No Start—Fried O2 Sensor and Main Relay

[alecd]

My '91 vert died on the freeway a few weeks ago, and my mechanic just gave up. Here's everything I know:


It died cleanly, just dropping in RPMs without any noises etc. From then on, it cranks but doesn't start. According to my mechanic, the O2 sensor and main relay were both fried. He also says that it fried the DME, but I don't think he's removed it, and when I took it out it the circuit board looks totally clean.


Mechanic says it's not the fuel pump or crankshaft sensor, so I've ruled those out. He says it's a short in the engine wiring, as you can hear a brief click when you plug in a good main relay. The click comes from the alternator/alternator wires—is a click normal, or evidence of the problem?


I figure if the alternator area isn't supposed to click on connection then that's my problem, and I need to figure out how to get the engine wiring harness off so I can rewire. But if not, I'm lost. Any advice is very much appreciated, and if pics or more details would help let me know.


Thanks in advance

[superj]

The engine harness comes off the engine, like any other. It comes off the round mount on the firewall too, just twist the top part off and undo the snaps and it drops out so you can pull it.


Not surr aboit your clicking but i imagine there should not be a click unless the key is on

[alecd]

Thanks! I got the harness off, and found a bit of exposed wire near the source of the clicking. It's the thick wire that goes to the alternator. My guess at this point is that it shorted to ground, thus frying my o2 sensor, main relay, and (according to the mechanic anyway) my ECU. Does that sound likely?

[superj]

I am not sure. I geuss its possible. See if anyone local will try your ecu out for you to see if their car runs

[richardodn]

I would repair the harness and replace all 3 relays (main, fuel, O2). Is your battery in the hood or in the trunk? If it's in the trunk, check the fusible link near the battery. It may have fried. I don't think they have them with hood mounted batteries. In either event make sure you're getting power to the relays.

[alecd]

I repaired and replaced the harness, no change. Battery's in the front. I'll figure out how to test those relays and report back--I know the main was blown before and I've replaced it.

[alecd]

Update: Relays are good, still no change. I'm at a loss folks.

[superj]

try to find someone else with a 318 and see if their car runs on your ecu. just because the board looks clean doesn't mean a diode or resistor didn't burn up. it could be your ecu stil.

- - - Updated - - -

is there some kind of reference sensor on a 318 that the ecu needs to get a reading from for the fuel injection to fire, or timing to be set off of?

[alecd]

I just stumbled across this and I realize I never explained what happened. Turns out my mechanic installed the wrong main relay which prevented it from starting... I guess the moral of the story is to do it yourself or go to a specifically European car place.

[superj]

at least you didn't have to buy an ecu. and your car is back on the road