Howdy,

I've been noticing this a lot more in the Fall/Winter weather; if I have the blower motor and lights going in park hardly any voltage drop at all. When in gear however there is a very significant drop at idle speed. If you're on the gas its not an issue as the voltage goes back up under load, but if I am stopped at a light or 4 way or idling through the parking light it drops enough to dim the lights or slow the blower motor significantly.

Is this a symptom of the duraspark 2 ignition system trying to use a ton of power? I have a newer 60 or 70 amp alt and a 1 year old group 65 sized battery.

I believe I have the generic Blue streak coil and a normal ignition box compatible dizzy with the module, no points.

The current duraspark module is Napa brand.

Thanks All.

 

What is the idle speed set at?

Have you a real volt meter, even if just a plug in the cig lighter type? Monitor the volts, look for 13.7 to 14.2 volts.

I too have a 60 amp alternator and a 65 battery, ... but head lights and heater blower if on high plus your brake lights at a stop light add up in amps. In gear the engine rpm drops to it's lowest ... and so does alternator output.

Is not the ignition.

 

Have rocketman voltmeter and random LMC meter both pretty much read the same. With load or in park got somewhere between 14.3-14.6. In gear with at idle speed or at stop she drops to about 12.3 ish.

I've played around with different things turned on and off and it'll still bog down pretty good even with lets say just blower on set to low.

 

Sorry I forgot what I set my idlespeed to last, I think I had it a hair higher for winter, Im gonna guess somewhere between 750-800? I'd have to check tomorrow, I could be way off memory not too good these days.

 

900 wouldn't hurt none. I keep a low amp battery maintainer on my parked vehicles, all the time. Maintainer, not a simple charger.

Two head lights at 55 or 60 watts each is 120 watts, near 10 amps. Two brake lights lit are another 60 wats, call it 5 amps. There goes a quarter right there. I don't know what a blower draws, but It's gonna be at least another 10 or so amps I think. If the battery is thirsty for some charge, maybe it's aged, etc .... , there goes more. It adds up fast ..... What does your volt meter show when you are idling and turn the blower or head lights off vs on?

I know what you are seeing .... I see it too. I do have my idle up 800 - 850 in drive. My volt meter hovers at 14 volts at the drive through, but usually it's day and my head lights are off, and I don't recall running heat then (to hear better). I usually have it in neutral or park then, but if I step on the brake the needle reacts.

 

Funny thing the guys that put my motor in had the idle set at 500 initally.

In park I can turn everything on to max, all at the same time, cigarette lighter, lights, hazards, blower on high, stepping on break, wipers, dome light and in park those volt needles will barely move. I tried that max load test a week ago or so.

 

Original Ford alternator, or did you swap in a GM alternator?

Is your "newer" alternator an original rebuilt by a local shop that has been in business for years, or a parts store lowest bidder reman?

 

I dont know what amp ALT I have on my 81 F100 but even without the blower motor on when I just back out of the garage and head lights on I can see them dim at idle on the garage door when closed.
Because I just started the truck the battery is low and with the RPM low at idle the ALT output is low and why they dim.
I should also point out I have a head light relay kit installed so they are pulling power right off the battery.

This dimming is normal at idle just after start up. Have you checked after you have driven the truck 5+ miles at high way speed, that is what it takes to charge the battery after starting, to see if they still dim?
After driving 45 min / 40 miles at high way speed mine do not dim and I have never had a dead / low battery, truck has been on the road 5 years now.

The 1G / 2G (fire starter) ALT all do this at idle and why when checking out put you need to bring the RPM up to 2500, never at idle with everything on.
If you are that worried about output at idle do the 3G ALT swap. This ALT is used on the newer cars & trucks as it dose put out more AMPS at idle because the computers on them need it or they freak out and cause problems. If you are running after market EFI or a killer sound system then you need to go with a 3G ALT.
Dave ----

 

Sounds to me like a heavy load on the alternator, which with everything running that makes sense. Not a bad idea to make sure your battery grounds are good though. If those have a bad connection it will make the already loaded alternator work harder. The alternator will put more load on an engine than most other accessories driven by belt when their is a heavy demand on them.

When temp drops here in the winter I bump my idle speed up to 850-900 range. When temps warm back up I drop it back to the 700-750. 500 rpm would be way too low in my experience with these trucks.

 

I'll bump up my idle a hair tomorrow, need to changle fuel filter anyway.

Alt is mystery brand from O'Reily's

I really like the older septup where the brain is separate from the alt cause everything lasts longer but I'm sure at some point I'll have to swap to the newer system.

I'll have to check hwy speed, I don't leave town too often.

I upgraded all the grounds last year.

 

Check the belt tension.

It could be the parts store reman was assembled with marginal components that are showing their quality.

Could be low idle.

Could be fairly normal.

 

Posted in the other thread about the short but I figured I would update this one, since fixing the short there is no more bogging down since its related.

 

Do you have the list of grounding points? I'm getting some of the symptoms on this thread and my truck has never gotten any real tLC and I'm in that process right now.

Thanks!

 

So shorts cause head lights to drop in and out?