Alternator wiring

[Chuck Hurd]

Can someone explain what the pink and purple wire on the alternator does?
And what is relay for that someone added to the 96 patriot wiring diagram in tech support
My alternator is overcharging at16.4 v and after the unit sits overnight the alternator it warm to the touch

[Chuck Hurd]

Ok I think I got it figured out found this link
https://www.dixie-electric.com//tsb/TSB_704.pdf…..
I’m assuming the diode are leaking the alternator

[Eric Maclean Co-Admin]

Chuck
After review that diagram is appears how ever made that add on to the wiring diagram to add a  relay  the only reason they would require that relay is
That they were replacing the origina  alternator with an aftermarket alternator
That in itself isn't a big deal but the reason for the relay is that the aftermarket alternator would have had a regulator set up where the field excite terminal on the regulator was set up to dobble as a light circuit in a vehicle with dash mounted charge status light. The way it would work is that when the ignition was turned on 12+ volts would be passed through a light bulb placed in series in the ignition feed to the alternator that 12+ feed would be connected to the regulator to excite the alternator to begin to charge .
Unit the alternator starts to charge the light would see the regulator as a ground and the light would light.
When the alternator starts to charge the regulator then sends 12+volt back up that wire which equalizes the 12+ coming to the bulb which effectively turns the bulb off.
This system works fine in it's design but in the wiring diagram your looking at in its original form if the alternator back feeds that ignition wire it would be also be feeding the engine ECM relay and once the alternator began to feed back that wire the ignition switch wouldn't be able to shut down the engine.
Therefore the relay was installed to separate the alternator from the ECM feed the same effect could have been had by using a diode to block the back feed.


Hope this helps
Eric

[Chuck Hurd]

Thanks, after I found that link my brain started working and I came to the same conclusion.

[Eric Maclean Co-Admin]

Chuck
actually after restudying the wiring diagram it appears the relay is used to isolate the purple wire which is the alternator sense voltage wire for the regulator
The only reason to isolate the sense terminal is to avoid a battery drain caused by a fault in the alternator .

That alternator would be a(  Duvac style ) alternator
Duvac was the trade name of the company who built the original battery isolator blocks which where used to isolate two battery banks
They did this by using diodes bedded in a heat sink which would isolate one battery bank from the other .
It is to be noted that the battery isolator diodes will have a 1 to 2 volt drop across them so even if the alternator was charging at 13+ volts the batteries would only see 11 or 12 which would not fully charge the batteries.
By using a battery isolator the hot or output terminal of the alternator is not connected directly to the chassis battery which means the alternator has no way to measure battery voltage accurately
This is where the Duvac alternator differs from a regular alternator the Duvac style alternator is set up with a regulator which utilizes a remote sense wire this wire is usually connected to the chassis battery positive to provide the alternator with an accurate battery / charging voltage .
On the original Duvac brand isolators the isolator had a dedicated terminal just for the sense wire but on ordinary isolators with three battery terminals the sense wire could be connected to the cable headed to the chassis battery or directly to the starter cable as in the Beaver wiring diagram it should be noted that the sense wire is much more effective the closer it gets to the battery it is supposed to be senseing.

Eric

[Chuck Hurd]

Replaced the alternator today and it’s fixed, move on to the next item