Why does my frame have a positive current? Wth is going on!

Jamesjames · Jun 4, 2020

Guys I am dumbfounded here. I have a 66 charger with a 440. When I Use a volt meter and attach the negative end to the negative terminal on the battery and a positive end to some random bolt On the car I get about 2-4 volts of positive energy. When I removed the battery cable from the transmission and attached it to the frame it did not read any positive volts but then the starter did not engage. Wth is goin on? I put a brand new cable on btw.

Demonic · Jun 4, 2020

What kind of volt meter? Digital Fluke meters and lots of imported ones can read stray emf and induced currents. It's not really 2-4 volts, the meter lacks sufficient impedance.

Jamesjames · Jun 4, 2020

Demonic said:
What kind of volt meter? Digital Fluke meters and lots of imported ones can read stray emf and induced currents. It's not really 2-4 volts, the meter lacks sufficient impedance.

It is a digital volt meter in all honesty.

Demonic · Jun 4, 2020

Put a little resistance in series, like a small marker light bulb, and I'll bet it goes away. Analog meter won't do that.

MiradaMegaCab · Jun 4, 2020

How’s the body grounded?
It’s my understanding that your negative battery cable is bolted to your transmission.
Is there a ground wire from the engine to the body?

If your negative battery cable only goes to your transmission and if your engine doesn’t have an ideal ground strap/ bonding wire to your frame, you also might see voltage on your DVM.

Perhaps measure the Ohms between the negative battery post and the body.

moparedtn · Jun 4, 2020

MiradaMegaCab said:
How’s the body grounded?
It’s my understanding that your negative battery cable is bolted to your transmission.
Is there a ground wire from the engine to the body?

If your negative battery cable only goes to your transmission and if your engine doesn’t have an ideal ground strap/ bonding wire to your frame, you also might see voltage on your DVM.

Perhaps measure the Ohms between the negative battery post and the body.

Depends on what car you have.
Most of these old Mopars have a main negative going straight to a bolt on the intake manifold on the engine.
There's usually a smaller one also making connection to the body around the battery somewhere.
Further, there's oftentimes a smaller (sometimes braided) one coming off the back of the engine going to the
firewall as well.
At least, that's how the factory typically did it....

khryslerkid · Jun 4, 2020

Yep, I like the heavy negative cable from the battery to the intake manifold bolt, smaller braided cable from the negative terminal on the battery to the radiator support and the same size braided cable from the rear of the engine to the firewall.

dvw · Jun 4, 2020

Very simple. You have just done a voltage drop test. The reading on the meter is the amount that you are losing through a connection or series of connections. In this case you are testing the ground side. Somewhere between the battery negative post and wherever you are testing has a poor connection. Start at the post itself and the cable terminal. Then while the meter is still attached to the negative post touch the other lead at each splice, crimp, bolt on connection in the circuit you are testing. Each junction will have some voltage drop. You may find one or more with large drops. My bet is if you were cranking the engine the number you are reading would be even higher. This test can be done on the hot side as well (+ post to the + feed terminal of the item you want to test the circuit for). I use this test all the time at work. Much better than an ohmmeter.
Doug

Thrashingcows · Jun 4, 2020

I had a reverse light switch that tested fine with the digital meter but it would not activate the reverse lights, lots of messing around and I finally decided to use my old test light with a bulb in it and low and behold the switch showed it was not working.

Jamesjames · Jun 8, 2020

Jamesjames said:
Guys I am dumbfounded here. I have a 66 charger with a 440. When I Use a volt meter and attach the negative end to the negative terminal on the battery and a positive end to some random bolt On the car I get about 2-4 volts of positive energy. When I removed the battery cable from the transmission and attached it to the frame it did not read any positive volts but then the starter did not engage. Wth is goin on? I put a brand new cable on btw.

Thanks guys. I put a resistor in line on the negative cable and it seemed to knock the current down to almost nothing. Now its time to move on to the EL lighting. Headbang

dvw · Jun 8, 2020

Jamesjames said:
Thanks guys. I put a resistor in line on the negative cable and it seemed to knock the current down to almost nothing. Now its time to move on to the EL lighting. Headbang

Why did you add resistance? The reason for current flow thru the meter is the resistance is to high in the circuit you are testing. You would never increase resistance un a ground circuit.
Doug

MiradaMegaCab · Jun 8, 2020

Pics of battery cables, especially the negative cable would help.

snooprob · Jun 10, 2020

Thrashingcows said:
I had a reverse light switch that tested fine with the digital meter but it would not activate the reverse lights, lots of messing around and I finally decided to use my old test light with a bulb in it and low and behold the switch showed it was not working.

Hi Thrashingcow I was also testing a circuit with my DVOM. I had power and ground so I installed a new crank sensor...still no start. I retested the circuit...still had power and ground. What I realized was the power wire for this sensor was corroded Upstream. The wire consisted of twelve individual strands and 11 of the 12 were corroded in half so that single strand that was allowing voltage to pass. that made me think that I had plenty of power and ground but that single strand didn't push enough amperage through it to allow the circuit to function properly.
I know load test the circuit with a 194 bulb before opening my wallet.

Superbeedad · Jun 10, 2020

Jamesjames said:
Thanks guys. I put a resistor in line on the negative cable and it seemed to knock the current down to almost nothing. Now its time to move on to the EL lighting. Headbang

Adding a resistor is equivalent to a bad ground. Please don’t do that. You need a proper ground strap between the engine and the body .. usually at the firewall and back of engine.

Why does my frame have a positive current? Wth is going on!

Jamesjames

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Demonic

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Jamesjames

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MiradaMegaCab

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moparedtn

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