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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!
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Old 05-15-2018, 10:41 AM
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get a test light and check the wiring again.

a MM can measure 12v and there still be a bad connection which reduces current.
a test light uses a light which requires current to light,.
test the TL across the battery first.

check the hot wire to ground and then connect the TL to 12v and check the NEG side for a good ground if you cant go from the hot wire to the NEG wire.
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Old 05-16-2018, 03:02 AM
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+1 - test the 12v harness under load w/ a lite as mentioned.

And thoroughly inspect female connector at the end of the harness for dirt or misalignment, frayed connection in the conn. backshell if you can.
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Old 05-16-2018, 05:31 AM
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These are the results of my tests this morning. Sorry it's sideways... I hope this helps someone with more electrical experience than me have a clue as to what's going on here. Thanks!
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Old 05-16-2018, 08:06 AM
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i don't know what your FP is suppose to pull as far as current. my 930 and my 77 911 were around 7amps. the list of FP's I have the lowest was 5amps.

I would still check the FP connection with the a light (test light). about $15 at advance
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 05-16-2018, 08:32 AM
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Sorry, should have mentioned that I tested the connection with a test light (probe on one end, alligator clip to ground on the other?) and am getting juice through the car's wiring. I'm following a lead right now from 911 S Registry. There is no resistance on the pump + and - terminals when there should be 1.3 or so. I have the pump apart and am about to test the armature for a short. Top part of the pump electrics (brushes and wiring from terminal) check out with no short. Break for lunch and googling how to check out an armature. Thanks for your help. I'll follow up with results.
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Old 05-16-2018, 09:51 AM
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if you have no resistance that is an open,.
some meters will display a O.L as an open so don't get that confused with 0ohms resistance and a short. BTW, shorts blow fuses and or melt wires.
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 05-16-2018, 10:22 AM
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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!
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Old 05-16-2018, 01:36 PM
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Success! After careful reassembly including repositioning the magnets and securing the speednuts that hold the insulating plate, the pump works properly. I did find during reassembly that I over-tightened the four screws that hold the actual pump part to the top, and found it no longer would run off the battery. Loosening the screws slightly freed the pump and it ran off the battery before final installation. I don't think that was the problem at first, as I hadn't opened it when it first acted up. I more think dropping it while cleaning it and making it pretty was the first problem. As I previously mentioned, when I first opened it up I found a small corner chipped off one magnet, lying in the bottom of the housing, both magnets had slid down to the bottom of the housing, and two of three speed nuts loose on the bottom, which allowed the insulating plate to move. On reassembling the pump the first time, I probably did have the top four screws too tight, which bound the pump from moving. I seem to have read another post here or on Pelican about that issue. Anyway, the car is running, no leaks! and I get to enjoy a newly rebuilt trans. I hope this helps others repair the$$$$$e pumps. Thanks for your help! (copied and pasted from Early 911S Registry)
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Old 05-16-2018, 03:50 PM
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Quote:
Originally Posted by Mike O'Meara View Post
Well done ! But that looks like it belong in some electrical gadgetry museum.

The current chart is in A not mA, correct ?
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Old 05-16-2018, 04:01 PM
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I can't vouch for those amperage numbers...I'm not experienced with the multimeter beyond ohms, so amps were new territory for me. I found the meter wandering all over with the numbers. As a last resort, I read the instructions (!) and later let the meter auto choose the scale of measure. Just touching the probes together had the meter wandering from 0.003 to 0.006, but I think that was because I was measuring mA. I was very confused and frustrated. Apparently a lot of us have trouble with digital multimeters, judging from a google search on the subject. So those are most likely A, not mA.
I need to take a remedial electricity and multimeter course.
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Old 05-16-2018, 05:21 PM
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take some 600 sandpaper and clean the part where the brushes make contact.
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ SOLD
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[__] RUNNING:[] NOT RUNNING:__2000 MER E320 WAGON [] WRECKED:[]RUNNING:
Old 05-17-2018, 02:42 AM
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Quote:
Originally Posted by Mike O'Meara View Post
I can't vouch for those amperage numbers...I'm not experienced with the multimeter beyond ohms, so amps were new territory for me. I found the meter wandering all over with the numbers. As a last resort, I read the instructions (!) and later let the meter auto choose the scale of measure. Just touching the probes together had the meter wandering from 0.003 to 0.006, but I think that was because I was measuring mA. I was very confused and frustrated. Apparently a lot of us have trouble with digital multimeters, judging from a google search on the subject. So those are most likely A, not mA.
Yes.

I use a $20 digital multimeter which has a separate setting on the dial for DC A and DC mA.
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Old 05-17-2018, 08:29 AM
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Quote:
Originally Posted by T77911S View Post
take some 600 sandpaper and clean the part where the brushes make contact.
+1
The brush to armature interface is critical.
Pencil eraser works well too
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Old 05-17-2018, 08:30 AM
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The pump is back together now, so I can't use sandpaper, but I did thoroughly degrease the armature and the brushes contact surface. It's been brought to my attention that the armature itself is very deeply worn where the brushes ride, and there may not be much life left to this pump, or the armature itself. Just when I was feeling accomplished! These particular MFI pumps are not complicated...just rare and expensive!! Thanks for the tips.
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Old 05-17-2018, 07:38 PM
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