AE86 fuel pump weirdness...

[jondee86]

Matt Dunn... this one is for you

The car was parked up for 18 months or so. Ran fine last time out
of the garage. This time when I go to fire it up, there is no fuel,
and when I check it out, the pump is not running. Make sure its
getting 12v, and it is, but nothing. Unplug the pump and test... has
zero resistance to ground. About 30 litres in the tank, but fuel
is not working either (worked before).

I've got an old analog multimeter, works fine but kind of hard to
measure anything less than 100 Ohms. So zero pin to ground and
zero pin to pin. I figure the pump has a short to ground, but don't
know why it should take a dump when it has just been sitting ??

So I cast around for another pump and yesterday I jack the back of
the car up to get ready for dropping the tank. Today I check the
plug again (before dropping the tank) and I have 3 kOhms to
ground

So whats the go ?? Gremlins in there ? Bit of moisture ? Worth
hooking a battery up and trying the pump before I drop the tank ?
Any rational explanation ?

Cheers... jondee86

[Crucible]

You can do a simple voltage drop test to locate the fault. "With the circuit under load plug connected" you should have batt voltage at pump and no more than .5v on the ground side. Any higher than that and you have a bad ground.

Or

Run another ground, if you have 12v at pump and still no go the pump is stuffed. Must be plugged in and underload to do a voltage drop test and make sure you also have a good body to battery ground also!

[allencr]

Power to the pump & mallet/rubber hammer on the tank to break through a little fuzz/smegma that's built up on the pumps brushes & commutator.

[Crucible]

Yeah...that works sometimes.

[jondee86]

Thanks for the replies. Did a bit of research, and it seems I made a
fundamental error I saw two wires, and assumed that they
both went thru the cover and all the way to the pump. But from the
look of the pics I have seen, one wire grounds to the pump bracket,
and then wire from the harness grounds to a tab on top of the lid.

So, the pump plug will show a "short to ground" when tested. The
12V feed wire would have a small resistance, maybe 30-40 Ohms,
only my meter is not accurate with readings that low.

I have already whacked the tank, but I can't remember is there was
power on the pump or not. I'll give it another shot before I drop the
tank. Still reckon there is something weird going on, as I got 3 kOhm
to ground on both pins

Cheers... jondee86

[Crucible]

First of all, take the ohm meter and throw it over the fence, a voltage drop test will lead you to the problem!

Do you have a volt meter? You can pick cheap ones up for nothing these days. Normally you will have four wires going into the tank, 2 will be for the fuel sender and other two will be power and ground for pump. The thicker gauge wires will be for the pump.

If you dont have a multimeter you can just use a simple test light, make one out of an old taillamp bulb and holder. Disconnect the pump plug and bridge the two terminals, if the bulb glows bright you have good power and ground and hthe pump is shot. If the bulb is dull run the ground of the bulb to a known good ground (back to the battery) if it glows bright you have a bad ground.

Make sense?

[Lloyd]

Pull the pump and bench test it. If it goes then see if you have a power and an earth at the plug (at the pump, nowhere else). Go from there.

[matt dunn]

The two wires that go to the pump, one is power, and the other is earth obviously.

The earth one goes to the top plate of the pump cover to earth it as the actual tank itself is not really earthed as it is held in by rubber uder the metal straps.
There will probably be some amount of earth on tank though.

The power goes through an insulator into the pump.

If the pump has been sitting for that long it may be buggered.
Giving it a whack with some power on it may fix it,
or it may not.

As for resistance, checking either wire to earth could give any readings,
from short circuit to open circuit, wont really matter,
onlyones that does is the resistance between the two pump wires,
which should be quite low I would expect,
too low for a non digital meter to accuratly read.

Most likely cause is a stuffed pump.

[jondee86]

Most likely cause is a stuffed pump.

Yeah... I'm ready for that, but I like to know what is going on
before I start replacing parts. So that's why I want to figure out the
circuits and what wires go where.

Thing thats throwing me off just now, is that when I lifted the lid
over the fuel level gauge, I found three plugs. A two pin plug for
the pump, and I have that circuit sussed now. Then there is a three
pin plug which I think is for the fuel level gauge and low fuel light.
But the wires for all three plugs run thru the rubber bung in the fuel
level gauge lid ?? So what's the third two pin plug for ??

Going to take a battery over and sort this out tomorrow

Cheers... jondee86

PS: I know where the 3 kOhms came from. Until I unplugged the
fuel level gauge plug, the fuel pump was using that ground. After
unplugging I'm reading the tank to chassis resistance.