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Testing engine earth


daverclasper
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Hi. Can't find much about this online/my books, so attempted this a bit blindly.

With my multi meter set at 200 in the ohms range (no continuity buzzer on meter), I am getting a reading  of 0.5, with negative  probe to block and pos probe to pos battery post.

Is this about right and is this the way to do it please.

Thanks, Dave

Edited by daverclasper
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No, earth test should be neg battery terminal to block. 

If you are going positive terminal to block, it is a dead short...

 

To accurately test low resistance, you will need something a bit more expensive in tgd way if a meter. My recently purchased Megger tester is very accurate down to fractions of an ohm, and calibrated. But cost is "substantial"

I wouldn't Worry, a decent earth lead from battery to block with clean connections will be fine.

 

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Hi Folks,

the standard earthing on my 4A is battery Neg to body then chassis to engine.

This means that that must be some mystical contact between body and chassis.

Therefore I also fit a battery neg to engine block.

 

Roger

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Uh.....I would use the lowest range you have, 0.5 ohms is quite high, this may be down to your meter, but this result suggests your earth strap has 0.5 ohm resistance.  V= IR,  0.5 ohms carrying 10 amps, say, means a 5 volt voltage drop! 

Short circuit the earth strap with a jump lead and retest?

Doug

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11 minutes ago, KevinR said:

To do the test properly you need an earth bond continuity meter with Kelvin probes.  With a test current of 10amps you should be getting a resistance of about 1 milli Ohm.

Exactly...

The resistanceoff a properly functioning earth strap is a tiny fraction of an ohm. A regular multimeter cannot measure it. If a regular multimeter shows a measurable resistance in the engine earth circuit, there is a problem.

Nigel

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its amazing over 55 years how things get basterised by PO, my daughters Mk2 Spit which we got in 2001 always started OK and had no issues it had been converted to neg earth sometime in its history, after about 5yrs I decided to pull the engine and undertake a minor reco, bearings, rings, oil pump, timing chain & sprockets & head rebuild, when putting the engine back in I had an Eureka moment where's the engine earth, THERE WAS'T ONE! the engine had been earthing by the choke cable!!

I fitted a MkIV set up Battery earth to body around the back of the battery box earthing to the body near coil bracket which as a one piece cable extended to the engine bellhousing bolt, I also fitted a Vitesse style engine to chassis earth strap plus a second body to bellhousing strap on the drivers side "belt and braces" well I had a stack of 9in 1in wide long woven style straps surplus from the refit of a 4MW very large metro water transfer pump where each pump had over 300 straps. The 4 river pumps pulled 20MW on startup enough to dim the lights in local outer metro townships!! Someone had stuffed the auto restart program they were supposed to restart with a 20min delay between each pump!. Depending on the storage res level (0 to 80mts) the 4 pumps could push 1000,000,000 litres per day (1000 MLD.) bit bigger than a Spit engine.

Peter T

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Quality meters will be fine, but cheap meters (I own some, expensive or cheap all break the same when dropped/trodden on/run over) just don't seem to cope with very low resistance. But maybe my cheap meters are too cheap...

 

Then again, for an earth lead, 0.5ohm is very significant. I guess a starter motor impedance is about 0.1 ohms (ballpark, 100A at 10V allowing for voltage drop) a 0.5ohm earth lead would restrict current down to 20amps! (or 24 as you wouldn't get the same voltage drop) but the starter would not be turning.

 

But this is all irrelevant. I have never even considered checking earth paths on a car, just use a decent earth strap and job done.

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  • 3 weeks later...

Hi,

 You can spend a bit of money on a four-wire low Ohm meter. You could save money and buy one that uses your cheap multimeters display.

Why spend a lot of money. Why bother?

You are worried about potential volt drops? Measure them.

Without the engine running and all loads on, measure the volt drop across the earth strap without actually touching the earth strap so that you are measuring any non-perfect connection points. Non cranking < 0.3 Volts? It would be nice. During cranking, I would love 0.5 volts.

You could measure the volt drop on +ve or -ve cables. Meter polarity makes no difference. Put the leads any way round you are only interested in the number.

I have a milliohm to millivolt 4 wire module you are welcome to borrow.

You could measure any voltage present between the coolant in the radiator to ground/earth to see if you have any internal corrosion going on.

Cheers,

Iain.

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