build the worst it got was 11v or so. It sat the last 6 months while I built the motor and now on first start this low voltage issue got worse. Thanks for the inspection procedure as well Dusty. I will most likely order the service from ECMTuning to get it all checked out. I will update the thread once I get a response from them on whats wrong with the ecu

 

[shinzon]

Sounds like you got a large amount of voltage drop between battery positive and and the ECU. I would first suspect those two white wires that run from the fuse box to the positive battery terminal, those love to corrode, even on my southern car. With the engine running, and the electrical system loaded, touch one end of your multimeter to battery positive, and the other to the terminal on the fusebox where the white wires connect. Basically, you're reading the difference in voltage between two ends of a wire. If the connection is good, you should read a couple hundred millivolts or less difference; if not, you should read 1-3v, or whatever amount of voltage you're missing. Repeat this procedure for any other suspect wires. The charge wire from alternator would be another suspect, also the wire to the MFI relay.

 

[steve]

Maybe I've missed some important data but it seem that there is a critical measurement missing. What does your multimeter say the voltage is at the ECU or at the MPI relay feeding the ECU.

If those are close to your battery voltages you clearly have an ECU issue. If it was a 1G I'd be looking at the CPU A2D reference voltage and the hybrid IC that buffers the input voltages. I don't remember ever seeing like problems on the 2G ECU's but the guys at ECMTuning would have more experience.

If the voltage is low at the ECU I'd measure the drop between the ECU and MPI and from the MPI to the battery to narrow won't the bad wiring segment.

 

[BrandonGST]

I already shipped it out to ECMTuning early this morning . But, if the ecu is not at fault I will definitely try these tests out

 

[BrandonGST]

Just to close this out, I just got the ecu back today. Turns out that some caps were leaking internally causing the low voltage at the ecu. They fixed the caps and now I am reading 14+v at the ecu

 

[DSMPT]

Just to close this out, I just got the ecu back today. Turns out that some caps were leaking internally causing the low voltage at the ecu. They fixed the caps and now I am reading 14+v at the ecu

Glad to hear that

 

[HaveBlue83]

I have this exact problem in my 99 GST......I have 13.8v at the alt and battery. The ECU shows 13.4v.

After 10-20min driving it goes to 12.8v-13.1v, wot is 12.8v and the dash lights for brake and oil come on of when doing a pull sometimes. With headlights on it's also a draw especially at idle, and I've seen down to 12.3 with headlights on, at night, at a stop light.

I've already checked grounds, and the batt and alt seem fine. Maybe .4v lower at warmed up than cold. I ordered a new 90A alt just in case. I also added a ground wire from alt ground post to the frame rail.

I don't see a batt to alt issue. I'm seeing a drop on the logs and via my volts gauge. Soooo it now seems this 95 EPROM is going to go to Tom next.

 

[motomattx]

I have this exact problem in my 99 GST......I have 13.8v at the alt and battery. The ECU shows 13.4v.

After 10-20min driving it goes to 12.8v-13.1v, wot is 12.8v and the dash lights for brake and oil come on of when doing a pull sometimes. With headlights on it's also a draw especially at idle, and I've seen down to 12.3 with headlights on, at night, at a stop light.

I've already checked grounds, and the batt and alt seem fine. Maybe .4v lower at warmed up than cold. I ordered a new 90A alt just in case. I also added a ground wire from alt ground post to the frame rail.

I don't see a batt to alt issue. I'm seeing a drop on the logs and via my volts gauge. Soooo it now seems this 95 EPROM is going to go to Tom next.

Yours sound more like an alternator issue, if its a cheapo parts store reman unit, no matter who its from consider it suspect and probably junk, they will act up intermittently and the parts store tests wont show it as bad even though it is.

 

[HaveBlue83]

Yours sound more like an alternator issue, if its a cheapo parts store reman unit, no matter who its from consider it suspect and probably junk, they will act up intermittently and the parts store tests wont show it as bad even though it is.

I have a new WEI 90A I just got in in prep for swapping it out. The car only has 58k on it and most all the parts are original on it. I will pull the ECU and check, but I'm thinking I'm going to find a bad board. If I don't, and the spare eprom I have gets socketed and tried out, and it's still happening, I will swap the alt. I have done all alt prolly 5+ times on these cars. I went to the 90A Saturn units on my mirage swap and I get a few years from them before they fry out .

I'm also getting some weird things.....AC fan turning on low when the car is cold, and the front O2 refuses to cycle since I got the car last month. And I put a new sensor in. So there is enough to tell me to check the ECU out.

 

[chrisman287]

I have a new WEI 90A I just got in in prep for swapping it out.

Hope you have better luck than I did. Mine was crap out the box. Voltage was good on my gauge but dummy lights stayed lit. Ended up putting a used OEM unit in.

 

[HaveBlue83]

Hope you have better luck than I did. Mine was crap out the box. Voltage was good on my gauge but dummy lights stayed lit. Ended up putting a used OEM unit in.

If it's crap, I'll return it to rock auto. I didn't get an ac Delco unit because the pictured plug was a 2 prong not a 4, so we shall see. It's the original alt in there afaik, I think it's just the ECU. I have a spare eprom that's going out tomorrow to get checked, caps, and socketed so next week ish I'll have that back and I can swap it out.

My dummy lights randomly come on, only faintly, when doing pulls and punching it. But when u read the alt and batt, it's FINE. so I say it's caps.