How is this happening?

TrickyWicky

thescouselander wrote: »

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

Depends on whether you think an isolated radiator that carries a voltage via the coolant is grounded or not.

When I stick the meter in continuity mode and dip one probe in the header tank or onto the engine and the other on the rad I get tone.

thescouselander

TrickyWicky wrote: »

Depends on whether you think an isolated radiator that carries a voltage via the coolant is grounded or not.

When I stick the meter in continuity mode and dip one probe in the header tank or onto the engine and the other on the rad I get tone.

I can categorically state that I do not consider this arrangement to be properly grounded. Usually water is a poor conductor of electricity although it will conduct some electricity depending on what else is in there which is why you get a tone. Have you measured the resistance between these two points?

onomatopoeia99

I'm curious as to why you won't tell us what the car is. I own two different cars, each of which had production figures under 500 (and in one case the specific model had no more than a few dozen).

I certainly wouldn't expect any vehicle to be wired in a way that expects the coolant to act as a conductor, no matter how rare it is.

TrickyWicky

thescouselander wrote: »

I can categorically state that I do not consider this arrangement to be properly grounded.

<snip>
Have you measured the resistance between these two points?

Thats besides the point and you're clouding the issue. The point is that I have -1.4V on the engine and radiator. The radiator being a side effect of conductivity via the coolant. It clearly is not however the cause so measuring the resistance for fun isn't going to help me here.

onomatopoeia99 wrote: »

I'm curious as to why you won't tell us what the car is. I own two different cars, each of which had production figures under 500 (and in one case the specific model had no more than a few dozen).

Erm because I don't want to? - I know plenty of people with this model, I run the only UK forum for it and quite frankly none of my extensive contacts are able to come up with anything - even those on the foreign forums where there are more members who know them inside out. Oh yes.. and I also don't want to be linked to other forums Is that really such a big issue for you? - I can pretty much guarantee you won't know this vehicle and even the specialists refuse to touch them because they're so quirky. Like I said.. not much point me mentioning the make or model.

onomatopoeia99 wrote: »

I certainly wouldn't expect any vehicle to be wired in a way that expects the coolant to act as a conductor, no matter how rare it is.

It is not wired that way - it is a side effect of the fault. The engine has -1.4V all over it. This consequently spreads via the coolant to the radiator. This is not part of the intended design but again you like the previous poster are clouding your vision and making more out of this than you need to.

My question is simply how the car can generate a negative voltage which I'm observing on the radiator and engine - not why it is on the radiator. Your post really has not helped me.

frisbeej

The alternator will be outputting 14 volts-ish, the battery will be 12 volts, you're probably measuring that difference, maybe in combination with some bad earths.

Rover_Driver

TrickyWicky wrote: »

My question is simply how the car can generate a negative voltage which I'm observing on the radiator and engine - not why it is on the radiator. Your post really has not helped me.

The electronics in modern cars can cause all sorts of similar problems like that, try using a jump lead connecting the battery negative to the engine and see if the voltage is still there, if it's gone, it sounds like a bad earth connection between battery negative and engine.

thescouselander

Trickeywickey - I really don't think you understand enough of the theory of what's going on here. There are a number of reasons (which a number of people on here have given you) as why this problem might be occurring and most of them point to a grounding issue.

You're not going to find the problem by only measuring the voltage between various bits of the car because you won't know if you are measuring the potential difference between two separate floating circuits or the effects of a current travelling through a resistive load or of there is something wrong with your meter.

You need to take some measurements of the resistance between the parts of the car that are at different voltages to give an extra clue as to what's going on.

Ectophile

Since nobody else has mentioned it (or I just wasn't reading closely enough)...

I wonder if you are putting the probes of your meter on two dissimilar metals. For instance, the radiator might be brass and the battery terminal lead. With water between the two, you could just be seeing a battery. If that's the case, then it's probably a complete red herring in your fault-finding.

Joe_Horner

Basic electrical theory (and practice) says that, if the engine and battery negative are properly connected by the earth leads then there cannot be a voltage difference between them.

So, you have a bad connection on either the lead fom the battery to the body or from the body to the engine. Starting there, apply systematic fault finding.

With the ignition and light son (to create a load - there's no point measuring voltages without a current flowing):

(1) Connect the meter from the battery terminal (the actual post, not the clamp) to the earth lead clamp. If there's a voltage then the connection between the post and clamp is bad.

If no fault:

(2) connect the meter from the battery negative to the terminal at the end of the eartt lead where it connects to the body. If there's a voltage then the lead is faulty

if no fault:

(3) connect the meter from the battery to the body. If there's a voltage then the connection at the body end of the earth lead is bad.

If no fault:

(4) connect the meter from body to the terminal where the engine earth lead attaches to the body. If there's a voltage then the connection at that point is bad

if no fault:

(5) connect the meter from the body to the termnal where the earth lead attaches to the engine. If there's a voltage then the engine earth lead is bad.

if no fault:

(6) the connection between the earth lead and the engine block is bad. Confirm by nmeasuring voltage from body to engine block.


Note that you may well find that there are several "bad" points through that sequence, all of a few 10ths of a volt, which are adding up to your 1.4v total (+ or - the difference is only in the way you connect the leads). So, when I've put "if no fault...." try the next step even if there was a fault unless the total voltages to that point make up the full 1.4v

Across any of the connections or leads in the earth circuit you shouldn't see a drop of more than maybe 0.05v testing like this under headlamp and ignition load. If you do, the connection or lead concerned is bad regardless of whether things seem to work or not.

The above advice isn't specific to any secret special model but is applicable to every single car ever produced with an electrical system.

Joe_Horner

thescouselander wrote: »

You need to take some measurements of the resistance between the parts of the car that are at different voltages to give an extra clue as to what's going on.

The problem with that is that the sort of resistances that can give problems (0.1 ohm or less) can't be reliably measured with most DMMs. But you get the same effect by measuring voltages with the circuit loaded.

0.1 ohms resistance in a battery earth, for example, will show a voltage drop of a full volt when a 10 amp current (typical high beams) is flowing - you can't measure 0.1 ohm reliably but you can easily measure 1v