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Hi, I have a 1978 Ford F-150, 4wd pickup truck. Have replaced the battery, the voltage regulator, and the alternator. when you disconnect the battery terminal the truck stalls and shuts down; when the truck should stay running when you pull the battery terminal. Don't know what else it could be other than another bad alternator from the parts store again for the second day in a row, I know it's not the starter; cannot figure this out to fix it. this morning, the parts store said there was nothing wrong with the new alternator I got yesterday. It did not stay charging and truck shut down when you pulled the battery terminal off of the battery,; the truck should still run and charge unless its a bad alternator don't know what else it could be
pulling the battery cable will NOT shut down the engine if the charging system is working correctly. Use to pull up to an irrigation engine, leave the truck running, remove the battery and move it to the irrigation engine. Once the engine was started, put the battery back in the truck (while it was still running on the alternator) and never had a problem.
New parts can be bad. If the alternator and regulator are working, you might have a wiring issue...
You could pull the battery cable and the engine would still run back when they had generators, not alternators.
Good chance you ruined your alternator whenyouremoved the cable with the engine running.
X2 AFAIK taking the battery connections off the battery (when the truck is running) is a way to take the voltage regulator (out of the elec control system) and then the alternator can to "full field" and ruin the internal works of the alternator. Sounds like you got a new alt that was bad or X2 you have a different wiring issue.
Using a multimetor is the best way to test/ troubleshoot the elec system/test the alternators operation.
You guys that think it is bad wrong are just that
You can run a car all the way to Denver with the battery disconnected with an alternator
OP check your alternator harness and the fuse link from the battery
* TEST = Full field the alternator at the regulator with one of those little testers or a jumper wire
True, it sends a voltage "spike" through the system when you disconnect and reconnect a battery cable while the engine is running
That is the ONLY problem and it's not a real problem
I do it in testing OFTEN, and have for about 40 years now with no problems ever
I agree with using a volt meter to check for charging. A marginal alternator might barely keep the engine running, but might not be able to put out enough voltage to actually charge a battery much or at all.
I also agree that disconnecting the battery while the engine is running won't normally ruin an alternator. While I don't recommend it, I've done it a handful of times in my life. It's probably not great for it on paper, and possibly in real life as well, but it's not an instant or impending death sentence for the alternator. If anything, I'd be more worried about the PCM and other modules with all of the un-damped voltage fluctuations that they will experience with the battery out of the loop. We don't have to worry about that part on a 1978 F-150.
Some cars will have their engines die without the battery connected when sudden loads are added, such as brake lights or turning on the blower.
Well I do know from 1st hand experiance, that when you use the one piece molded negative (lead clamp and wire cable as one) battery cable and it intermittently works. It can cause wild fluctuations in your alt gauge when driving down the road and cause for a difficult time to trouble shoot.
And that issue (of lack of sudden internal connection, when the truck was idiling) in my buddies drive way, cause the black material inside the voltage regulator to melt and make for a white smoke show, to the point I though the truck was on fire. And that material in the VR stinks like crazy.
I managed to find the originial issue by luckily having the hood up with door open and the dome light on. I was doing a wire wiggle check and noticed when I pushed and pulled on the battery cable, that the dome light came off and on. Wala bad connection that LOOKED ok from the outside.
I now have battery cables that the lead connection is clamped to the cable end.
I had a 76 f150 that had this issue with the regulator it came with you had to rev it a touch to turn (ignite) the alternator on so if you're just starting it and then popping the cable would kill it not that this was ever really a concern for me but consider trying to rev it to 1500-2000 before doing that
You could just watch the volt meter I had go from like 12 to 14-15 as soon as you touched the gas pedal on that and then return to like 0 with the key removed the important thing was key on engine off and key on engine running voltage was the same number until the alternator ignited and it started charging
These things I'm almost sure are supposed to be self igniting so this shouldn't be a fix but it doesn't mean it isnt the cause of your problems and if it is you're pretty much good it'll charge on it's own with nothing unusual needing done
This is however standard operating procedure on some older vehicles
Last edited by Wheezingthunder; Apr 26, 2026 at 04:40 PM.
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