rpm drop when lights are on

[obdulio39]

this one has me going crazy. been working on my spider 80 carb for 4yrs. everything works well except when lights come on; it drops 200-300 rpms. moved from 55amp to 70amp or so on a rebuilt. cleaned all ground pods. what could be causing this. anybody... tks

[azruss]

an rpm drop like that can be created by a heavy electrical load. If it was a short, you would blow a fuse or find your wiring melting under the dash with associated smoke. If it is a connection that is causing a heavy draw, there would be lots of heat generated. look and sniff.

[TX82FIAT]

I agree that an RPM drop like that can be caused by a heavy electrical load. You mentioned you have cleaned the grounds already. I suspect you still have a portion of that circuit with significant resistance someplace at a connection point, fuse box, ignition switch, ground, someplace. Did you replace the main ground strap in the trunk as well as make sure you have a good engine ground strap.

[obdulio39]

thank you for the input. so far no smoke or smell. the ground cable at the battery is new with good contact. I (brown wire) the car with a hvy gauge cable (electrical) and crimped a new positive end on the cable at the battery. let me keep on looking. again tks

[hayesbd]

I have a '73 with a mechanical voltage regulator. It was dropping rpm in a similar fashion (maybe not as much as 200-300 rpm, though). I found that the regulator was trying to keep the system voltage around 16 volts. I cleaned up the regulator and adjusted it so it's right at 14.4 when cruising along anywhere above 2000 rpm and now there is no problem. You might have something similar going on?

Brian

[majicwrench]

The headlights draw a lot of current (more if you have fancy aftermarket types) It takes HP to produce current. So anytime your headlights are on, the alternator works harder, slowing the engine down. Modern cars, the computer system compensates for this, raises the idle etc.
LIke was said, having a system that is trying to produce 16v will make it work even harder.
Not sure you have anything wrong?? Did you verify RPM drop with a seperate tach or are you trusting the dash tach? I could imagine the volt decrease from turning on the lights may make the tach change it's reading.
Keith

[obdulio39]

thanks for the advice. im not an expert on this issue; but this problem happens at idle. I was thinking using a smaller pully on the alternator so it will run faster? at idle. or switch to 95amps. or use a headlight wiring kit. the headlights are hella on the car but idk what bulbs they have inside. actually the rpms go down as soon as the courtesy lights are on; not necessarily the headlights. im at a lost on this one...

[majicwrench]

Do the rpms actually drop or is the tach simply showing a drop?? Have you verified with a separate tach??
Keith

[obdulio39]

the rpms actually drop. and even worst when cooling fan comes on. the car does not stall; but it goes to 500 rpms when fan and lights are on.

[majicwrench]

Sounds fairly normal to me. The more stuff turns on, the more the alt has to work. Is your idle stable??

[TX82FIAT]

You said you moved from a 55 amp to a 70 amp alternator. I know they had a 45 amp with external regulator, a 55 amp and 65 amp with built in regulator. A lot of folks have upgraded to the 95 amp alternator. This is a long shot, I wonder if the voltage regulator on the 70 amp is cycling properly to account for the change in load and the RPM given the design of the alternator/regulator. The alternator needs to constantly find the balance between engine speed and electrical draw. If the 70 amp is designed to have a higher electrical draw at idle or idle at something like 900 rpm instead of 500-700 rpm then when sitting at idle it may be struggling to fulfill your voltage needs. Therefore, as you increase the demand on the system the alternator works harder drawing power from the engine. As long as the car is not stalling our when you stop I think you are OK. Maybe set the idle to 900 RPM and see if that changes anything so the car runs at 700 RPM with lights on.

Again, long shot... I'm not familiar with the 70 amp alternator. Did you have the same RPM drop before changing alternators?

[obdulio39]

thank you tx82fiat. i have not verified the drop in idle; but you can tell when an engine rpms go down. i'm ok for now, but i wonder if a 95 amp will cure the problem. i remember the car did not do it with the original alternator; it all happened when i got this one from autozone., unfortunaly i had to give it to them or pay core charge. I've heard some controversy with the 95amp so i'm not sure if i need one.