bad earth, testing and diagnosis help please

Hello there, I am certain that my landy has developed an earth fault.

I would appreciate some tips on diagnosing an earth fault on a landy. I would also be grateful if you guys could tell me where the main earthing points are behind the dashboard.

I have poltergeist activity going on when i turn on my lights, or when the heater fan is turned on - typical reactions are one swipe of my windscreen wipers and the fuel and temp guages rise dramatically.

I know that i can use my multimeter to test resistance between pieces of metal to gauge how they are connected. low ohms show good connectivity, and high ohms usually shows up potential earth faults.

could anybody tell me what should be the average resistance for a good connection in ohms and what level of resistance should show up an earth fault.


Thanks

keny

Most multimeters are unable to measure the kind of resistances which could be bad enough to cause an earthing problem (they can't measure sufficiently low resistances). I think that a more reliable way of checking for earthing problems is to measure the voltage accross the suspect area of the circuit. connect a cable from the negative battery terminal to the negative probe on your multimeter (set to test voltage), then put the positive probe onto the earth terminal of the component which you suspect may have an earthing problem. Switch on the power to that component. If the multimeter reads more than 0v then you have an earth problem. Work your way back along the circuit from the earth terminal on the component towards the negative terminal of the battery to identify exactly where the problem is.

Thank you for that - my own view initially was to just check resistance between parts of the body, the engine and the chassis with the neg terminal of the battery.

As an academic point you say that i would need to measure small amounts of resistance; what do you call low resistance?

I ask this because i have had reading of up to 38 ohms between the engine and the body - i assume that this is definitely too high.

just an experiment i tested my meter to ensure that it was functioning correctly and just tested a piece of copper wire - this tested as 0.1ohms

I apologise if this sounds more like a master class in resistance measurement.

I just tried measuring the resistance of a 21W 12v bulb with my multimeter and it gave a reading of between 3 and 0.9 Ohms. I did a quick calculation using Ohms' law - it should have been reading about 6.8 Ohms so the meter was out by a factor of more than seven! Check out the 'measuring low resistances' section of this page

8886 Reference: Measuring resistance - ohmmeter

To check if there is an earthing problem between the battery and the body set your multimeter to read volts, switch on the your lights, connect the necative probe to the negative terminal of the battery and connect the positive probe to a part of the bady which you have cleaned to bare metal. If it's reading much more than 0v then you need to clean up some of your earth connections. I don't know what vehicle you have, but on a series Land-Rover the earth from the battery goes to the chassis first then to the engine and body, so I would measure the voltage drop between the negative of the battery and the chassis first (with the lights turned on) On a seriesIII there is an earthing strap between the chassis and bellhousing on the gearbox.

oh thanks for that - for the record i have a landy 110 300tdi.

I have taken on board all that you have said. I have a bit of a landy novice; my last vehicle being an old discovery.

The problem with 'testing' bad earths in the Ohmic way is that a multi meter just cant do it.

for a very low current connection, like what a multimeter test will do, it will conduct fine. As soon as you pull some heavy(er) current through the earth, the poor connection cannot carry this and so the 'resistance' goes up. (The current is limited, and must find another route.)

I would be inclined to not worry so much about testing, rather than just fixing. You won't do any harm by cleaning a good earth.

Take the 4 screws out that hold the console in (bit with speedo, temp gauge etc) and unplug the wires and speedo drive from behind it.

The earth should be visible, right hand side, down a bit, into the dash. Unscrew it, clean it, apply some vaseline, and screw it down with a fresh screw and ring terminal if needed.

If this still dnt fix it, then trace the earths back from the back of the fuel gauges, or from the fan switch.

Also, you then KNOW what state the earths are in behind the dash, so they can be eliminated in the future!

fenby1976 said:

I just tried measuring the resistance of a 21W 12v bulb with my multimeter and it gave a reading of between 3 and 0.9 Ohms. I did a quick calculation using Ohms' law - it should have been reading about 6.8 Ohms so the meter was out by a factor of more than seven!

Click to expand...

that's due to the resistance of the wire within the bulb changing when it gets hot

i still think the quickest way to fault find bad earths is by using a temporary flying lead to bypass the suspect links - using a jump lead connected from the battery to very near the starter often shows up a poor connection

footoomsh said:

Thank you for that - my own view initially was to just check resistance between parts of the body, the engine and the chassis with the neg terminal of the battery.

As an academic point you say that i would need to measure small amounts of resistance; what do you call low resistance?

I ask this because i have had reading of up to 38 ohms between the engine and the body - i assume that this is definitely too high.

just an experiment i tested my meter to ensure that it was functioning correctly and just tested a piece of copper wire - this tested as 0.1ohms

I apologise if this sounds more like a master class in resistance measurement.

Click to expand...

if you think that was a master class i'd quite like to see your dummies guide

footoomsh said:

Hello there, I am certain that my landy has developed an earth fault.

I would appreciate some tips on diagnosing an earth fault on a landy. I would also be grateful if you guys could tell me where the main earthing points are behind the dashboard.

I have poltergeist activity going on when i turn on my lights, or when the heater fan is turned on - typical reactions are one swipe of my windscreen wipers and the fuel and temp guages rise dramatically.

I know that i can use my multimeter to test resistance between pieces of metal to gauge how they are connected. low ohms show good connectivity, and high ohms usually shows up potential earth faults.

could anybody tell me what should be the average resistance for a good connection in ohms and what level of resistance should show up an earth fault.


Thanks

keny

Click to expand...

try re-earthing the heater earth in the dashboard this will sort out your lights heater fan windscreen wipers and the fuel and temp guages

Had lots of malfunction of electrics on my Def 200 tdi some of those you mentioned to.To start with i put it down to it been a well used ex camel T truck but the ultimate solution was to run a good clean earth wire from the bulkhead to the chassis and be done with.I am no electrician mind but nor is my Landy startrec enterprize. Had its nose pointing to the stars once though..

I made some enquiries with LandRanger about having a new earth put in my '86 90 and they said it would only take them about an hour for the full thing - can't imagine it being very expensive

I've had similar issues to you, dodgey lights as well as the temp gague rising unexpecidly.
Since having these issues I recently replaced the gearbox-battery earth, put an earth to
the engine-chassis as well as earthing the body under the seatbox to the gearbox earth as
the body and engine weren't earthed beforem

Although the last earth wasn't strictly required earth straps only cost a few quid so its
worth it for piece of mind really and since replacing them all the electrics have been fine

Hope this helps

Ollie