April 14th, 2012, 03:34 PM
Always with the confusion. The 73 F100 351C 4v quit charging. Replaced the alternator. Drove 70 miles with about 10 stops and restarts, battery discharged. Zero. Zip. New battery. Made the same trip, battery half discharged. Swapped old battery back in just for curiosity's sake. Same trip. Battery about a quarter discharged. Now she seems to be running fine, except the battery never gets charged over about 75%.
Plan on completely rewiring this spring. Do the electronic VRs work very well? Is it necessary to use a field relay like the later model truck? How does one mount a heavy duty fuse to replace the fusible link? What should such a unit be rated?
April 18th, 2012, 09:37 PM
sorry, just saw this... I'd be curious about the grounds. Are you getting voltage off the BATT terminal on the alternator? You should see 13.8+ volts coming off of it I believe.
Originally Posted by boxer3main
May 16th, 2012, 03:34 PM
Thanks. I'll check. I have checked across the posts running and not running and got the same figure, 12.8. I checked continuity on everything with no problems showing up. It has to be producing SOME juice or the bat would completely discharge, right? The dash gauge doesn't do anything, which is kinda Ford standard. It doesn't matter how often I start it, how far I drive or what draws juice (lights, heater fan, etc) it's always the same. I've gotten to where I carry an extra charged battery just in case.
May 16th, 2012, 03:44 PM
When you rev the engine, the voltage at the battery should be higher than 13 volts. Sounds like the regulator is bad, the alternator is bad, or the wiring is bad. Since you already replaced the alternator, you only have a couple more things to throw money at before you need to actually start diagnosing the problem.
Does it have an ammeter?
Last edited by squirrel; May 16th, 2012 at 03:45 PM.
May 16th, 2012, 03:56 PM
Well, it's got a dash gauge. May I assume that is an ammeter? It doesn't do anything. The needle sits just below the middle running or not. I just checked and got 12.4 at an idle off the alternator BAT pin. Argh! More chasing. It must be charging SOME at higher revs, I guess, or it would discharge even more than it does. Thanks for the help.
May 16th, 2012, 04:07 PM
I don't know how the ammeter in that truck works, it could be a shunt type. The shunt type ammeter works by measuring the (small) voltage drop in a bigger wire, such as the wire connecting the alternator to the battery.
The first thing I'd do is measure the voltage at the alternator output pin, with the engine revved up. then measure the voltage at the battery with the engine revved up the same as it was for the first test. If the alternator is putting out more than 13 volts, and the battery has less than 13 volts. then there's a wiring or ammeter issue.
If the alternator is not putting out more than 13 volts when you rev the engine, then you need to diagnose the regulator and it's wiring.
Do you have any manuals that tell you how to diagnose it?
May 16th, 2012, 04:09 PM
Lord God King BangShifter
My stock ammeter only shows on the charge side for the first few minutes after start up. Other wise it just sits in the middle. It only shows a discharge if the engine is off and I have the lights on.
You need to check the voltage at the back of the alternator.
BS'er formally known as Rebeldryver
May 16th, 2012, 04:13 PM
Lots of manuals. I'll take a look this evening. I just got 12.8 on the battery posts with the engine off. 12.4 at an idle. 12.9 at 2000 rpm.
Adding to this: 12.4 BAT terminal on the alternator to ground (at battery) at idle. 12.9 at 2000. 13.12 at 3000. Could this be a field related problem? Possibly even though the wire shows continuity either the regulator or the wire is not passing enough juice? Could it be an RPM problem related to pulley size? The alternator is new and a few amps higher than the original. The one that came out was a 40-45 something like that. This one is 65. I swapped the old vee pulley on replacing the wide belt one that came on it. Gotta go to town. I'll pick up a bit of 6 and 8 gauge wire and fiddle a bit. Thanks for the help.
Last edited by Yardpilot; May 16th, 2012 at 05:04 PM.
May 17th, 2012, 03:10 PM
The alternator is not putting out enough voltage. Like others have said, it should put out 13.5 - 14 + volts when charging. You need to figure out if the failure is in the new alternator, or the external regulator. To do this, hook up a temporary jumper wire from the positive on the battery to the field terminal of the alternator, with the engine running at a high idle and a volt meter connected to the output terminal of the alt. When you hook up the jumper, it will bypass the function of the regulator and kick the alternator into max output mode. If the output voltage reading jumps way up, the alternator is good and the regulator is bad. If the voltage does not go up, your new alternator is defective. Good luck, Joel
May 18th, 2012, 08:33 PM
That was interesting. I hooked a 14 gauge jumper from the field on the alt to the positive battery post. That's as far as I got. The wire smoked and burned off instantly, adding a neat blister on my thumb. Gotta think on this. Pull the existing regulator connector first, perhaps?