89IrocZRagtop

My 89 RS Camaro w/305 TBI won't start. It cranks over good. It has good spark. It has fuel pressure. The TBI injectors have no pulse. Does anyone know what would cause the injectors not to pulse. The car starts and runs fine most of the time. But when i park it outside and there is cool moisture in the air it will not start. Has been doing this on and off for a year now.
Thanks

RFmaster

Ragtop

Check the dizzy to harness connector. If you are loosing DRP signal from dizzy to ECM (Circuit 430, Pin B5, PPL/Wht wire) ECM will assume that engine has stalled. When you get DRP drop out in addition to timing retardation injector control pulse is also cut off. This is a safety feature since ECM must have dizzy reference signal indicating that engine is rotating. The only true way to see this is with oscilloscope - if you have one. Check for flaky wiring harness, pins inside ECM connector(B5), dizzy to harness connector (4 pin connector). Check for loose ignition module (inside dizzy) mounting bolts, any rust or dirt build up as well.

C430 (ppl-wht) to ECM B5 is a reference pulse from dizzy to ECM. This signal tells ECM when "points closed". When this line measured with DVM at idle (depending on DVM circuitry) it may read 1.5 Volts.


C453 (blk/red) to ECM B3 is a ground reference between dizzy and ECM. This line should be at 0.0 Vdc when measured to ground!

C423 (white) to ECM D4 is EST circuit which triggers the ignition module. The computer (ECM) does not know what the actual timing is, what it does know is when it gets a reference signal. It uses this reference signal as a base point and advances or retards the spark from that point.

Report your findings...

//RF

89IrocZRagtop

Thanks for the help RF. I traced the problem to the Vats System. Before I bought the car, someone bypassed the vats system by removing the starter relay from the vats to the starter. Then they took a resistor and just twisted it loosely on the two tiny wires that went to the key switch. So with the starter relay bypassed the car would always crank over. But with the loose twist connections on the resistor it would not fire the injectors everytime. I first thought it might have been the vats, but everyone told me if it was the vats the starter would not crank. Now I am just wondering why someone bypassed the vats.