Hi,
Having some issues with my electrical system and I am looking to be steered in the right direction. Here is my set up"

70 roadrunner
383 / 727 - both freshly rebuilt
edelbrock heads, rpm intake, 750 holley
MSD ready to run distro
powermaster 75 amp alternator
new engine harness and dash harness

Here is the problem. The car runs ok around 900-950 rpm in park. It even runs ok while in gear at around 850ish RPM. my voltmeter shows about 14.5 volts. Then when I turn on the headlights the volts drop to around 12.5 or 13. The car starts running rougher and rougher until it finally dies.

The blower motor / heater is not hooked up so I can't really test it under another load. The battery eventually dies and will not restart the car until I throw it on the battery charger. The battery will be dead after its been running a while. (alternator not charing?)

I have checked all of the grounds and they are good, but I am going to go over them all again just to make sure.

I feel like the care is idling too high at 950 in park and 850-875 in gear. But if I go lower than that the volts drop to 12.5 or 13. I am going to take the battery and the alternator over to a parts store to get them checked, but any other ideas of what is going on? Thanks for any help!

Hey mods, I accidently put this in the wrong forum. Can you please move it?...sorry

Quote:

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take the battery and the alternator over to a parts store to get them checked

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That's the first step.

When you say you "checked the grounds" How did you do that? Without removing the connector and cleaning you can't be sure it's ok. Even with an ohm meter you can miss corrosion issues that get worse with heat.

seems like the vr is doing its job. My bet is alt bad since it can't keep up with the load/amps being pulled.

We have had similiar problems on cars before. When you use a high amp alternator the factory wire can't handle the job. We had this problem on a car with a 100 amp alternator, and nothing would stay charged. Ran a battery cable off the alternator to the battery and all problems were solved. This is now the way we wire all cars. If your alternator and battery check out, give it a try. It would be real easy to throw a heavy wire/cable on with a couple zip ties to check before you go through alot of work.

I checked the grounds by just pulling off the VR, ballast resistor, headlights ground and looked for corrosion. Didn't see any so I put them back on.

The next step I am going to pull them off and sand the area down to metal and put them back on.

I ran home on my lunch break and was looking it over and I noticed something odd. The taillight housing on the inside of the trunk where the light bulbs socket pushes in; there is another "ground wire" running from there to the body. The wire is bare on the end and shoved in between the housing and the bulb socket. Could that be causing part of my issue? I will pull those housings off and clean them up and see what that does.

I have actually been meaning to run a cable from the alt to the battery and just haven't done it yet.

Make sure you fuse it, nobody wants their car to burn.

ok, i won't have much time to do anything until this weekend but I went played with it for a few mins last night.

With the idle around 850ish in gear I realized that I can have the headlights and the volts stay around 14.5. as soon as I hit the brakes (brake lights go on) that the volts start dropping. It seems the system won't handle both.

But I upped the idle slightly to 900-925ish in gear and everything is fine.


That idle seems a bit high to me. How do you determine what the idle should be at? Just to where everything is running good?

I am still going to have the alternator and battery checked out, but this seems to have done the trick for now.

The engine and dash harness are brand new. (bulk head) and I have already bypassed the amp meter and gone to a volt meter.

the only part that I have not done yet is running a wire from alt to the bat.

I would have your battery load tested along with everything else mentioned and if you can, substitute a known good battery into the car and see if it improves. My Fury was doing what you describe and it turned out to be the battery which was only eight months old even though it checked out ok. Good luck to you.
John

Ok - so i finally was able to get the parts store and have my battery and alternator tested. They both tested good.

So after triple checking all the grounds and I charged the battery up until my charger showed it was at a "full charge" and started the car up. Still having the same issue, but I went and drove it around for about 20 mins. (at night so my headlights were on) My voltmeter in the car showed 14 volts pretty much the whole time.

After I got home the car would not restart. Through my battery charger on it again the the charger showed it only had about a 50% charge.

So at this point I assume that the battery is bad. Since the car was running fine the whole time I was driving it says to me that the alternator is working fine since it powers the electrical system while running.

Sound logical? How reliable/accurate are those parts store testers?

you should be able to have them test the battery too.
Pretty much they try to make the parts store testers idiot proof. But they keep making better idiots.
They just look for an ok or bad. Don't even tell them what they are doing.

I had them test both the battery and alternator..both showed good....

Make SURE you have a good ground from the battery to BOTH The frame and the engine block, and prefferably a ground strap from the block to the frame.

If the alternator indeed is producing amperage when tested, but the battery is not getting a charge, the problem is likely in the grounding between the frame and the motor.

With the engine running, pull one of the battery cables off battery. If it dies, you are NOT getting power from the alternator.

Scott