fuel pump relay question

[Farrar Hudkins]

I'm thinking of adding a fuel pump relay to my car, mounted alongside the other relays so it won't be sticking out like a sore thumb using that weird "RPM Relay" pigtail. One of the things I've seen on other classic car forums is to use the signal from the oil pressure sensor as your trigger wire for your fuel pump relay. I've had the pump keyed to ignition (green/white wire) for a while and it has worked fine, but when the engine stalls and I hop out to check something, I usually forget to turn the key, and the fuel pump keeps running, straining against the needle valve and draining my battery. The oil pressure sensor thing sounds like a perfect solution. All I have to do is piggyback onto something that's already there.

The DeLorean basically has two oil pressure sensors: the level sender and the idiot light sender. Which one should I use?

Farrar

[Bill Robertson]

Don't use either one. The relay will give your dash gauges a false ground.

Bill.

[Farrar Hudkins]

Bill, thanks for the reply. I don't understand. Can you explain how that will happen?

Farrar

[Bill Robertson]

Dash gauges work by being grounded through the engine block. Oil pressure light is fully grounded, like the courtesy light door plungers, oil pressure gauge is variable grounded. If you use either gauge ground wire as the trigger signal for a relay, as soon as the relay is tripped the oil pressure light will come on or the oil pressure gauge will peg (straight down IIRC).

(Remember you don't want to leave an analog gauge pegged for very long otherwise the wire inside will burn through).

There's an unused port of some sort on the passenger side of the engine, right next to the oil pan, but I don't know if it's under pressure.

You could run a length of hose to a manifold on one of the pontoons with two pressure senders, one for your relay trigger and the other for whichever dash gauge.

You also could put an elbow into the oil galley port on top of the engine (plug closest to the oil pump) and run a hose to a remote pressure sender.

Bill.

[Bill Robertson]

Correction: oil galley plug is closest to the *WATER* pump.

Bill.

[Farrar Hudkins]

Sorry, but I'm still confused. I was planning to piggyback a new wire from the oil sender, not from the gauge's ground wire. I need +12v to trigger the relay, not ground. The relay will have its own ground. Are you telling me the oil pressure sender sends a variable ground? I've never heard of a variable ground before.

Farrar

[Bill Robertson]

The sender wire is the gauge's ground wire. The sender itself works like a rheostat. If you provide a zero resistance path to ground (by pulling the wire off the sender and touching it to the block, for example), the gauge will peg. The trigger circuit inside a relay would have the same effect (only take a couple of milliamps to trigger a relay).

Bill.

[Farrar Hudkins]

Thanks for that explanation! Now I understand. I was hoping, I guess, that it worked the other way: that the sender grounded to the block because it was metal, and its wire was +12v. Well, I guess I'll just use the green/white wire after all, just in a different spot.

Farrar