This post is hopefully of interest to all those experiencing problems with this series cars (F-body Firebirds and Camaros, and maybe others).

Recently, my brother who runs a car shop got a 1990 Firebird in that another shop had had for two months and could not resolve the problem. I got involved because I am a retired Electrical and Electronics Engineer and sometimes help my brother with eletrical problems he runs into.

Here's the info:

1990 Firebird 3.1 Liter V6 received into shop. Would crank but would not start and run.

Mechanic had replaced distributor and ECM. Also had replaced TPS. Determined fuel supply good to injectors, and ignition system OK.

Researched problem on Internet. Conclusion was VATS keeping injectors from firing.. Common problem was broken wiring in the ignition passkey switch circuit. Also information found on VATS bypass modules. Found schematic of one using 555 timer (http://newrockies.ca/).

Found bad connection at steering column in Passkey wiring connector fixed by previous mechanic. Now had a good VATS square wave at computer. Turned switch off. Signal went away. Would not come back. Conclusion was bad VATS module.

Removed VATS module, Tough. Behind RH dash near ECM.

Decided to bypass VATS. So built one per Internet schematics. Internet info mentioned both 30 and 50 hz vats signals used this model. Most common was 50hz. Set module to this freq.

Spliced into ECM. Still no start.

Decided to buy commercial version of VATS bypass.

Using scope and Fluke VOM, checked ECM sensors and grounds. All OK. Made jumper test leads for Injector on ECM. Checked for injector pulses. Found none. Measured injector resistance.

Found 1.6 ohm RH rail, 4 ohms LH rail. Removed plenum and checked each injector. LH all 12.6 ohms. RH was 11.2, 2.0 and 11.4. Replaced all three RH injectors. So ECM was seeing about 7.5 amps instead of normal 4 amp pulse.

Later discovered internet article (www.asashop.org/autoinc/feb97/techtips.htm) stating ECM had internal circuit to protect injector drive transistors due to short in injector wiring. Felt better about our approach.

Installed commercial VATS Bypass (www.bakerelectronix.com) which was cheapest one found. Got dual freq version (both 30 and 50 hz) since not sure which one needed. Cost about $40 plus $30 overnight FedEx. Wired in the bypass at the VATS module connector. Set VATS freq to 50Hz since most common (blue wire on bypass). Connected Starter Enable Relay wire (BL/Yel Stripe )and Bypass BLACK wire to GND (BLK/WH Stripe), Red on Bypass to IGN (Pink/Black Stripe), Blue on bypass to VATS wire (DK BLU) on the harness. Checked VATS signal with scope. All OK.

Car tried to start, but backfired. Found previous mechanic had spark plug wires crossed. Corrected and car started, but run rough and would not stay running.

Switched VATS bypass to 30Hz (green wire on bypass). Car started and Ran fine. Problem solved.

Summary:

1) Bad Passkey wiring.
2) Bad VATS module
3) Shorted and defective injectors
4) Incorrect spark plug wiring.

Hope this helps with YOUR similar problem. Thanks.

Joe