How does the ECM calculate RPM?

[89mulletbird]

OK my car (TTA) will start backfiring hard randomly on the freeway and throws a code 42. When it starts backfiring I see on the tach and scanmaster that the RPMs are erratic. It should be showing 2500 or so but the needle is dropping down to 1000 or near zero.

I have already changed:
ECM
Ign Module
Coil Pack
Crank Sensor

I checked the cam sensor cap and it looked OK but didnt change it.

What else should I try??

 

[Razor]

Normally I would suggest ignition module or ECM but since you swapped it, next move is the chip itself.

 

[89mulletbird]

Man.. what could be wrong with the chip? I'll have to talk to Eric at turbotweak to get a spare to try out.

 

[novaderrik]

it doesn't cost anything but some time and effort to check all connections for corrosion.

 

[TurboBob]

The ECM calculates RPM from the amount of time between crank sensor pulses.

The crank sensor feeds the ignition module, which then feeds the ECM.
The ignition module also feeds the tach. (The ECM can't affect the tach)


The connector at the ignition module is known to get flaky with age...

Bob