[Thedeputy]

Hi,
I've got a problem of over-charging on my friend's 1968 road runner. After a long drive, the ammeter needle hits the right corner...fully. Measurement reads 17+ volts on the battery, on idle. So, this should be the voltage regulator right? Replaced it by another one, same result after a long drive (about an hour). Besides this, my fuel gauge is not working properly. It reads more than empty (but it isn't) and during the drive, the needle rises. Does this have any connection with the overcharging problem?

Alternator is new (dual field). there seems to be a resistance in the car when the ignition is on (not running),measured between battery - and chassis. About 60 Ohm. Ignition off reads 0 Ohm.

Does anyone have a clue?

[dave-r]

The regulator is not charging for sure when the car is running. But why?

It sounds like an earthing problem as you suspect.

Is the regulator body correctly earthed to the body of the car?
Is the engine correctly earthed to both the chassis and the body?

Switching the ignition on should have no effect on the earth from the battery to the chassis. They should be directly connected!

I think the wiring is all screwed up.

[dave-r]

Here is a simple diagram of the charging system.

The problems in the dash might be earthing problems too.

There is a voltage regulator in there that puts out 5v for the gauges. I think it needs a good earth to work. But if the battery is not earthed nothing will work right.

[Thedeputy]

That's my guess as well.
We grounded every thing with an extra wire, just to be sure.
The strange thing is, that when the ignition is off, ground is perfect everywhere in the car, but as soon as the ignition is on there is a resistance, between the car's body (or engine) and the negative pole of the battery.

I think we'd best pull out the entire dash,

Again.......

Thanks

[dave-r]

That's my guess as well.
We grounded every thing with an extra wire, just to be sure.
The strange thing is, that when the ignition is off, ground is perfect everywhere in the car, but as soon as the ignition is on there is a resistance, between the car's body (or engine) and the negative pole of the battery.

If the battery -ve pole is connected directly to ground that is impossible!

[Thedeputy]

?????
What do you mean? In my challenger the original negative cable is mounted on the engine, and with a smaller wire to the left front side of the radiator support. So that's directly to ground.
Or do you mean something else?

Antoon

[ianandjess]

i edited this it was wrong

[ianandjess]

gday last post is wrong sorry just went & checked the main earth is to the engine then the engine is earthed to the body via an earth strap perhaps your engine to body earth is weak could be worth checking
cheers ian

[dave-r]

perhaps your engine to body earth is weak could be worth checking
cheers ian

It MUST be weak or bad. If the -ve of the battery had a good connection to the chassis and engine (and engine and chassis to each other) then you would have almost ZERO resistance no matter what you did to the keyswitch or anything else.

If the keyswitch makes a difference then the battery is earthing another way and it is doing it badly. Make sure there is no corrosion or paint under the connections.

[Jon]

You may want to check your field wires from the alternator. If there is a short the regulator tells the alternator to continue charging. Sometimes if the wires are bundled and taped a short or broken wire cannot be detected easily.

[Thedeputy]

Well thanks you guys.

I'm afraid the only way to solve this problem, is take out the dash and engine bay wiring, and completely check it for shorts and breaks.

Thanks again.

Antoon

[71_DEPUTY]

Antoon-measuring resistance with voltage applied to any circuit will give you a false reading as the ohm meter supplies the voltage for measuring resistance.

I would check the wiring between the alt and the voltage regulator- this might involve cutting the tape to visually inspect the wiring. It sounds like as the wire heats up is shorts out to ground or another wire.

John Mac

[Thedeputy]

That might very well be the problem. I will get right on it.
Thanks

[dave-r]

Ah. I didn't click that you actually had the battery connected when measuring resistance.

Good job it was not mains voltage you were measuring.
Every electrician makes that mistake once. My AVO meter exploded when I made that mistake and almost blew my nuts off.

[Thedeputy]

Aaaaaahhhh, you don't want to loose that set of tools