Electrical Starting Issue

Hi All

I think either my alternator or starter, maybe even both are f*cked.

Last week after ripping out a ton of useless wiring. I moved the truck under its own steam into the workshop. The next day I came back to a lifeless truck. Battery read 6v and there was a current leak which I tracked down to the D+ wire to the alternator.

Disconnected the alternator from the 12+ side and tried to jump start the truck. The starter would turn ever second or so, got a 5v drop while trying to start. Still lifeless.

Checked the engine was earthed, checked the starter wasnt earthing and all is fine.

The D+ wire to the alternator read 6.4k ohm to ground. Got 4k ohm to ground when I turned the ignition on which leads me to think its not a good sign. Am I not supposed to see +12v trying to make its way back to the alternator? The alternator is meant to, once the engine's on, supply +12v and when it meets the opposing voltage stops flow. Thus the dash light goes out as the voltages push against each other?

The third terminal for the tacho is not used on my alternator as I dont have a rev counter.

Im currently lost as to why this has happened.

My wiring for my truck in its current state is.

Chassis > Battery > Isolator Switch > Starter
Ignition to starter.

Yes, its that simple.

Alternator has been removed from the circuit as has any other 12v device. When hooked up to the 12v supply my ARB compressor ran very badly, drawing in excess of 10A. I suspected earthing but every thing is earthed!

Go earth on starter to earth on battery and crank it over. Basically u should see less than 0.5v like this any more and you have a bad earth. Easy way to test is get a jump pack and put poss on battery and negative to engine block if it starts it proves a bad earth. All of what you describe sounds like a bad earth to me

The D+ wire to the alternator read 6.4k ohm to ground. Got 4k ohm to ground when I turned the ignition on which leads me to think its not a good sign.

Don't try measuring resistance with power applied, you will get all sorts of strange readings and could possibly bugger your meter up.


drawing in excess of 10A. I suspected earthing but every thing is earthed!

Poor earths wont generally increase the current, actually the opposite as increased resistance will reduce current flow

Oh and a word of advice don't run the engine if the alternator is disconnected as this can actually damage the alternator.

Worth charging your battery on a proper charger and then measuring current being drawn from battery with the ignition off and report back.

have you just got a flat battery /poor isolator switch

Tonights been fun.

I used a fully charged battery which I ran in my old truck. Isolator switch is spot on, its been checked.

Started with your suggestion Racandy. Nowt, no voltage drop, no engine struggling to turn. The only thing happening was a small click of the glow plug relay. I couldnt hear the starter solenoid so I decided to fire a few volts down the exciter wire using a flatter battery while my fully charged battery was hooked up to the motor. Big click from the solenoid on the motor but no turning. Tried it again with the ignition being turned but didnt get a look at the meter. This was due to my attention being focused on the now smoking negative terminal of the battery that was hooked to the starter motor. Decided that wasnt a good sign so stopped.

Serpentine tensioner pulled out the way I removed the belt. I find the alternator has next to no resistance as I turn it by hand.

Thought i'd try the starter again but nothing happened apart from the loud click of the starters solenoid as I turn the ignition. I dont know if I freed it up but it fairly clicks now.

It was at that point I lost my mind and decided just to rip the lot out. Alternator was a breeze, the starter motor was a bit of a pest. That nut behind the solenoid is a pain to get to. Both now sitting on the garage floor.

Kwakerman, those measurements I gave are with no voltage going through the system. I havent measured resistance on anything live in quite some time. There is no current leak through the isolator unless I attach the alternators D+ connection to it. I believe D+ to be the heavy gauge wire that leads back to the 12v side and not the dash light feed, right?

I'll take both to a auto electrician to have a look over. If they both come up fine then the head scratching begins.

It mentioned a double earth here but the only earth I could see was this one. Tried earthing the motor at different points to no avail.

Here's a pic, you can see the only earth and the cable hanging over the cooling pipes is the live to the starter.



Im off to bed, had enough!

Sounds like the starter could be the cause, as you've got it out anyway it may be worth trying it on the bench with your good battery.

Get ya battery booster and put positive on and earth to body then drop feed across to solenoid to test it. 2 mins and u will know more

Well Racandy I did try your suggestion but the motor wouldnt spin while on the vehicle. Solenoid fired but nowt else.

So got it over onto the floor and rigged it to my purpose built testing rig. Well the instant I touched the exciter wire against the terminal the motor started to spin. At full whack the voltage only dropped by a volt if that. So it looks to be in great shape.

Although I was pleased to see this. If its not the earthing, which im now praying it is. Then there's something else a lot more expensive gone wrong. I had near enough the exact same setup on the truck as I did on the floor.

I'll fire the starter back on the truck on Thursday morning. I'll wire it up exactly as I have for testing except for a second battery in parallel as my single can only fire out ~360A, Im expecting a heavy draw. I cant think what it could be causing the engine to act as if its almost hydrolocked. If the engine doesnt turn, i'll rip the glow plugs out and try turn the engine again.

The only other thing I've done to the engine is top up the oil. It wasnt even showing on the dipstick and tonight it was sitting bang in the middle between high and low. As I did suspect perhaps I'd hydrolocked the engine. Would anyone happen to know the length of a 300tdi dipstick by chance?


Here's a pic. The oil you see is from topping up the front diff. Also points for identifying the brand of hideously loud and slow 12v compressor.

have you tied turning engine over by socket and bar

360a battery ain't anywhere near enough to turn a tdi over u need close on double that. Think u may of just found your issue.

jamesmartin said:

have you tied turning engine over by socket and bar

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If im looking at the front of the truck. Which direction to I need to turn the crank? With no glow plugs in this should be an easy task if its not hydrolocked right?

Im wondering if the dipstick may be from another engine. As when I first got the truck and checked the oil. It hardly touched the bottom of the stick. I then topped it up and the trouble started.

Racandy said:

360a battery ain't anywhere near enough to turn a tdi over u need close on double that. Think u may of just found your issue.

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Im aware of starting current draws. I tried two batteries in parallel when I first couldnt turn the engine over. The ~360A battery im using for testing and the battery in my car.

Preid said:

If im looking at the front of the truck. Which direction to I need to turn the crank? With no glow plugs in this should be an easy task if its not hydrolocked right?

Im wondering if the dipstick may be from another engine. As when I first got the truck and checked the oil. It hardly touched the bottom of the stick. I then topped it up and the trouble started.



Im aware of starting current draws. I tried two batteries in parallel when I first couldnt turn the engine over. The ~360A battery im using for testing and the battery in my car.

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do what james said and make sure your engine turns over fine by hand. then forget the alt, it's irrelevant atm.

when you jump started it from another running vehicle, was it still turning over slowly or was it normal?

trax said:

do what james said and make sure your engine turns over fine by hand. then forget the alt, it's irrelevant atm.

when you jump started it from another running vehicle, was it still turning over slowly or was it normal?

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Alright, which direction should I turn the engine if I was looking towards the front?

It was hardly turning over at all when I tried jumping it. Same as when the battery drove the motors.

Clock wise is the correct rotation. I think if I was local we could have this diagnosed in about 30 mins lol. I'm really hoping for ya it's not a knickers engine buddy

Preid said:

Alright, which direction should I turn the engine if I was looking towards the front?

It was hardly turning over at all when I tried jumping it. Same as when the battery drove the motors.

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clockwise, afaik most engines are clockwise

Also a jump start off another car will only produce what a set of jump leads or alternator can supply so max 200a.

Preid said:

Alright, which direction should I turn the engine if I was looking towards the front?

It was hardly turning over at all when I tried jumping it. Same as when the battery drove the motors.

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assuming the engine isn't knackered. if it made no difference to crank speed when you jump started it, then either the wiring is knackered or the starter is knackered.

wiring will be easy enough to check, and i know you tested your starter, but that wasn't under load. I'd be tempted to give the starter a good clean, then reattach it and try again.. assuming the engine hand cranks ok.

Racandy said:

Also a jump start off another car will only produce what a set of jump leads or alternator can supply so max 200a.

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how's that? it'll pull whatever amps the other battery has as well, won't it?

you'll soon find out if the jump cables are underrated

trax said:

how's that? it'll pull whatever amps the other battery has as well, won't it?

you'll soon find out if the jump cables are underrated

Click to expand...

You would think so yes but jump leads are very poor quality and obviously they don't have the best connection to either vehicle either. You would be surprised how many breakdowns I go to and people say oh it's not my battery I've tried a jump start off the neighbours v8 on my fiesta and it won't go. But ya jump leads cost 2 quid from aldi is what's going through my heAd. Drop my jump pack on and it fires into life. They just look amazed at ya lol. So hard to diagnose a car over a forum but least with everyone's input and sensible help you will get there I'm very sure. The joys of Landrover ownership