Hi,
I have a 1989 GTA TransAm, 5.7L TPI.
I think that my Oil Pressure Switch has gone bad - it has 12V at pin D of the MAF Power Relay all of the time (0.7A is been drawn from the battery without the ignition on) - I've taken this voltage measurement with the connector unplugged from the relay itself.
Looking at the workshop manual circuit diagram (6E3-A-40, for diagnosing a code 33) one side of the switch is at 12V (orange wire) and the other feeds pin D of the MAF Power Relay (along with the ECM's fuel pump signal).
To me it looks like the Oil Pressure Switch has failed with a 12V output all of the time. Before replacing, it would be great to have someone confirm my diagnosis. Is the switch in the lower part of the engine?
Many thanks. Ken

 

Does the fuel pump run without the key on? It could be that the oil pressure switch is jammed on, but short in fuel pump relay or any related wire could cause same. Just due to ease of access, with key off I would check tan wire at pin A of fuel pump relay with connector attached. If no voltage, then it's probably a bad oil pressure switch.


Switch is located just above oil filter.

 

Diagram.

 

There are 3 relays near the power brake booster. Fuel pump relay is nearest the engine.

 

what happens if you unplug the switch?

 

Hi,
Thanks for the replies. Some extra info:
1. ignition on - fuel pump is running continuously (is that correct?)
2. ignition off - fuel pump does not run
Tomorrow, I'll check the tan wire at pin A of fuel pump and have another look to find the switch.
Thanks again everyone.

 

OPSU is supplied 12V from the BATTERY. If it was clicked on all the time, the FP would run all the time, even with the key out of the ignition (ask me how I know!).

It is the backup path to power the fuel pump in case something isn't working with the ECM-triggered fuel pump relay.

 

Your problem is not the OPSU. It's the fuel pump relay or related wiring.


The OPSU is not a backup, it is the main power supply for the fuel pump. The fuel pump relay supplies power for prime and cranking only.

 

That's not correct. The FP relay supplies power to the pump in the start position and in the run position when the ECM sees a signal pulse from the distributor (i.e. when the motor is actually running). It is also cycled on by the ECM for 2 seconds when the key is first turned into the Run position to prime the system.

The power supplied through the OPSU is the secondary/backup/alternate fuel pump power pathway.

Regardless, I agree the OPSU is not the problem. If it was triggered on all the time the pump would run all the time, regardless of key position (or even if the key was not in the ignition).

 

Hi everyone,

Thanks for all of your inputs. I've had a close look at the fuel pump relay wiring and think that I've found the problem. The Orange (12V, "E") and Red ("D" to ALDL connector) wires (pins 1 and 2 on connector) had no plastic insulation extending 8mm out of the end of the connector.
They were touching - which would mean that 12V was always present at "D" on the MAF power relay (hence my 0.7A current draw with ignition off).
This would also explain why the fuel pump was always running with ignition on (but engine off).
I'll be repairing the wiring harness next weekend for a final test to confirm this.
So, thanks all for your help and pointers to the fuel pump relay wiring being the likely problem.
I'll update next weekend after the test to finalise the thread.
Cheers, Ken

 

Hello, did you sort out your problem? I have similar problem on my 1987 5,7 TPI and browsed forum into your thread.

 

Yes, replacing the oil pressure switch did fix the problem. After replacing it, the 0.7A current draw from the battery stopped.

I had other issues, the ECM was faulty - and the car was running in open-loop mode.

Also, the Fuel pump was hardwired to the ignition switch - hence running all of the time when the ignition was on. This was probably done to bypass the ECM not working!

I sourced one from the USA, it cost $52USD almost 5 years ago.

ACDelco 10222130 Oil Pressure Switch.

Good luck, I hope this helps you solve your issue.

Regards, Ken.

 

Thanks for your update, I'll have to dig into it. Glad you sorted it out! My car's fuel pump runs continously when igniton is on, that's how I learned of your thread.

 

Don't want to hijack your thread so I opened another thread under TPI forum regarding my problem. Thanks for you inputs and time.