Headlight Question

[Big Blzn]

When I am parked in my garage with my '72 K5 running (not trying to kill myself..........yet) I notice when the headlights are on that at idle they seem to be on the dim side but when I rev them engine the get much brighter as the RPM's increase. Is this typical? It doesn't bother me to much but just want to know if there is a problem or the begining of a problem that I should fix. Should I test the starter? My stock volt meter seems to running where it should be too.
Thanks
Hoby

 

[4X4HIGH]

That is common. You can fix this by installing a relay.

 

[Chevy305]

quick fix is to turn up your idle

 

[dremu]

When I am parked in my garage with my '72 K5 running (not trying to kill myself..........yet) I notice when the headlights are on that at idle they seem to be on the dim side but when I rev them engine the get much brighter as the RPM's increase. Is this typical? It doesn't bother me to much but just want to know if there is a problem or the begining of a problem that I should fix. Should I test the starter? My stock volt meter seems to running where it should be too.
Thanks
Hoby

Well, this means the voltage in your electrical system increases with RPM, which is more or less normal. Get a decent DVM and check the voltage at the battery at idle vs at RPM. Should be over 13V all the time; if not, either you've got some pretty serious drain from accessories, or your alternator ain't doing its job. (But, see below about wiring.)

Could be a sign of a wimpy alternator, could be a sign of the regulator on its way towards the great alternator graveyard.

Scott, I don't mean to disagee, but I donno that a relay will do much good. The relay / big wire mod allows more juice to get to the lights ... but in this case I don't think there's more juice to get there. Mind you, I think the relay/big wire thing is one of the easiest mods to do to our trucks; bang for the buck it's better than replacing the auto hubs with manuals =))

Thinking of big wire, you should also check your wiring. The battery-to-ground wire could be corroded or crappy, as could the other ground straps, or for that matter the hot wire coming off the alternator. Don't take this as a directive to go rewiring your whole truck, but it's worth a check.

Check the voltage at the back of the alternator, both with the negative on the battery, and the negative on the alternator case, and the voltage across the battery. If there's a noticeable difference (umm, say, more than .1V), then I suspect that the wiring is whacked, and its additional resistance is reducing your alternator's ability to charge and regulate.

The charging system is pretty simple, but you gotta check each piece. Make sense?

-- A

 

[dyeager535]

Agreed. Alternator output is pretty bad at idle on the 10/12SI alternators, worse as they wear out. They don't compensate for load very well. Low voltage at the headlights would be constant dimness, varying output from the headlights means the voltage input is changing.

Headlight wiring is poor too, but I bet if you put your fan on high, then hit the headlights, you can hear the fan slow down. Not enough output from the alternator.

These are big reasons to convert to the CS130/144 style, as they A)have much higher output than early stock alternators, B)have much better output at idle RPM's, and C) are much better about keeping voltage output constant based on electrical load and engine RPM. At idle or cruise, voltage output is near the same no matter what accessories are running.