Alternator confusion !!!!

davesh · Aug 15, 2008

Newbie !!!!

Defender 300TDI

Yesterday my ignition light/battery warning light on the dash started to come on and off intermittently whilst driving. Today it remains lit constantly whilst driving.

I have put a meter on the battery and the voltage remains constant when I start the engine, and then dip the accelerator ( well maybe drops .2v to 12.6v ).

I decided to put the meter on the terminals on the back of my alternator and this is where my confusion started. I have 3 terminals : the first is at the bottom of the alternator, is unused, and has a rubber cap on and is presumably for the tacho ; the second has a largish nut, appears unlabelled, and a thickish cable departing it, reads the same voltage as the battery, and is presumably therefore the charge wire ; the third terminal is labelled D+, has a smaller nut than the other used terminal, and has a thin wire attached, presumably the no charge wire ?

My confusion arises because I have read Mad Hat Mans excellent guide to split charge systems and this states that the D+ terminal should be the big charging wire !

What gives ? Do I not have a stock alternator ?

Any other advice regarding further diagnostics appreciated, but I guess a new alternator could be in order ?

Cheers

Dave

david451 · Aug 15, 2008

davesh said:
Newbie !!!!

Defender 300TDI

Yesterday my ignition light/battery warning light on the dash started to come on and off intermittently whilst driving. Today it remains lit constantly whilst driving.

I have put a meter on the battery and the voltage remains constant when I start the engine, and then dip the accelerator ( well maybe drops .2v to 12.6v ).

I decided to put the meter on the terminals on the back of my alternator and this is where my confusion started. I have 3 terminals : the first is at the bottom of the alternator, is unused, and has a rubber cap on and is presumably for the tacho ; the second has a largish nut, appears unlabelled, and a thickish cable departing it, reads the same voltage as the battery, and is presumably therefore the charge wire ; the third terminal is labelled D+, has a smaller nut than the other used terminal, and has a thin wire attached, presumably the no charge wire ?

My confusion arises because I have read Mad Hat Mans excellent guide to split charge systems and this states that the D+ terminal should be the big charging wire !

What gives ? Do I not have a stock alternator ?

Any other advice regarding further diagnostics appreciated, but I guess a new alternator could be in order ?

Cheers

Dave

If it were charging, before dont worry about the wiring. See if you can get the back off and access to the brushes, might be a cheap fix.

Griffdowg · Aug 15, 2008

with the engine running you should be getting around 14v without any accessories on. put them all on, lights, radio, wipers etc and see if the alt can maintain a charge. i wouldnt worry about the wiring, esp if its only just started doing it too.

G

davesh · Aug 16, 2008

Typical. The alternator was definitely not putting out any charge so I gave the battery a slow overnight charge and drove the car up a mountain this morning to walk the dogs. Battery warning light remained on all the way there. On the return journey the battery warning light went off, and now the alternator is giving 14.2v at idle again.

I hate intermittent faults.

What would you recommend now ?

Cheers

Dave

david451 · Aug 16, 2008

davesh said:
Typical. The alternator was definitely not putting out any charge so I gave the battery a slow overnight charge and drove the car up a mountain this morning to walk the dogs. Battery warning light remained on all the way there. On the return journey the battery warning light went off, and now the alternator is giving 14.2v at idle again.

I hate intermittent faults.

What would you recommend now ?

Cheers

Dave

Must have some tyres on your vehicle to get up a mountain.

If it were me I would check all connections and failing that get the back off it and check the brushes, could be they are worn right down and likely to fail again.

davesh · Sep 25, 2008

I am tempted to swear !!!!! I replaced the alternator yesterday and initially all was fine - battery warning light went out and it was reading 14.4v at the battery when idling. Then I went out today and the battery warning light came back on. At idling the voltage at the battery is now back down to 12.63v and the same on the output terminal of the NEW alternator even when revving the engine.

I am baffled ...... any thoughts ?

Thanks in advance.

Dave

Griffdowg · Sep 25, 2008

check the cable connection for the batt on the starter motor. this links the alt to the batt too. maybe they are loose/coroded etc. worth a look. also check your engine earth strap is secure and rust free. you seem to have a prob with engine elccy circuit.

G

davesh · Sep 25, 2008

Thanks G. I'll take a look at the connections you have suggested but in the meantime another possible clue. If my understanding of the battery warning light circuit is correct ( largely gleaned from here Wiring colours on Land Rover - LR4x4 - The Land Rover Forum ) if I turn the keys in the ignition until the warning light comes on and then disconnect the wire from the B+ warning light terminal on the alternator I would expect the light to go off as the circuit would be without an earth through the alternator. Correcto ?????? In my case the light stays on.

Cheers

Dave

davesh · Sep 25, 2008

Re-seated and cleaned the earth from the negative battery terminal although it seemed to be fine anyway. Can't find the engine earth strap

, but this doesn't mean it isn't there. Any clues where I should be looking for a 300TDI ?

Other weird and probably related fact is that my chassis is live !!!! I disconnected the earth from the battery and put a meter between the negative terminal and the chassis and it shows around 5.5v. The meter also indicates that the +ve terminal and the chassis are connected as the continuity test produces a nice beeping noise.

I am way out of my depth here .... any guidance appreciated.

Thanks

Dave

boydy1989 · Sep 25, 2008

Continuity isn't always a good measure - it can be very high resistance, but still continuous.

5.5v between the -ve and the battery is odd though.

I would get your alternator tested out of the vehicle by an auto electrician.

Also, check the full route of the wires for chafing, burning etc. The earth strap goes from the back of the starter to a hole in the rearmost plate of the nearside engine mount.

Also, if your ignition switch is faulty, it will break the charge circuit and stop it charging - alternators are funny beasts, i.e. if the no charge light blows, they will stop charging. So, check for continutiy in that wire too - if the light stays on with the wire taken off the back of the alternator, then it is earthing somewhere else, so it will stop charging.

Again, check wires for chafing.

davesh · Sep 26, 2008

Unfortunately the problem has again disappeared this morning, but I did determine that the engine earth strap was missing and have now fitted one. This may resolve the problem but only time will tell.

If it doesn't I suspect that the diode/resistor/connections between the battery warning light and the alternator need checking out as it seems most likely that the alternator is failing to receive its "exciter" voltage. Anyone know whereabouts they actually are ?

Thanks

Dave

davesh · Sep 28, 2008

Unfortunately the earth strap didn't fix the problem so I have been poking around looking for bad connections/bad earths between the alternator and the charge light.

The most obvious suspect so far is the plug ( photo attached ) which is from the rear of the dash behind the warning lights ( there are 2 similar plugs and this is the one that attaches to the connector nearest the speedo ). As you can see it has a thickish white wire attached which definitely has something to do with the charging light according to my Haynes manual.

Can anyone who has a better ( or even any !!! ) understanding of the wiring diagrams tell me what purpose this connection serves ?

Cheers

Dave

boydy1989 · Sep 28, 2008

That connection is part of the charge circuit - it is the wire that connects to the No Charge light. Basically, if that connection is poor, the light will come on, the battery won't charge. There is corrosion on that, clean it up, and the corresponding connection is goes on to, and try again.

Burnsieboy · Jan 2, 2021

Hi, I am new to the site although I have had my Defender 90 300TDI for about 7 years - it never misses a beat - well until now!!!! The battery warning light has started coming on while the engine is running although the on-charge voltage across the battery is a healthy 14.3V and off charge reading is 12.8V. Having done some checks on volt drops etc I suspected that the diode in the feed from the dash indicator light to the alternator may be short circuit (not a common failure mode for a low current diode but not unheard of - they usually fail open circuit). I rigged up an additional diode in series between the dash indictor light and the voltage reference terminal on the alternator - Indicator light off with engine running and a healthy 14.3V at the battery terminals!!!! So it would appear that the problem was the diode failed short circuit allowing the alternator to raise the voltage on one side of the dash indicator light sufficiently to make it glow. I would like to investigate this further but cannot locate the in-line diode between dash indicator light and alternator - can anyone tell me where it is located please?

Simon

tottot · Jan 2, 2021

Part of the regulator on the back of the alternator I recon.

Alternator confusion !!!!

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