`76 F-100 Supercab 390 2wd
I have a new battery, new alt--had it tested as well, and a new voltage regulator. I Started her up and drove for about 5 mins on Sunday no problem. Today, it turned over and she acted like it wanted to start, but never did...until I jumped it.

Using a voltage regulator, and with the engine running, I got 12.30 volts. I held the meter there (to the battery terminals) for a minute and had a steady drop in the hundredths place, so: 12.30, 12.29, 12.28, etc to 12.06. Then I shut it off.

I know the alt should be charging the battery while running--I think I've seen on here I should get 14 volts or thereabouts. I'm charging the battery now, but I feel this will get it to start just a few times, then the battery will drain, as it is not being charged, and I'll be back to a no start situation.

Ideas? Bad battery? Thanks in advance.

 

The battery sounds fine if it will start and run for a while. Voltage is low which indicates no charge. Starting and driving just pulls the battery down. With the engine running at fast idle and no unusual loads you should see between about 13.5 and 14.5 volts.

It could be a bad regulator or wiring problem or even the alternator as AP store tests are not always decisive.

Within the last week or so there was a very good thread on this with step by step diagnostics with a volt meter to narrow down the problem. Suggest you search and find it. Visual inspection of the wiring would also be a good place to start.

 

Thread with good diagnostic guide.

https://www.ford-trucks.com/forums/1...questions.html

 

Yes, the steps in that thread are what I would tell you first. If you have an ALT light (instead of an ammeter like the other guy), probe the 'I' terminal of the regulator connector instead of the 'S' terminal, under the same conditions outlined in that thread.

 

Will do tomorrow. Thanks guys. I'll report back my results.

 

See if you have a short to ground. Here's a simple way of Checking, Had to do it myself this morning.

Disconnect negative Battery Terminal.

Connect A test light to both the negative terminal and the negative lead.

If your light lights, you have a short to ground.

Disconnect your BAT cable on your alternator, If the light goes out, you have a bad alternator.

If the light stays on, reconnect your cable and move to the fuse box.

Pull each fuse and replace them one at a time until the light goes out. When the light goes out, you have a short in that circuit. Check wires for Breaks and repair as needed.

 

That's a useful test if you have a drain on the system that's killing the battery overnight, but in this case, the OP has observed that the battery is dropping significantly while the truck is running. Here, the alternator isn't even charging to begin with.

 

I brought the truck to O'reilly after charging the battery overnight. Upon starting at home: a lot of smoke, not usual, and, most importantly, I retested the charging system. At battery 14.5 volts.

O'reilly said they could test all charging system components inside the truck. Battery, v-reg, alt, and starter all good according to their diagnostic tool.

Made a stop while out and had a hell of a time getting it going. I think my 12v key on source for my dizzy is intermittent, or has a short. After cranking it over about 6 times, I popped the hood and basically jostled the dizzy and 12v key wire a little and it fired right over on the next turn.

When I got it home, I popped the hood again and measured voltage at battery:14.5.

I have not performed FMC's diagnostic yet, sorry. Should do that 1st I guess. I also am sure that I don't have a engine to body ground since the rebuild. Would that cause a draw?

 

It sounds like you are fighting wiring and maybe ignition switch issues.

The body does need to be grounded. A 10 gauge wire from the negative battery terminal to a clean chassis point should do it since negative is also attached to the engine. Another ground from engine to body wouldn't hurt. A missing ground won't cause a draw but some things mounted to the body, like the voltage reguator, may not work properly.

 

I've got the first ground you've mentioned but no the second. I'll have a lot of time coming up to clear all of the wiring/ignition probs. Winter break for teachers!

I think the housing of the ignition switch could be bad. When I first got the truck I had to rig it to work.

FMC's tests tomorrow morning. Then the wife is making me go to church. I consider working on my truck going to church.

 

How did the testing and Church Service go?

 

12 volts does not run to the distributor. BLACK is ground to the ignition module and the sink for primary return current. ORANGE and PURPLE are for the variable reluctance sensor which serves as the trigger to the ignition module.

That measurement indicates the alternator is charging, but your previous measurements did not. What has changed between the two instances?

 

Hey FMC,
My dizzy is the one wire hook up, well, two. One goes to the 12v power source, and one goes to the tack.

I don't know what's going on. It's been cold, so I haven't gone out to do more testing. I did go to Oreilly's and they hooked up a testing unit to the truck--battery, starter, alternator, and voltage regulator all checked out as working properly.