itzdave

Hello, first time poster. I recently put a 2002 lm7 in a Mazda Miata using a home made harness and it ran great for a few weeks. Then the power wire from my alt grounded out on a header, and fried the harness. I bought a new harness from PSI and wired it up and now the car doesn't want to run right. The first few times it started, tach worked, oil pressure worked, but the fuel pump relay kept clicking on and off. I realized that I didn't have the motor grounded so I fixed that, and double checked that the car was getting proper power. Then when I tried to start it, the tach and speedo quit working (autometer speedo sweeps when car is turned on, now it doesnt) and my oil press gauge pegs high when cranking. Fuel pump relay still flickers on and off at almost anything over about 2k rpms. I checked all fuses and relays, none are blown. Pcm shows no codes.
Is it possible that my pcm has been fried? Has anyone experienced anything like this before?

More info about car: 1990 Miata
2002 lm7
Walbro 255 f/p
Corvette fuel filter/reg
Pcm has been tuned and vats/junk removed.

LS1-450

iTrader: (33)

Yes, it's possible. Checking ref voltages to sensors from PCM can reveal a fried PCM. Also, sounds like a ground issue. Should be 3 grounds to back of passenger side head. You're just gonna have to keep testing everything until the issue(s) are found. Electrical problems are frustrating, to say the least. Good-luck

BP Automotive

iTrader: (3)

Make sure you have everything grounded. Batt to chassis chassis to engine, etc.

For checking the pcm make sure you use a digital multi meter and grab up some GM diagrams. Start checking your reference outputs/signal circuits.

A scanner would be worth its weight in gold in this situation.

Sounds like still may have a ground issue, you can always use the GM schematics to trace out your fuel pump circuit to make sure the pcm isnt putting out an intermittent signal. It is the dark green/white stripe wire at red 9. Good luck!

Bill

itzdave

Thanks. Yea I'm going to double check the other grounds in a little bit. The harness doesn't have grounds on the passenger head, but on the drivers side. I connected them to the valve cover because it was just easier to access, so ill change that as well.

BP Automotive

iTrader: (3)

More than likely this is your problem, it must be grounded to the cylinder head.

ls1nova71

iTrader: (4)

Yup, I agree. Definitely don't ground them to the valve covers, since they're isolated by the rubber gaskets and rubber seals around the valve cover bolts.

itzdave

So that one (probably bad) ground can cause all those issues? Haha I'm dumb...

itzdave

Mini update:
Thank you everybody for the advice on relocating the harness ground. Car fired right up and runs great!
However, still no tach, oil pressure gauge, or speedo. Its like the cluster isn't acknowledging that the car has been turned to the 'on' position. Must have popped a fuse or something but they all check good visually and with a meter.

ls1nova71

iTrader: (4)

Bad grounds can do all kinds of screwy things. What do you have your gauges grounded to? If the speedo isn't doing the initial sweep like its supposed to, I would check it or the power to the gauges.

itzdave

Yep. I forgot about the fuse block under the dash for the cluster, it had blown. Fixed it, and its all fine now!
Thanks again for all the help, I'm sure ill be posting again next time I have problems, Haha.

itzdave

This is my car btw.

ls1nova71

iTrader: (4)

Nice! Glad you got it going.

itzdave

Drove it finally. Still keeps popping the fuse for the cluster. And when I did a hard pull, at the top of fourth gear, power shut off on the car very rapidly kind of matching the rev limiter. Scanner showed a few codes.
Fuel pump primary circuit.
Mass air flow low voltage.
Fuel injector.


Time to do some some more troubleshooting, next track day is a week away!