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Alternator Wiring


Chris Scott
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Hi all, I need some help!

Car now runs, however the alternator is not charging the battery - I have no idea if it worked before tear down, so fighting blind…especially as I’ve rewired the car too. 
 
The wiring loom has two positive wires to the alternator, a 6mm (main charge) and a 2mm called ‘positive connection for charging’. Currently I have them both to the +ve post on the alternator but it’s not charging.

Have I wired it right or wrong? Could the 2mm be a “battery sense” wire and need to go to the regulator? Could that be why it’s not charging? 
 

It’s a Zetec Silvertop. I’ve included the wiring guide and a diagram of the alternator plug (type 2 or 3) 

 

D42B9A93-2A5D-44C7-A0B9-FDBA2B6A1448.jpeg

D289CFE5-4CE1-4697-AD51-D1ADEF2FB145.jpeg

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A couple of thoughts.

The 6mm one carries the main charging current which would be the IG and should go to the battery.

The 2mm one on a type 2 would also go to the IG connection OR on a type 3 would go to the S connection. Safe says leave open for now.

1mm to the warning light. The other terminal of the warning light goes to switched live.

Then do not forget the 4th wire. Every electrical circuit requires a conplete circuit so the battery negative needs connecting to the alternator body this is the 4th connection, it is normaly achieved by the mounting bolts of the alternator to the engine block and hence back through the starter earth strap to the battery.

So if the warning light is wired wrong the alternator will not start charging as some current through the warning light is often used to boot start the alternator.

OR painting the alternator brackets thus preventing the negative connection so no charge, on older alternators this used to fry the control electronics in the alternator, most modern ones live through this but any form of arc mig tig welding with the alternator connected can still fry the alternator.

If you have a volt meter measure the battery terminal voltage just before starting and then with engine revs above 3000 rpm if the voltage rises then the alternator is trying to charge, if not there is a problem in charging.

I hope that this helps

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6 hours ago, IanS said:

A couple of thoughts.

The 6mm one carries the main charging current which would be the IG and should go to the battery.

The 2mm one on a type 2 would also go to the IG connection OR on a type 3 would go to the S connection. Safe says leave open for now.

1mm to the warning light. The other terminal of the warning light goes to switched live.

Then do not forget the 4th wire. Every electrical circuit requires a conplete circuit so the battery negative needs connecting to the alternator body this is the 4th connection, it is normaly achieved by the mounting bolts of the alternator to the engine block and hence back through the starter earth strap to the battery.

So if the warning light is wired wrong the alternator will not start charging as some current through the warning light is often used to boot start the alternator.

OR painting the alternator brackets thus preventing the negative connection so no charge, on older alternators this used to fry the control electronics in the alternator, most modern ones live through this but any form of arc mig tig welding with the alternator connected can still fry the alternator.

If you have a volt meter measure the battery terminal voltage just before starting and then with engine revs above 3000 rpm if the voltage rises then the alternator is trying to charge, if not there is a problem in charging.

I hope that this helps

Very helpful thanks! Both the block and bracket are painted so well worth looking at the earthing, I’ll also give the voltmeter check a go when revving. 
 

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You need to make sure which alternator diagram is correct.

The 6mm should be ring terminal to battery, it may need to bigger wire depending on alternator output.

IG should connect to a switched live.

L wire to warning lamp and other side of lamp to IG.

S depends on alternator but would be 2mm to battery.

If you have a small Denso type alternator (40amp) it may not give enough juice if you're fuel injected.

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I’ve done some more digging, based on what’s been said on here and I think I have a 4 wire alternator like the picture.

I have a 6mm and a 2mm wire for the alternator (both going to battery) and both attached to post on alternator(4).

I have a warning light wire too(1) - so 3 wires only.

I don’t have a switched live going to the alternator at all, do you think this could be the issue? 

AE47E1F1-D304-49BE-AB64-1C23C96A553F.jpeg

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Quite likely the issue then. Modern engines have so called ‘smart alternators’ where the charging can be controlled by the engine’s ECU via a single feed wire to the alternator (all part of a CO2 reduction effort so the alternator does most of its work when the car is decelerating). I don’t know much about the Ford engines but on an MX5 engine the alternator was ‘smart’ from about year 2000.

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