I've been converting an '87 harness to work with a '94 series I. It's for a track car, so we are not even attempting to integrate with the stock Fiero harness. In looking through the schematics, we found several circuits supplying 5-volts to various sensors. The all seem to find a common ground among all the 5-volt circuits. We've taken care to do exactly that, but are still having the conversation whether this is required or should we in fact be grounding these circuits to the same point as any/all other circuits? what have those of you who have converted wiring harnesses done about these circuits?

Thanks.

The sensor 'return ground' should not be connected to engine ground. They should run back to the PCM, I shown WC6 as the ground for A of the IAT, G of 4T60E, and B of the EGR. Also BE5 as A of the CTS and C of the TPS.

I also keep the Black/White PCM grounds (black) separate and ground them to a separate bolt from the relay and injector grounds(Tan/White).

The reason?

If any of these grounds ever becomes loose, corroded or otherwise slightly imperfect, you do not want your relays, or injectors or other power requiring devices to be altering the sensitive sensor inputs and thus causing poor driveability issues in the future.

Words to live by!

Most fail to realize that everything has resistance, and that resistance gives rise to differences in potentials.

During cranking, the voltage of the engine actually increases when compared to the voltage at the chassis, or the ECM. Not by much, but more then enough to screw up readings from the TPS, IAT, or MAP. This is why these sensors MUST be grounded directly to the ECM as opposed to the fluctuating voltage of the engine block. This also highlights why it is important to have a STURDY ground from the battery to the chassis (powers ALL accessories on the vehicle) the chassis to the engine (to common grounded sensors like engine temperature guage, and for the alternator to have something to work against) , and the engine to the battery (for cranking), and the ECM to the engine (for single wire sensors like the knock sensor). Otherwise, these things will have erroneous readings, or just malfunction.

quote Originally posted by phonedawgz: The sensor 'return ground' should not be connected to engine ground. ....

Thanks for the input. That all falls nicely in-line with our reasoning. (One of our crew is a licensed electrician) We've had some high level discussions with the shop reprogramming the computer who points to our not going to a common ground as a problem. They "have always done it that way."