Low Voltage to ECU and Rev stutter problem possibly fixed

[Reichart12]

So let me just start by saying, hopefully this is the right section for this post.

And a little back story, I have a 1990/1? m20b25 stroked and bored to 2.9 swapped into a 1987 m20b27 car running on megasquirt 2. I'm not an electrical wizard but Im learning. I took my car for some dyno time in June and they told me they couldn't do anything above 5k rpm because lack of fuel. Really it was just that the CPS was too far away giving loss sync problem. Fixed that and replaced some fuel system items just to be safe. For about 1-2 weeks the car would pull smoothly to 7k (7300 redline). Then after that and since the car sputters when it reaches 4500 rpm and will continue to rev up but is choppy and pretty lackluster. I thought it was just lack of tuning in that range. I go back to the shop to finish the tune and they tell me after a couple hours, I have an electrical problem holding it back now.

What I've tested so far is a red/blue wire running into the ecu that should be the power from the main relay. This will only read 8V. The battery post in the engine bay reads 12V, the starter and alternator are both 12V. I figured I would check the main relay next and it was 8V in and out. I changed it out because it looked beat but no difference.

So now I'm here looking for clues and suggestions. Would there be another place to check? Could the pin going to the relay be bad? A bad spot somewhere in the wire? I'm hoping to solve this relatively fast as now I have a cold hard start problem in the mornings (although this could be unrelated).

[b*saint]

What's the voltage on the common side of the relay? Before the contacts?

[Reichart12]

When testing the relay I left it plugged in and turned the key to the second position. Had the probe in the bottom side of the block where the wires go in. 8V in, 8V on the connector wire, and 8V out to the ecu. Having the car running may have changed it to read 9V.

[b*saint]

So its coming from the battery 8v. I would keep probing back until you find the drop in voltage.

[Reichart12]

No the battery is 12V. I thought you were talking about the main relay. When I get home from work today Im going to do some more poking around and maybe try a different battery that I have.

I read somewhere about an inline fuse that is part of the wire for the main relay and dme, any idea where that may be located?

[Kershaw]

Are you talking about the fusible link? It's in the trunk.

[Reichart12]

Yeah that's it. Thanks. Im going to start in the back and work my way up trying to find exactly where the drop occurs. Before I was thinking in the engine harness but who knows.

[RobDog]

I thought power for the main relay came from the ignition switch to the OBC relay then to the main relay. I have read where someone was having similar problems and it was the OBC relay. The wires you should probe should be the green and green/red wires going to/from the OBC. These green wires are the largest of the wires going to the OBC relay.

[Kershaw]

That's a good point, the OBC can kill power with the code entered so power definitely runs through there. Only if it's a 13 button though.

[Reichart12]

Oh awesome lead. I have the 6 button OBC and its always been funky. The display shows random shapes and flashes around a lot and have never really worked.

[b*saint]

If it was the fusible link, wouldn't you have low voltage on the whole car? Or you do?

[Reichart12]

I thought the fusible link was inline between the ecu and main relay at first, obviously not. So after driving about 1.5 hours home I checked the voltage of the battery, battery post in the engine bay, main relay, and the wire to the ecu. All read 13 or 14V. That looks normal to me but it makes it even more confusing that the other day half were 12V and the others were 8 or 9V. Maybe its drained slightly when the car is parked and turned off.

I tried to datalog driving around and the battery voltage on megasquirt was 11 with the car off, dropped to 9 cranking, and then maintained 11.5-12.8 while driving. Heres a screenshot of a higher rev where the rpms get choppy over 4500. (I know the afr is funky too)

[b*saint]

Voltage regulator?

[jeffnhiscars]

If it was the fusible link, wouldn't you have low voltage on the whole car? Or you do?

You'd have no voltage anywhere. Fusible link is a one time circuit breaker

[Reichart12]

So fusible link is good. I changed the fuel and o2 relays as well just because. I checked out the voltage regulator, the posts are shaven down and one is lower than the other but not all the way down. Will order one because its cheap anyways.