MFI Fuel Pump Electrical Problem

[EsGeht]

The pump will consistently operate when jumped from a battery charger, but not when connected to the car's wiring. There is 12v at the + in the car wiring plug and continuity to ground in the - wiring of the plug. Draw drops to 10.5v in the car wiring when the key is on. The pump shows 0.00 resistance across the + and - terminals. I even ran a jumper wire from the + side of the car battery to the pump while jumping the - side of the pump to the ground strap. Still nothing. I don't get it and am stumped. My electrical experience is limited. What am I missing? Thanks

[edmayo]

Just for grins try the pump across another 12 v battery.

[EsGeht]

Thanks Ed will do.

[EsGeht]

Tried jumping the pump with another good battery and no dice. There is more to the story (see below) but I don't get what's going on here. Could it be an amperage issue? The charger is a 6/2 amp Schumacher and it's set on 2amps...spins the pump every time! Here's the back story from my post on the Bird:


Fuel Pump Electrical Help Needed
The fuel pump in question is from a 72 with MFI. The pump works on the bench with a battery charger for power, but not hooked up to the plug in the car's wiring. Plug shows continuity to ground on the negative side and power to the plus side!!?? How can this be?? Car battery has sufficient juice to crank the motor easily. What!?
This information for background: I just finished replacing the fuel lines, resealing the motor and installing a rebuilt trans. The pump didn't turn on when I tested it after re-installation. I took the pump out, tapped with a soft mallet, tried it on the bench, and lo! it worked. Cool. Installed it, didn't work. Took it out, tapped it again, and it worked. Reinstalled it. Tested it multiple times after, and it turned on with the key. Finally finished reinstalling the trans and motor, and test drove the car yesterday and all was well, in fact it ran better than before and the trans is fabulous. Happy! But after the car sat yesterday, the pump refused to kick on with the ignition on. I pulled the pump and this time opened it up and discovered a small corner of one magnet had broken off and had apparently wedged the armature. (I had dropped it while cleaning it and I assume that's what caused the damage. The pump had worked flawlessly for years.) I cleaned everything, saw no other apparent damage, and closed it back up, testing it on the bench at each step, and it worked each time. Feeling very proud of myself, I attached the fuel lines to it under the car, attached the wiring and thought I better test it first before actually nestling it back into it's mount. Turned the key, no pump noise! I tested for power at the plug...good. Tested for continuity to ground...good. I hooked up the battery charger with alligator clips to the pump and lo! the pump works with juice from the charger. What!? Back and forth, under and out from under the car, hook up the alligator clips, etc, multiple times, and the pump works with juice from the charger but not from the car. The battery has plenty of juice and cranks the engine over easily. What is going on here? Anybody? Help, please! I am absolutely baffled!

[edmayo]

Sounds like an amperage problem. measure resistance all the way from the fuse to the plug at the pump. Could be a simple as tightening fuse connections,,,,,or more difficult,,,,,,but what ever it seems like the pump isn't getting the amperage kick it needs even though the voltage is there. You could put an ammeter in series with your charger and see what amp the pump draws when it first runs, then do the same in your car. Take out the fuse, put your ammeter across the fuse connections, see what amp draw there is when you try to run pump.

[62S-R-S]

Have you tried swapping in fresh relays with the meter close at hand while it reads 10.5..

Also, with the meter brought into the car with spare length, ignition switch wires can be checked, and a host of connections pulled and cleaned. If you see a jump in volts, the problem may be found.

[304065]

So you've actually tried using the car's battery to run the pump? And a different battery also? There should be no shortage of amperage available in that context. And yet your battery charger spins it fine.

Imagine you had an oscilloscope and could put the battery output on the screen. It would show up as a flat horizontal line.

Now put the charger output on the screen- it will have very small ripples in it, caused by the rectifier in the output circuit. There are specs for how much ripple is allowed, but it's definitely there.

So how can it run on rippled current but not on straight line?

A short in the rotor.

When you measured 0.0 resistance in the rotor, that seems strange. The rotor is a long piece of coiled wire. When you connect DC to it, it becomes an electromagnet, which causes the charged poles to spin away from the fixed magnets in the motor case. When the rotor spins, the brushes come into contact with new spots on the commutator, reversing the polarity, pushing the rotor again, and so it goes. That's an oversimplification but you get the idea.

You should measure SOME resistance in the rotor-- if you don't, that probably means that there's a short circuit somewhere, allowing the current to bypass a section of the wire.

By introducting alternating current into the shorted rotor, you create enough magnetic flux to get the motor to spin.

That's the only explanation that fits the available facts. It's weird, but it fits.

See also here: http://forums.pelicanparts.com/porsc...ml#post4490489

[EsGeht]

Thank you! That makes sense to me with my limited understanding of electricity. Here are tests I made this morning, I hope it means something to the more knowledgable... Sorry it's sideways.

[EsGeht]

And yes, I studied this link http://forums.pelicanparts.com/porsc...ml#post4490489 before opening up the pump. The pump is half way installed right now. I'll pull it out and retest with another battery, but it looks like the pump is going to be coming apart again. Thanks, I have a least a clue!

[EsGeht]

Progress! After disassembling the pump and taking many measurements with the multimeter, not knowing for sure what I was doing, I put the armature back into the housing and tried it on the charger and directly off the car battery and it spun off the car battery, too. When I disassembled it (both times) I found loose "speed nuts" (see photo) in the bottom of the housing and the magnets were too deep into the bottom of the housing. I reattached and moved everything into place, partially reassembled, and now have operation. I'm off to get new speednuts, reassemble the pump completely and try it again. Main point of progress - it runs directly off the battery now. Cool! (Copied and pasted from Pelican)