does anyone know what inputs tell the pcm what rpm the motor is running at, or if there is more than one input?

 

I would guess the high and low resolution pulses from the opti.

 

Low pulse resolution on the Opti.

 

You all know that or just guessing? I would think it would be coil driven...

EDIT: I guess the coil would be fired off the opti though...

 

in most cars RPM is figured by using refference signals from cam and crank sensors.



so yeah, I woud bank on opti refference pulses.

 

The injectors wont fire until the PCM sees a crank signal.

 

im trying to figure out why the pcm sees spikes of 5000rpm while im starting the car and for a few seconds after car is running. Its cutting fuel to the injectors (i.e. noid light dont shine) and making it a real pain to start and very rough idle for about 10 seconds.

 

Med I didnt see the year of the car listed. Are you getting any codes? Any mods lately? What about the VATS system?

 

its a 95. Im getting two codes, 18 for injector circuits and 21 for tps curcuit (high voltage).

I dont think its the vats cuz no light comes on and the car does start. Nothing has been done to the car since last year.

 

The tach is ran off of the low pulse res. off the Opti.

 

so if the tach shows a few hundred rpm during startup, but the pcm senses 5000, would that indicate a separate input to the pcm?

 

Well if the computer is seeing a high tps voltage during cranking, its not going to pulse the injectors. Something to do with an anti flood mode.

 

the tps sensor has been adjusted and is no longer showing high voltage, but the problem persists. Wondering if it could be crank sensor, camshaft sensor, or opti, or if there is any way to test these guys before replacing them. One of them is causing the 5000rpm spikes that the pcm reads.

 

In theory the only thaing that tell the pcm the RPM is the optispark unit sensors. If they are dirty or damaged I think it would give you some real funky reedings. As i recall the tach gets its signal there to however the tach may be slower to respond than a scanner. Are you using a known good scanner?

 

From the codes you are quoting, appears yo0u have an OBD-I car (why not add a simple "signature" with year, trans and major mods, to help people answer your questions?). There is no discrete crank position sensor on a 93-95. And the CKP on the 96/97's is only used for misfire detection, not for timing, ignition or RPM.

The "cam sensor" is the low resolution pickup from the Opti. Have you been getting any Opti optical sensor related codes - DTC 16 or 36.

The Opti feeds the PCM, the PCM counts the revs and feeds the tach. How are you reading PCM RPM to compare against the tach reading?