kelcor507

We are finishing up a 69 Camaro conversion using an LS1 from a 99 Firebird. We used and modified the stock PCM, relays and wiring harness. Everything seemed to work fine when running for short periods in the shop (Except running rich). However now that we are road testing we found a problem with the fuel pump turning off randomly.

We traced the problem back to the ignition (Pink wires) all read 9.3 volts. All battery (Red Wires) read 12 Volts. We have tried 3 batteries, removed all unused wires from the PCM harness. The problem only stops when we pull the PCM BAT or PCM IGN fuses - Turning the PCM off.

Do you think it is a bad PCM, or should we keep looking at senors /wiring?
Do you think this may cause the engine to randomly run rich?

Old Geezer

iTrader: (3)

Is the testing done w/ the engine running??
What does the batt read?
What does the alt output read?
Where are you powering the pinks from?
Where is the batt located?

kelcor507

Testing has been done with engine off, ignition switch on.
Batt Reads 12.3 volts
Alt will read around 13 volts.
pinks are being powered by the ignition relay.
battery is located in front right fender.

TomM

iTrader: (56)

Did you use the braided resistor wire from the ignition switch? It appears to be a cloth wrapped wire, either white or tan. If so, remove that wire and replace it with a correct red wire. It is reducing the voltage to around 9 volts as you mentioned. This was done from the factory on most early model cars with points type ignition system.

T,

kelcor507

We have 12v from battery to the ignition switch (new wire). The switch runs back to the stock Firebird ignition relay. We are getting 12 volts back to the ignition relay from the ignition switch.

When the PCM is powered, it lowers to 9.3 volts. With the car running I measured 6 volts to ground.

Also - We measure 115 ohm from pink wire (Ignition) to ground with the ignition on and off. I assume this is not normal?

kelcor507

Correction the 115 ohm to ground on the ignition wires is only when the ignition is off.

TomM

iTrader: (56)

You have voltage drop, your wire is either too small, or overloaded. Use the igniton wire to power the relay coil only. Run additional fused wires for the loads to and from the relay. The PCM should not be on the same circuit as the switched wire. It should be powered by a wire that is hot all the time.

T,

kelcor507

When we disconnected Pin C1-19 (IGN) from the PCM all voltages go back to 12 volts.

The incoming voltage to the PCM 12 volts (Disconnected). When we reconnect to the PCM Pin C1-19, all IGN voltage drops back to 9.5

We connected pin C1-19 directly to battery power from a fused output. Now all IGN Voltage and battery voltage read 12 volts... Seemed correct, however the car will no longer start... This is very puzzling.

We switched back to the original IGN wire and it starts fine, but back to 9.5v on all ign wires.

We are using the stock wiring harness for the engine and PCM. We made no changes to the connected devices but removed EVAP, EGR, Rear O2 ect..

Very strange!

pdsq98gt

Have you tried a new ignition relay? Sounds like you have a volt drop. See what the voltage is at the both sides of the secondary sides of the relay with the circuit energized (back probe). You should have 12v on both sides since the only loads on the circuit should be down stream of the relay. How where does the power to the relay come from? Directly from the battery or a fusible link or fuse block? Could be a volt drop there. Open circuit voltage testing won't tell you if there is a volt drop since there is no load on the circuit.

DWC

Another good way to check for a voltage drop is to probe the circuit (while powered) from 2 points measuring the DIFFERENCE in voltage.
Example: Positive lead of meter at pcm 12v in and negative lead of meter on the relay 12v in. Meter should read 0volts or maybe 0.01. I don't really remember how high it can be but VERY close to zero. I think you would see your 2.5 volt difference. Narrow it down by probing between different connections etc.
Just throwing it out there, it's helped me in the past

TomM

iTrader: (56)

One other thing....check all your grounds, even the factory original grounds for corrosion or good connection. Run new ones close to your load(s) if need be. +12V systems are notorious for grounding issues.....you can never have enough.....

T,