Alternator 'key on' electrical and physical path?

[pybyr]

In the process of troubleshooting my intermittent charging problem with the 165 Amp Leece Neville Alternator in my 89 F350 7.3 I decided to double-check the voltage at the "key on" wire that feeds power to the alternator's regulator/ field when the engine is running.

I'd previously checked and found 12 volts there, and assumed that all was good with that.

I've since checked it when the system is acting wacky, and found that the voltage bounces around- sometimes going below 12 volts, down into the 11 point something range, which matches up with the times that the alternator output drops.

I see that the "key on" wire to the alternator goes down into the part of the harness loom that goes right over/ by the alternator-

I have the Ford shop manuals for the truck, but not the wiring diagrams.

Can someone fill me in on the wiring diagram- and even better, if possible, the physical path of the "key on" wire to the alternator so that I can start chasing where the bad connection may be?

Thanks!

 

[itsacrazyasian]

Ford has used the green/red 12v key on for their charging systems for years. It was the same when i did the 3g swap on my 87/89/91/94. Well the 94 had it stock. I can pull up a diagram for you in a few,.

 

[itsacrazyasian]

Email me and i'll fwd the diagrams. theres 8 in total.

 

[redmondjp]

You have a chicken-and-egg situation here! The voltage on that wire is what is passed through the voltage regulator and then through the field winding of the alternator. It is system voltage, so of course it will drop when the system voltage drops as a result of the charging system intermittently failing.

Now, is this lowered voltage to the regulator causing your issue, or are you simply measuring the symptom of the charging system not working properly (caused by something else)? That is what you must figure out.

I would first suggest doing a full-field test in which you bypass the regulator - there is a terminal on the back of the regulator with words something like "GROUND TO TEST". Doing so will completely bypass the regulator and provide maximum charging. If you do this during a period in which no charging is occurring and it causes it to start charging, this points to a bad regulator.

I'm thinking that you may have a bad connection somewhere, and I would include the vehicle grounding (battery to frame, battery to engine block, engine block to frame, engine to body, alternator to engine block, etc) in the list of suspects. Your alternator also has the failure-prone connector on the side of it which should definitely be inspected carefully.

Good luck, and post back with your progress!

 

[franklin2]

As I said in a previous post, the signal for the alternator excitation comes from the instrument cluster. It runs through the light bulb, and there is a resistor that goes around the light bulb in case the bulb burns out.

If you want to eliminate that as a possible cause, just jump the "I" terminal ( or whatever it is on your setup) over to the battery +.