2000 Merc 5.7 MAG MPI stopped running for no apparent reason

[2100V]

Boatwakes,

do you have any idea exactly how the ECM tells the coil to fire when it receives the signal from the sensor? Does it send a positive voltage to the coil through the - terminal/white wire or does it ground it?

[2100V]

Well guys I replaced the ignition sensor and the coil, and I have no spark whatsoever. Everything else works, I turned the switch off and on and cycled the fuel pump/injectors so many times there's raw gas coming out the exhaust so I'm pretty sure it's isolated to the ECM not telling the coil to fire. Here goes this year's Christmas bonus.

[boatwakes]

As you've looked at it before, check out section 5G-39 and follow the diagnostics flow chart. From what your telling us, circuit 430 (purple/white going into ECM) needs repair. On page 5G-40, it also suggests disconnecting the tachometer and then trying to start the engine. If it runs, theres a short in the tach circuit. Don't give up yet. If you're in So Cal I've got a diagnostics computer.

[2100V]

Circuit 430 has 5V in it with the key on, which is the normal sensor reference voltage coming out of the ECM. Correct me if I'm wrong, but the tach wouldn't work if it had a short would it? It actually sits at about 300 RPM while the starter is spinning and drops back to 0 when I let go of the key. I'm in Knoxville, TN but I wish we were closer because I would love to see what the scanner says without paying the dealer $98.50 to tell me.

[2100V]

It's at the dealer now. I'll let everyone know what it turns out to be for your future diagnostic needs.

[Wickedcummins]

Not sure on the start algaryhm but have you checked TPS?

Most start tables Use TPS and CKP to fire injectors and ignition! A bad map and IAT and ECT will start off Limpin Base table!

[2100V]

Well guys it was a short in the tach circuit under the dashboard somewhere. I spent more money on a coil and ICM I didn't need than I spent at the dealer. They diagnosed it by hooking the coil hot straight to the battery (which was my next move, go figure) then chased the circuit until they found the short. Hopefully this info. will help someone else down the road. Thanks...

[NICKYPOO]

It always seems like its some stupid little thing like that. At least its fixed. Sheesh, what a bitch.

[Wakemaker]

So did you replace the tach or find/fix the short? I am having a similar problem and thought I would try disconnecting the tach at the back but then not sure if would run without tach. Thanks for sharing

[2100V]

Two of the wires on the back of the tach rotated just enough to touch each other when I hit a bump I guess, and it was grounding out the coil.