How is this happening?

TrickyWicky

Iceweasel wrote: »

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.

I do see your point but I actually run the forum for this particular car and none of my members have been able to help - one even works for the dealers of that brand and has access to all the tech spec.

This was why I was asking the question in general so that it gives me a rough idea of the cause and what i'm looking for.

Slowly_does_it..

The MG VA is a positive earth, could be one of those!

Honestly though, it sounds like a dirty connection somewhere on an earth strap. I would be going through all earth connections, giving them a good clean, quick coating of vaseline, and put back nice and tight.

My old fiesty the earth strap from the shell to the engine fell apart, did not bother me, might bother you.

When I had a "large" sound system, the strap from the shell of my car to the negative side of the battery was not thick enough, an upgrade fixed that nicely, maybe an earth cable of yours has given up mid cable?

thescouselander

Here's another thought. You didn't say where you were applying the meter probes but on the assumption one is on the battery -ve terminal and the other on the engine and there is current flowing you might see a voltage drop between the two due to electrical resistance in the ground path between the engine and battery. What do you get if you measure the resistance between the two points on your multimeter?

facade

If the negative terminal of the battery is bonded to the engine, via the earthstrap, then there are only 2 ways you can read -1.4V by touching the engine and battery negative. (3 if you include broken wires to the meter)

1) the part you are probing is electrically isolated, and the DVM is picking up interference or more likely displaying nonsense as a consequence of how it works.
2) there is a tremedous current flowing from the engine to the battery, and to get 1.4V across the earth strap I would expect it to be on fire.

Are you probing the cam cover or something that is isolated by a gasket and rubber washers?


As to your problem: if the ecu grounds the fans, but they don't run, then you need to be looking for +12V on the live side of the fan motor.
Then grounding the switched side of the motor should see it running.

colino

I think you simply have a spurious reading, but don't, as many do, start replacing expensive bits to get the fans running. It could be the PCM, the CTS, the IAT, the electric fans or the VSS. I'd check the harness first, then change the relay.

thescouselander

facade wrote: »

If the negative terminal of the battery is bonded to the engine, via the earthstrap, then there are only 2 ways you can read -1.4V by touching the engine and battery negative. (3 if you include broken wires to the meter)

1) the part you are probing is electrically isolated, and the DVM is picking up interference or more likely displaying nonsense as a consequence of how it works.
2) there is a tremedous current flowing from the engine to the battery, and to get 1.4V across the earth strap I would expect it to be on fire.

Are you probing the cam cover or something that is isolated by a gasket and rubber washers?


As to your problem: if the ecu grounds the fans, but they don't run, then you need to be looking for +12V on the live side of the fan motor.
Then grounding the switched side of the motor should see it running.

Just a add a third/fourth way - a high resistance across a dodgy connection would cause a large voltage drop with relatively little current. V = I x R and all that.

facade

thescouselander wrote: »

Just a add a third/fourth way - a high resistance across a dodgy connection would cause a large voltage drop with relatively little current. V = I x R and all that.

Indeed, but if the engine is bonded to the battery via the earth strap, it can't be high resistance. Also the negative voltage at the engine block would mean a current flow into the battery.

My money is on the voltmeter probe floating because it is touching something isolated from earth.

The early Discovery was prone to losing the earth strap from block to chassis, it used to set the handbrake cable on fire when you started the engine as it earthed the transmission.

Slowly_does_it..

facade wrote: »

Indeed, but if the engine is bonded to the battery via the earth strap, it can't be high resistance. Also the negative voltage at the engine block would mean a current flow into the battery.

High resistance in the thin layer of rust between the strap and the block?

thescouselander

facade wrote: »

Indeed, but if the engine is bonded to the battery via the earth strap, it can't be high resistance. Also the negative voltage at the engine block would mean a current flow into the battery.

My money is on the voltmeter probe floating because it is touching something isolated from earth.

The early Discovery was prone to losing the earth strap from block to chassis, it used to set the handbrake cable on fire when you started the engine as it earthed the transmission.

Yes, under normal conditions there shouldn't be a high resistance but that can happen if there is corrosion where the grounding strap connects or if has simply some loose.

I agree though the op might be probing an ungrounded object.

TrickyWicky

thescouselander wrote: »

Here's another thought. You didn't say where you were applying the meter probes

Yes I did - see post 4.

facade wrote: »

If the negative terminal of the battery is bonded to the engine, via the earthstrap, then there are only 2 ways you can read -1.4V by touching the engine and battery negative. (3 if you include broken wires to the meter)

Touching black probe on -v of battery and red probe to engine block / radiator.

facade wrote: »

1) the part you are probing is electrically isolated, and the DVM is picking up interference or more likely displaying nonsense as a consequence of how it works.

No it's not interference, it has to have direct physical contact and this negative voltage can be found anywhere - engine, starter, engine mounts, radiator (via the coolant) etc.

facade wrote: »

2) there is a tremedous current flowing from the engine to the battery, and to get 1.4V across the earth strap I would expect it to be on fire.

Thats just it, last time i checked the current consumption it was minimal. 20mA with the ignition off from memory and about 500mA with it on (my memory could be wrong but it was low).

facade wrote: »

Are you probing the cam cover or something that is isolated by a gasket and rubber washers?

Radiator is isolated but the voltage is spread via the coolant. Again the engine matches this voltage.

facade wrote: »

As to your problem: if the ecu grounds the fans, but they don't run, then you need to be looking for +12V on the live side of the fan motor.
Then grounding the switched side of the motor should see it running.

I swear the thing is haunted because I have tried this - with two witnesses and the fans didn't work. Oddly though sometimes when turning off the ignition they will spool up for 3 seconds and then switch off again. I've tried a spare set of fans and had the same result.

facade wrote: »

Indeed, but if the engine is bonded to the battery via the earth strap, it can't be high resistance. Also the negative voltage at the engine block would mean a current flow into the battery.

My money is on the voltmeter probe floating because it is touching something isolated from earth.

The early Discovery was prone to losing the earth strap from block to chassis, it used to set the handbrake cable on fire when you started the engine as it earthed the transmission.

Car has one earth strap from battery to chassis and then (shockingly) just one to the engine from the chassis. Nothing gets warm (other than the engine when running) and nothing smokes. It's all very odd.