1987 Charging Trouble Shooting

[Justin Fleming]

I am helping a a friend trouble shoot his truck charging issue. He sent me the attached photo and two things jump out at me.

1. shouldn't there be a lead going from the positive post on the alternator to the battery or a wire coming out of the wiring harness that connects to it

2. It appears that someone spliced the white wire on the alternator plug, but it appears that it is spliced to a black wire, that did not look correct. I also thought it was odd that the red wire looped back to the post, but I did find diagrams online that kind of showed that....

At the end of the day, the alt gauge in the truck is not working, and he is running off battery only. The alt post have 14v and is a new alternator.

 

[diesel4me]

The stud connection has a thick red wire that (eventually) connects to the battery positive--the factory harness has that wire run back to the firewall where it "T's" off ,one side goes to the starter ,under the positive battery cable,(and there will be at least one fusible link in that wire there)...
...the other side runs behind the engine to the junction block up next to the brake booster,and another red wire from there runs to the fuse block to power the ignition switch and rest of the cab..

Some guys who upgrade to high amp altenators will add a short wire from that stud to the positive battery terminal to reduce the load on the original harness,that has a good length of wire to get battery charging current through..

The other two wires on the alternator on the plug are wired as such--the red wire or #1 (marked on the back of the alternator) has 12V present at all times...the other wire,usually a tan or white one,needs to get 12V only with the ignition "on" or "run" position ,this energizes the field windings to allow a charge to be produced..

It sounds like someone just jumped the #1 wire right to the stud on the back of the alternator in order to get it to work..this "might" work,but the other wire #2 needs 12V too,and it wont charge until both wires on the plug have 12V with it running...jumping the #1 wire that way usually works,but the alternator wont be able to "sense" how much charge the battery needs correctly,it may not put out enough--and it could also run "full fielded" and put out maximum charge all the time,and boil the battery or harm the alternator.

 

[Justin Fleming]

So this all being said, should there be two additional red wire coming out of the factory hares where the black wire is coming from that is spliced on to the white wire that goes to the two prong connector, (1) to go to the stud and (1) to go to the red wire on the two prong connector?

Does anyone have a picture of this off of there truck?

 

[K85 Octane]

Test the black wire for 12v both key on and key off. Maybe you'll get lucky and it's key on 12v. If so, make sure it's in position #2 like Bob said. Then add a sizable wire to the battery from the charging post on the alt. Start the truck and check all wires for 14.4v.

 

[Justin Fleming]

What should I do with the red wire looping to the post.

Also where does volt gauge pick up its reading in this mix?

 

[K85 Octane]

The red wire goes to #1 post as long as you run another wire to the battery from the alt charging post.
The volt gauge gets power from any key on source, so it's probably just tapped into the same wire that feeds all your other gauges. Part of the printed circuit.

 

[diesel4me]

There should be only two wires on the plug...a red one and a tan or white one..any others were likely added on by someone,or perhaps if the black one appears factory and was in the harness loom,it probably goes to another component like the electric choke,as mentioned..

Those 2 wires are all you need for it to charge properly...provided the red wire on the plug is hot all the time,and the tan or white wire on the #2 plug terminal gets 12V with the key on,it should charge...

Far as I recall the volt meter is fed by the fuse box via the dash cluster printed circuit,but its possible not all trucks were set up like my '77 that had a voltmeter...

I have to go plow my driveway before the winds ramp up to 50 mph here in a few hours...be back on later today if you need more help..