L134 Fugitive Voltage in Engine Compartment

Guys: I have two Willys Jeepsters, both with the L134 engines. They both start, run, and accelerate just fine, but one has a great deal of what I will call fugitive voltage in the engine compartment. I put my volt meter prongs into the engine compartment on the one and I get erratic and high readings on the volt meter. On the other one I get nothing at all. The one with the fugitive readings charges the battery fine at about 7.8 to 8.2 volts on idle. To test for the problem, I disconnected the fan belt and ran the problem child for a short while, and still got the high and erratic readings. This tells me that the generator and voltage regulator are fine, which lead me to think it must be either the distributor, coil, or plug wires, or so combination. I was hoping someone on this forum might help me decide the next course of action. Because I have the two I can swap out parts one at a time. Any thoughts. Thanks.

Kind of hard to take a guess without knowing what you are putting the probes on.

Congratulations on having two Jeepsters. Jeepsters are the Unicorns of the Jeep world – rare and marvelous beasts! Let’s take a look at your issue:

You commented:

“I put my volt meter prongs into the engine compartment on the one and I get erratic and high readings on the volt meter. On the other one I get nothing at all.”

The generator, battery, starter and regulator are the four major components of the low voltage side of the electrical system. The problems you describe are all low voltage issues, so I don’t think that you need to look at spark plug wires, plugs, distributor or anything on the high voltage side. The generator and starter are all electrically grounded to the engine block through their mounting bolts. The battery negative terminal is grounded to the engine block through a BIG battery cable. When you make a voltage measurement, use a clean spot on the engine block as a ground reference (black lead of the meter). The engine block is electrically connected to the frame by woven braided grounding straps that bypass the motor mount vibration isolators.

https://www.kaiserwillys.com/braided...71-jeep-willys

There should be one of these to bypass each vibration isolator because lights, gauges and accessories use the frame and chassis as their electrical return (ground). The engine block needs to have a very good electrical connection to the frame. On the one Jeepster where you get no reading at all, if you are using a point on the chassis or frame as a ground, and there is no ground strap, there is no electrical connection.

A question; what voltmeter are you using to make your measurements? This may sound strange, but a Harbor Freight digital meter can drive you crazy when you use it on an old Jeep. It has to do with how the digital meter samples the voltage being measured – let’s just say that in this case old analog meters give you a better idea as to what is going on. A digital meter could be the reason you are seeing erratic readings. (A Simpson 260 is probably the best all-round meter for old Jeep use.)

Here is a good enough analog meter on Amazon; Mrs. Google can find it for you:

Gardner Bender GMT-312 Analog Multimeter

On the Jeepster that has high and erratic readings, I would rethink your claim that the voltage regulator is operating normally. 8.5 volts at idle is a pretty high reading. As a quick check, disconnect the Field wire from the generator (it is the smaller of the two studs), start the vehicle and measure the voltage directly across the battery posts – don’t use the engine block or chassis for the black lead. The battery voltage should be down around 7.2 volts. Turn on the headlights and it should drop a few tenths of a volt. Turn off the lights, reconnect the Field lead and measure directly across the battery again. If you read 8.5 volts, I suggest that you have a regulator that is charging a bit too hard. Just about every source has a different number, but 7.6 volts seems to be the most common voltage for a battery charge voltage at normal (about 75 degrees) temperature.

On the Jeepster where you get nothing at all, read the battery voltage directly across the battery posts – not the terminal clamps. You should get a voltage reading. Leave the red lead on the positive battery post. Make the same reading using the clamp for the black lead. You should get almost the same reading as you did on the post. If not, you have a bad connection there. Move the black lead down to a clean spot on the engine block – a head bolt stud works nicely. Again, you should get about the same reading (within a tenth or two of a volt). Finally, move the black lead over to a place on the body or frame (don’t measure through paint or … gasp . . . rust). If at this point you lose your voltage reading, you have verified that the engine block is not well connected to the frame and body. Check the ground straps and make sure that there isn’t paint under the place where they get bolted to the frame under the vibration isolator.

Troubleshooting electrical issues by internet is tricky, but these are some hints that should help you get a better idea as to what is going on. Don’t hesitate to ask more questions if this doesn’t help.

And – send us pictures of your herd of Unicorns!

Maybe I did not explain the situation right? I am not putting the volt meter prongs on anything, just pointing them into the engine compartment. When I put the prongs low (closer to the pan) and high (above the air cleaner) in the compartment the volt readings are lower, when I put them closer to the distributor the readings get higher. Again this happens with and without the generator attached to the fan belt. Thanks.

This sounds like RF interference to me. Shielding gone bad or something not shielded on one that is on the other. A weak ground can cause it too.

OK, now I understand the phenomenon you are describing. It’s called ignition noise and ham radio operators fought it from the very beginning of the automobile era and still fight it to this day.

Your meter is reacting to electrical energy in the air – kind of like a radio receiver. In this case the source of the energy, the transmitter, is the high voltage side of the electrical system.

It would be interesting to take an AM radio receiver and hold it close to the Jeepsters with the engines running. I would expect that the Jeepster with the phantom voltage would cause a lot more noise in the radio than the vehicle that doesn’t. The noise will be a whine that varies in pitch as the engine speed varies.

The most likely source of the electrical noise is the spark plug wires. Small breaks in the internal conductors will arc over and the arcs will radiate noise. A new set of spark plug wires might greatly reduce the noise. “Resistor” wires will reduce noise more than a set of standard wires. The M38 series of Jeeps used shielded ignition systems for noise reduction - everything was shielded. But for civilian Jeeps that level of shielding wasn't considered necessary.

Other sources of noise can be the generator itself if there is any arcing at the commutator and brushes, but in the case of your Jeepster, the noise was there even with the fan belt removed, so the generator is not a noise source. Voltage regulators also caused noise – interference reduction was one reason they were mounted in metal cases. Without the generator operating, the voltage regulator won’t be active, so it probably isn’t the problem either.

“Back in the day”, Willys sold Interference Reduction Kits which were mainly condensers (we would call them capacitors today) that were attached to various points. Good grounding was also part of a noise suppression project. Those ground straps I described are important for that as well. The early Jeeps even had a ground strap from the hood to the body.

https://www.kaiserwillys.com/braided...b-gpw-cj-2a-3a

The number of ground straps on the WWII Jeeps was impressive. You can bet that they made a difference around radios because they would not have gone to this trouble unless it was important.

https://www.kaiserwillys.com/bond-st...s-43-45-mb-gpw

But, the good news is that unless the spark plug wires get so bad that the engine misfires, you really don’t have a serious problem. Enjoy your Unicorns!

I doubt it needs to be a AM receiver. I fly a 1942 L4 Cub that generates so much electronic noise that it is impossible to hear anyone on my hand held FM radio. I can get bits and pieces if flying perpendicular to the signal but if the engine is between me and the transmitter zero signal gets through. Needless to say, I don’t fly much around airports with control towers. I am not much of a radio guy but I should figure out how the Army had it quieted down. The L4 would have had radios mounted in it when they had it.

Guys: Thanks. You have supported my thinking as well. I'm going to swap out the spark plug wires one at a time to see if it is only one or the lot. I will start with the coil to distributor first. I'll keep you posted.