Fuel Pump 2 seconds, Fuel Pump voltage Options

[thompson-mfr]

I have a 74 2.0L original FI and the fuel pump has been moved to the front trunk. I converted to the in and out style (75-76).

My fuel pump used to go on for 2 seconds to prime, then off until cranking or running. I know this is how it is supposed to work.

Mine now is on constant with the key in aux position (before starting/cranking or running)

What controls this? The computer?

I also have only 7 volts to the pump and 9 volts at the fuse on the relay board that pump power goes through. Good 12+ volts at battery.

Finally to the question: Does the computer control this 2 second prime?

[SirAndy]

Finally to the question: Does the computer control this 2 second prime?

Yes, the brain controls the ground for the pump relay to do the 2 second prime ...
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[Cap'n Krusty]

Yes. If you want that to happen, you need to power the pump from the harness for the original location, or make a new one from the relay panel in the engine compartment.

The Cap'n

[76-914]

What size wire did you run to the pump? That's quite a drop in voltage!

[Spoke]

I also have only 7 volts to the pump and 9 volts at the fuse on the relay board that pump power goes through. Good 12+ volts at battery.

Where did you measure the voltage at the fuse? On the lug or on the fuse? Clean all connections and retry.

That kind of voltage drop is not good and needs to be fixed. Likely is connections.

[thompson-mfr]

Finally to the question: Does the computer control this 2 second prime?

Yes, the brain controls the ground for the pump relay to do the 2 second prime ...
(IMG:style_emoticons/default/smile.gif)

So, can I deduct if that is not happening there is a problem with my brain ecu?

Do you know the wiring (battery to fuel pump)?

I traced it from the fuel pump to the main harness coming through the firewall to the relay board. Guessing the brain interacts with the relay board to control the FP relay?

[thompson-mfr]

I also have only 7 volts to the pump and 9 volts at the fuse on the relay board that pump power goes through. Good 12+ volts at battery.

Where did you measure the voltage at the fuse? On the lug or on the fuse? Clean all connections and retry.

That kind of voltage drop is not good and needs to be fixed. Likely is connections.

I measured on the lug.

[SirAndy]

So, can I deduct if that is not happening there is a problem with my brain ecu?

No. They rarely ever go bad.
I would suspect just about everything else before you start looking for a new computer.

Wiring, relay, relay board, connectors. All of these are much more likely to be your problem.
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[Dave_Darling]

So, can I deduct if that is not happening there is a problem with my brain ecu?

Not necessarily--it is more likely that when you moved the pump you didn't hook into the original wiring.

The original wiring goes:

Battery to relay board, through fuse on board, through relay to pump. And pump directly to ground, because that's also important.

The relay is controlled by the FI brain, and also by the relay that powers the FI.

--DD

[thompson-mfr]

So, can I deduct if that is not happening there is a problem with my brain ecu?

Not necessarily--it is more likely that when you moved the pump you didn't hook into the original wiring.

The original wiring goes:

Battery to relay board, through fuse on board, through relay to pump. And pump directly to ground, because that's also important.

The relay is controlled by the FI brain, and also by the relay that powers the FI.

--DD

I didnt move the pump the po did. It has worked for years. The 2 seconds has worked for years. Now all the sudden it doesn't. I know the ecu is robust and rarely the cause of anything, but if it controls the 2 seconds and now that is stuck on. What else could this be? Thanks for your suggestions.

[Dave_Darling]

Ah, if it has worked for a while then my assumptions above were wrong! (IMG:style_emoticons/default/smile.gif)

Making another assumption: If nothing electrical was done to the car about the time the behavior changed, the ECU and relays and relay board and wiring are the primary suspects.

It is quite odd that any of that should fail such that the pump is on the whole time the key is on. I can see it happening if a jumper wire is added, or if the wire the ECU grounds that triggers the pump power relay gets shorted to ground. I'm thinking that may be the most likely cause.

Check for ground on the relay socket terminals for the fuel pump relay back in the engine bay. If either 85 or 86 (I forget which one, sorry!) is grounded full-time, that will make the pump run when the key is on.

--DD

[Tom]

86 is the controlled ground for the fuel pump relay. Remove the relay and hook up a multi-meter to 86 and ground with the meter on OHMS scale. You should get continuity for about 2 seconds when key if switched on, then it should go to infinity. If this works correctly, check out the relay, it may have stuck to on. With the meter on OHMS check from 30 to 87A and you should get continuity with 30 to 87 being open. Anything else means a bad relay. You can open them up and check the contacts and perhaps clean them up.
Tom

[thompson-mfr]

Thank you everyone that chimed in here! I am always amazed at the fast and intelligent responses. A special thanks to Dave Darling and Tom. The electrical test should pin down if it is the relay stuck, or the ECU.