How is this happening?

TrickyWicky

On a car which generates on DC voltage (positive and ground), how am I seeing -1.4V on the radiator and across the engine?

Make and model isn't really important here before anyone asks - rare as hell and nobody knows about them (specialists refuse to get involved).

I'm just wondering how I can be getting -1.4V when the lowest voltage possible on a DC system is (as I understood it) positive or ground (0V). I understood the alternator to only generate a positive voltage and ground?

If I can get to the bottom of it, I believe this will fix my cooling issue - the fans are electronically grounded by the ECU to turn them on!

facade

How are you measuring this?
where exactly are the meter probes- is the black lead attached to the battery negative, and the red lead stabbed on the engine block to read -1.4V?

bigjl

Negative earth or positive earth?

TrickyWicky

facade wrote: »

How are you measuring this?
where exactly are the meter probes- is the black lead attached to the battery negative, and the red lead stabbed on the engine block to read -1.4V?

Yes the black meter probe is on the - terminal of the battery. The positive is then used as usual - on the radiator, engine block, positive terminal etc.

The car overheated a year or so back due to something being amiss. The cooling fans didn't kick in and I ended up with 10V all over the block and radiator (something with the autobox connector - eventually when I found this a wiggle, disconnect and reconnect solved it). Now the engine and block have a negative voltage and it should be 0V :mad:

I am seriously stumped!

TrickyWicky

bigjl wrote: »

Negative earth or positive earth?

As far as I'm aware, earth is the negative supply or a 0V if there is a negative supply too. As the car battery only has a + and - I'm assuming that the earth should be 0V / negative earth.

Strider590

Your meter is probably picking up interference from the ignition/coil/plugs/leads.

It'd be a good idea to check the resistance from block to battery - terminal, should be less than 1ohm.

Nodding_Donkey

TrickyWicky wrote: »

I understood the alternator to only generate a positive voltage and ground?

An alternator actually generates AC (hence the name) which the rectifier then converts to DC.

Don't know if that helps any.

marlot

TrickyWicky wrote: »

...I ended up with 10V all over the block...

That suggests the earth lead from the engine to the body has a very bad connection.

I'd start by replacing it, making sure that the metal it's attaching to is clean and bright. I'd also check the battery to body connection at the same time.

If you're making a circuit through some autobox wiring you could soon burn it out - especially when trying to start the car.

thescouselander

Probably just a bad ground connection somewhere. 0v is a relative value so if the bonding strap connecting the two is not good the -ve terminal on the alternator and hence the engine could be floating at a different potential to that on the battery and anything else with a good ground connection to it. What readings do you get when the engine is not running?

Also, I'd check the connection to the -v terminal on the battery is good.

If you want to know more of the theory look up floating ground on the internet. Basically when the bonding is faulty you end up with two floating grounds and your multimeter is picking up the potential difference between the two.

Iceweasel

TrickyWicky wrote: »

Make and model isn't really important here before anyone asks - rare as hell and nobody knows about them (specialists refuse to get involved).

Your car may be are rare as hell - but by not mentioning what it is, you are denying yourself the possibility os someone who does know about it from assisting.

Would it be a 1924 Calthrope? - I have one in the shed!

You never know who could google the name at some point and find it and you.

Good luck.

TrickyWicky

Strider590 wrote: »

Your meter is probably picking up interference from the ignition/coil/plugs/leads.

Yes but this -1V is there when the engine is not running but the ignition is live so I don't think the plugs / coil/ leads will be the cause.

Nodding_Donkey wrote: »

An alternator actually generates AC (hence the name) which the rectifier then converts to DC.

Don't know if that helps any.

That helps a lot - and highlights my own incredible stupidity

marlot wrote: »

That suggests the earth lead from the engine to the body has a very bad connection.

I'd start by replacing it, making sure that the metal it's attaching to is clean and bright. I'd also check the battery to body connection at the same time.

If you're making a circuit through some autobox wiring you could soon burn it out - especially when trying to start the car.

No it suggests that as per my post, that was PREVIOUSLY an issue that I resolved by tinkering with a connector. The autobox ECU wasn't being grounded properly and consequently the engine was being sent high voltage wise. With the auto ECU out of action it had no idea what gear the box was in and wouldn't let the car start. As soon as I solved that the ECU could see the current gear and the car started. As I said in my original post.. that part WAS previously a problem that is now fixed but I do feel it's semi related to this hence i mentioned it.