The Stovebolt Site Forums The Shop Area The Electrical Bay Should I see any voltage on the minus pole?

Okay, I'm not really into electric stuff - so bear with me.

Today I found out, why my electric fuel pump stopped working, the minus cable slipped out of the connection, so I put it back on a new eyelet and reconnected it (basically a screw in the sheet metal).
While doing this (mind you, the ignition and everything was off) I saw some sparks.
As far as I know, one should'nt see sparks coming out of a minus cable - right?

So I disconnected the minus cable from the battery, and measured between the minus pole and the loose cable.
Low and behold - I saw 12V on my multi meter.

Maybe I'm just dumb, but IMHO - with everything turned off - I shouldn't see any voltage there, or am I missing something?

Thanks Frank

You'll always see a spark in a live circuit when a wire is connected or disconnected.

In the future, disconnect the battery before doing any work that involves the electrical system to avoid accidentally damaging circuits or accessories.

My question is this. Why was the circuit on the fuel pump hot, presumably with the ignition off? Normally an electric fuel pump should be wired to only be hot when the ignition key is switched on.

Exactly my point Otto - that's why I tested it on the minus pole of the battery.
The fuel pump only starts working when I turn the key - ignition was off, and the pump didn't work - still I had sparks.

That's why I'm so puzzled, it doesn't make any sense to me seeing voltage on the minus of the battery, when everything ist turned off.
No hot cables, no blown fuses though.

Modern radio? They use 12v all the time to keep memory alive.

Once again we are wondering what truck you are working on. With 40000 members +/- we can’t possibly remember everyone’s vehicle! Please let us know with every new thread by putting your vehicle info in the subject line. Thanks

Nope - no Radio at all.

For the record: it's not my truck, but the 41 sedan.

12v or 6v?

Negative ground or positive ground?

When you connected the two with your meter, you completed the circuit. Something in that circuit is drawing power. Start disconnecting components, one at a time to isolate the offender.

Sounds to me like a backfeed situation from another electrical component which shares the same power supply as your fuel pump. That other component may have a weak ground.

12V negative ground.

Just to make sure I understand this - when I disconnect the ground strap from my battery, and than measure between the ground and the negative pole of the battery (given everything is turned of) - I shouldn't see any voltage at all- right?

We have to straighten out the words here first to be sure we don't have a misunderstanding. Words are all we have to analyze. We can not see or touch the problem.
Here in the USA we call the "minus" side of a circuit, negative. Also referred to in the USA as ground. We call the other side "Positive".
Also please use the term "wire" for a wire..... and "cable" for a very large wire like the battery cables.
Please ALWAYS use a modifier in your explanations..... don't say "the minus cable slipped out of the connection"...say the ground wire of the fuel pump slipped out of it's crimped on terminal that was attached to the chassis with a sheet metal screw.
We don't want to misconstrue what you did or found or corrected. Otherwise we will go off in the wrong direction (which may happen anyway).

So.....QUESTIONS:
1. "As far as I know, one should'nt see sparks coming out of a minus cable - right?" Was the spark coming from the ground terminal end of the fuel pump wire which is attached to chassis metal with a screw?

2. "So I disconnected the minus cable from the battery, and measured between the minus pole and the loose cable." So you took the negative battery cable off it's battery post and measured between that post and the battery ground cable that you just took off or..... did you measure from the post to the fuel pump loose screw end?

3. Do you have a relay in the fuel pump circuit?

Can you check to see, that when key/ignition is off, if there is 12V coming to the fuel pump positive supply wire/terminal? *
I can better help you after these are answered.

Obviously the 41 has had a 12V conversion. Was this done by you? and when was it done? Have you done any wiring anywhere recently?....or replaced any components?

Phil is heading where I am thinking of heading, once I get clarification and the test. *

You should not see voltage between the post and ground. Something is "on" that should not be "on". There is resistance somewhere, creating a load. The battery is trying to service that "load"/resistance. You are doing something similar to a voltage drop test. Parasitic draw test. Do the same test but set meter to amps.

I seems like a stuck relay/VR, bad fuel pump or bad ignition switch....or you recently miswired something....Carl's backfeed.

Does it all work Now? Starts? Runs? Fuel pump not clicking or humming?
\What motor is in the 41? Is the 41 a Chevy? Alternator or generator?

Sorry, English is not my first language, so please cut me some slack - but yeah, I will try to stick to the right words.

I also appreciate your elaborate questions, and will try to answer them as good as I can.


"Was the spark coming from the ground terminal end of the fuel pump wire which is attached to chassis metal with a screw?"

Yes it was.


"So you took the negative battery cable off it's battery post and measured between that post and the battery ground cable that you just took off or..... did you measure from the post to the fuel pump loose screw end?"

No - just from the negative post to the loose ground cable, and than (just to make sure) from the post to a bare metal clamp on the fender.


"Do you have a relay in the fuel pump circuit?"

No - it's attached directly to the ignition.

"Can you check to see, that when key/ignition is off, if there is 12V coming to the fuel pump positive supply wire/terminal?"

I will, when I'm im back in the garage next weekend, but I doubt it since the pump only runs, when I turn the key.
I go and check it never the less.



"Obviously the 41 has had a 12V conversion. Was this done by you? and when was it done? Have you done any wiring anywhere recently?....or replaced any components?"

Yes, I have done the conversion this summer - no additional wiring, except for the programmable distributor, which is pretty straight forward (I attached the wiring diagram).



Components: I swapped the 6V fuel pump for a 12V pump and the alternator, but kept the wiring from my 6V alternator.
Also all the bulbs of course and a Runtz voltage drop for the fuel gauge.

Bevor that, I overhauled my overdrive, and when I put it back in, I remember that I had a hard time to figure out, how I wired it the first time - so maybe I have done something wrong there.
The only relay on the car is also in the overdrive circuit.


"Does it all work Now? Starts? Runs? Fuel pump not clicking or humming?"

Actually it's running fine, the electric fuel pump makes a typical fuel pump ticking sound though.
At least I think it should sound like that, since the 6 volt pump sounded the same.

Still the voltage on the negative side of the battery bothers me.


"What motor is in the 41? Is the 41 a Chevy? Alternator or generator?"

It's a Chevy, the motor is a 261 and it is an alternator.

Thanks, Frank

Before, during and after test, measure battery voltage between positive and negative battery posts and write down the values each time and report after all testing is done.

If everything is working OK. First do the check for voltage at pump input with key off. This is a very important test.

Then do a parasitic draw test with all ignition and any other lights, security or battery/memory hold-up turned off.
Parasitic draw= something is on that should not be on and is drawing current while you sleep.

With all things turned off, take off the negative battery cable at the battery post. Set multi meter to amps reading mode. Start with high scale on meter, probably 10A.
Touch red meter lead to the negative cable clamp and the black lead to the negative battery post. This places the meter in series with ground. Read amps. Should be near zero. Change to a more fine scale as you go. Get to milliamp scale. Should be no more than about 50 milliamps. If higher, it means there is a draw of some kind. Report findings. If a draw is detected, please state the amp value. We will then go over next steps to identify the draw source.

TESTING NOTES:
1. Leave all electrical connections connected except the negative cable at battery post as mentioned. Do not remove positive battery cable.
2. I assume the other end of negative battery cable is attached to a motor bolt or to a starter bolt.
3. Be sure you know how to set up the multi meter for amp and milliamp readings.
4. Do not run motor during tests.
5. Please state when fuel pump stopped working relative to any electric wiring you did. How many times did motor start and run between wiring and pump stopping. First time................many times.......before pump stopped.

Thanks a lot for your effort.

I will do the tests as soon as I'm back at the garage.

Regarding point 5. The pump stopped working during a test drive - but it was a mechanical thing.
I accidentally pulled the ground wire when I opened the air vent.
The wire got caught up in the air vent linkage, since I hadn't tucked back the harness under the dashboard at that time and all was still a big spaghetti monster.

Since this is a brand new pump and I haven't done any driving (since the 12 volt conversion) before this test ride, I started the motor maybe 4-5 times for ignition timing.

Could be a leeky alternator diode
Steve

After a lot of testing and measuring - turned out it was the little Bluetooth receiver/amp.

I thought if you turn it off it is actually off, but I just learned, that "off" is just some sort of standby, and if I really want it off I need to install a switch.

Anyway - thanks for your input guys.

Frank

II just love to quote myself. Bill Hanlon mentioned radio hold-up also.

"Then do a parasitic draw test with all ignition and any other lights, security or battery/memory hold-up turned off.
Parasitic draw= something is on that should not be on and is drawing current while you sleep."

Glad you found it.

Yeah, I first ignored it, cause I thought if I turne it off, it is actually off - little did I know.

Only after unplugging everything - piece by piece and measuring each time I found it.
Would be nice if that behavior would have been documented somewhere in the manual though.

Frank