HI, this question is for my 82 F150 4x4 with the 351. When I turn the lights and/or heater on in my truck my engine RPM drops considerably. The lights and heater seem to be fully functional however.

I have installed a new 100amp alternator w/internal regulator, I have 14.something volts at the battery with everything turned on I can.


I have already installed a HD harness w/relays (from LMC/Bronco Graveyard) I have a new headlight switch, havn't installed yet.


As for the heater/blower motor I havn't explored any possibilities yet.


I was looking for some input as to what may cause this to happen? The battery in my truck is a pretty small battery, could I need a larger capacity size? Any help would be appreciated.

 

You should quantify "considerably". But, I'm betting that you are dropping a couple hundred RPM and that doesn't surprise me with a good alternator. The original alternator couldn't throw much current at idle, but the new one probably can and that's what is pulling the engine down. Nothing to be worried about.

 

Well I don't have a tach installed yet so its hard to give a number.


With just the motor running itself. I can dial in what I think a standard idle should be. With all the accessories turned on, the idle was too low so I had to adjust my idle with all the accessories on.


Now with the motor running itself, it idles a bit high in my opinion, but is lowered by turning everything on again. Is this something I should live with?


Also, with the higher idle now, when turning the truck off, it diesels a little.

 

Automatic or manual transmission?

 

Its a manual

 

Where is your distributor getting the vacuum for the advance? Manifold or ported? If manifold try ported. The reason being that manifold vacuum drops when the RPM drops, which lowers vacuum, which…. Vicious circle. Ported won't do that.

 

Well here is an update on my problem.


I have replaced the headlight switch. And I have tried switching the vacuum lines from manifold to ported. No difference. The motor still bogs down with the headlights on and/or the heater blower. I have to keep my idle high enough to run these accessories without my truck shutting down...this in turn causes my idle to be too high with no accessories turned on and if I shut my truck off it diesels which makes starting it again a little difficult.


I did notice that the main ground cable from the battery is not mounted in an optimum spot...the PO mounted it to a bolt on the exhaust manifold So Im going to grind a bare spot on the frame and install a new ground there and see if that helps.


Any other suggestions would be greatly appreciated!

 

The main ground point from battery should be somewhere on the engine block itself, not the frame.

 

Alright, good to know, Ill find a place on the block. Thanks

 

On my old pile I ran a thread chaser a couple times into an unused pad above the oil dipstick port, then filed the pad nice and shiny bright. That perked everything up pretty good. A good block to frame cable is essential too, don't misunderstand, and from the block to cab/firewall. What happens is the engine is often semi-insulated with rubber mounts and the like.

 

Factory ground spot was on the block. On my 302 it is situated below the head on the side of the block there is a small raised pad that mine is bolted to.


I hope you can figure out the cause, I will be following this as I planned on upgrading my alternator in the future.


Just like Tedster9 said above, the engine is where the battery grounds but between the frame and the engine is not that great of a ground. This is the main reason why I mounted a dual terminal junction block on my core support and I will be running a 10ga ground wire from the battery cable mount on the side of my block to the junction block for use in grounding my horn and headlight relays. They work fine as they are now but I am the kind that prefers to go over kill on good grounds as they are very important when it comes to how bright a light is or how loud a horn is.

 

Ok what Ill do is put a cable to the block from the battery. Then Ill do a cable from the frame to the same spot on the block to cover all my bases.


As for the alternator. I do have great voltage. Like I said above, I get 14 something volts at the battery and solenoid with everything turned on I can turn on lol


Hopefully someone can come shed some light on some more possible causes.

 

First, you are supposed to have a ground to both the block and the frame. The ground to the engine, but obviously not to the exhaust manifold, powers the starter. So, if you move the ground to the frame it probably won't crank and, if it does, you'll probably cause other problems.

Second, you are supposed to have a ground from the engine to the cab. Typically this is from the rear of the intake manifold to a screw to the wiper motor, but can be to a screw into the firewall.

Now, let me see if I understand. When you bring the or the blower on the engine slows down, and if the idle speed isn't high enough it dies. Right? I can only see one option for the engine slowing down - the voltage to the ignition box drops so low the resulting spark won't fire the plugs. (The alternator cannot pull enough power to slow the engine down that much IMHO.) So, let's try this - start the engine and put a volt meter on the positive side of the coil and note the reading. Then have someone turn the lights and/or blower on and note the reading. I'll bet the voltage drops dramatically.

If so, we need to ascertain why the voltage goes down so much. But, let's make sure that's the problem first.

 

This doesn't seem right.
What do you have for lights?

Stock; high beams, tail lights and backup lights wouldn't draw more than 200W

1 horsepower is 746W

How on earth can a quarter horsepower load your idle down to where it stalls the truck?

 

I think the draw is lowering the voltage to the coil and/or DS-II box. If it is idling the alternator can't put out much and the voltage drops below the working point. And if the engine is spinning faster the voltage stays up just enough to run.

Having said that, the bit about having just installed a headlight relay bothers me. If done properly there's no draw from the cab. If done wrong it might pull the voltage down, but then the lights shouldn't work.