another fuel pump problem ....

[malibud]

1975 280, I had it running great for about an hour after sitting for 3 years , Problem much like the other post. I have no power at the fuel pump . I can the fuel pump directly to a battery and it pumps. I can start the car for about 15 seconds before it dies . I opened the FSM black side box and the switch is moving when i start the engine . Any ideas ? any way to test the big relay? something is preventing the fuel pump from getting power.. Can I bypass that big relay by putting a new one between the ignition power and the fuel pump?

thanks a new car I would like to drive some more

Dan in NC

[FastWoman]

You've just got to trace the power from point to point. The particulars of fuel pump wiring differ from year to year, and I don't know the way a '75 is wired. However, I would bet it's the same as my '78 from the firewall and back. There's a big wiring harness connector (actually two of them) just in side the passenger firewall. You may find a crusty/arcing/burnt connection in one of them for the fuel pump wiring. (The connection is somewhat inadequate for the fuel pump current.) It could be any number of things, of course, but this one connection was particularly troublesome on my car. I eventually solved it by snipping out the wire and connecting via a bullet connector.

BTW, to test a relay, just remove it from the car, identify the two coil contacts (should be about 65 ohms between them -- not 0 or infinity). Connect 12V across those two contacts, and measure the resistance between the contacts that should switch on the pump. Resistance should go from infinity to zero'ish.

Of course this could be a really fancy relay with multiple coils and contacts, in which case the above might not apply. In that case, identify the fuel pump contact, which should connect to a wire of the same color code as the one you see connected to the (+) side of the fuel pump. On my '78 it's a blue and green wire (blue with green trace, as I recall). Then put the connector partially onto the relay -- enough to make electrical contact, but still leaving enough space to poke at the electrical connectors with a multimeter probe. In the V mode, hook your black probe to ground, and touch the fuel pump contact with the red probe. You should have an indication of power there when you first crank the engine (for 5 sec, as I recall -- but could be different with the '75).

[Zed Head]

I think that 75 is wired like 76 and 77. The 15 seconds of running part sounds like either the fuel pump contact switch in the AFM isn't making contact (might be dirty at the contact points), or the fuel pump relay in the combined relay (combined with the fuel injection relay) isn't working. The combined relay is above the hood release handle inside the car.

I had the contact switch in the AFM get a piece of dirt or hair or something stuck between the contacts and the car would start, run for a few seconds, then die. I didn't understand how things worked back then so I jumped the fuel pump from my defroster circuit at the back window with a spare piece of wire that the PO had dangling under the car.

The Start circuit bypasses the AFM switch and sends power directly to the fuel pump relay. I think that the AFM switch powers the relay through another circuit.

You can check the AFM switch by turning the key on and moving the vane in the AFM (take the black cover off of the side and move the weight or remove the big air hose). If the switch and relay circuit is working, the pump will turn on. If the pump doesn't turn on, you could jump the AFM switch at the contact points or clean the contacts. If the pump still doesn't run, then check the relay under the dash.

[malibud]

Man it looks like I am in the right place .. THANKS for the quick replies . I moved the AMF switch but nothing happens at the fuel pump, but it does move when i start the car . I will check for power at the realy next I begining to think that there is the problem. I kind of like the idea of running a wire from the defrost (temporary) just to raise my spirits and go for a drive. I assume this would be safe >

I down loaded the EFI manual and it points to the connection on the AMF switch that turns on the fuel pump. it looks like it is working and is clean . thanks again you all have been a big help

DC

[malibud]

ok the problem is in the afm the fuel pump contact point will not make contact after the potentiometer swings . so i tried to wedge it shut . not good. got power at the fuel pump which was disconnected , but it melted the top of the fuel pump contact. there is still some there but not a whole lot . it seems like this would always be abad conection . why is it hot now ?

[Zed Head]

Can't really figure out what you're saying, but it sounds like you closed the AFM contact switch by hand and the power wire at the pump shorted on something, melting the end of the wire. Is that about right? You might have blown a fuse or melted a fusible link also.

If you're asking why it had power, that's what is supposed to happen if the key is on. When the AFM contact switch closes, after the vane opens, the switch powers the relay and the relay sends power to the pump (or another relay, not 100% sure right now, either way the pump line gets power).

[malibud]

you are about right . i closed the amf switch by wedgeing a piece of rubber hose behind it . i dont think the contact was all that great so it arced at the switch and melted the top of the switch. itried several times to bend the switch in order to make contact but it did not work . so wedged something behind it. now i need to try to attach a piece of copper strip to whats left of the afm fuel pump contact switch. can you buy the little control box that is on the side of the afm without buying the whole afm? thanks

[malibud]

i got it fixed thanks ... i bent the fuel pump control arm, what was left of it so that it would engage . off for a drive ... thanks again..

[Zed Head]

Something's not quite right if the top of the AFM switch melted. I'm pretty sure that it's designed for very low amperage,just enough to drive the relay solenoid. If you had enough current flowing to melt the top of the switch, you might have a short somewhere, maybe in the relay. If you're not stuck out on the road somewhere right now, you should re-check what's happening, the wires to the switch are pretty small and you could end up melting more things. At least you have more clues now.

[malibud]

i think it melted because the wires on the fuel pump were disconected i have been monitoring it and no hot yet . i will keep checking...

[malibud]

the switch post melted because it touch the spring switch on the afm and arced it causing it to get hot and spark. i think that may have weaken it . after bending it many times to get it to work the tip of the fuel pump control switch broke off. so ... i now have the conntact points zipped tied together so that the fuel pump runs with the key in the on postion. 1. is this ok? 2 can i replace parts inthe afm if so where do i get them or do i need to replace the whole thing ...

thanks

dan