Voltage drop with headlights on

[craiginaustin]

I have an electrical gremlin that I'd love some help narrowing down. When the headlights in my 64 are on, the reading on my voltmeter drops down to around 11 volts (sometimes less). The gas gauge also reads about half of what it reads with the headlights off. The same problem, but to a smaller extent, happens when the indicators are on (you can see the voltage / gas gauge drop in sync with the blinking). The voltmeter is wired straight to a spare IGN 12V slot on the in-cab fuse panel.

I plan on checking the alternator output voltage to make sure it's not toast. How can I tell if it's an internally or externally regulated alternator? If externally regulated, how would I find and test the regulator?

Could it also be a bad ground somewhere? The truck seems to be lacking grounding wires / straps in general -- I added one from the engine block to the frame (with a block to battery ground already in place), but I don't see any cab grounds at all. I believe there should be one from the firewall to the passenger-side head? Where would I check for the presence or lack of grounding to the in-cab electrical systems?

Thanks in advance for any advice. I'm relatively electrically savvy, but automotive electrics are pretty new to me.

[vince1]

I don't know about the 64 but on the 66 the external regulator is IIRC just to the inside of the horn relay. Pull off the connector and reinstall? If the horn relay acts as a junction block I would redo the connections from there to the battery, the alternator and the fuse panel. With the truck running, voltage across the battery terminals should be around 14 to 14 1/2. I can't remember where the grounds are but yes, seek them out and redo them too. Are there some at the headlight buckets? Pull off and reinstall the connector at the dimmer switch? Bad headlight switch? If the signals are also doing it and only 11 volts at the headlights, I think it is more likely a power supply break down more than poor ground but maybe a combination of both.

[craiginaustin]

I took some voltage readings today:

At battery, truck off: 12.3 V
At battery, idling: 12.6 to 13 V
At alternator, idling: 13.5 V

I guess this means the alternator is bad?

[franken]

Originally Posted by craiginaustin

I took some voltage readings today:

At battery, truck off: 12.3 V
At battery, idling: 12.6 to 13 V
At alternator, idling: 13.5 V

I guess this means the alternator is bad?

The higher voltage when running means the alternator is good.
The battery voltage is low so it needs a charge, or the meter is off.
What does the battery voltage do when the engine is above idle?
Chances are you have bad grounds and poor connections all over the place, which is typical of an old vehicle. Go through and clean connections wherever you can--on the + and - sides. If grounds are missing, put them back.

[craiginaustin]

Originally Posted by franken

The higher voltage when running means the alternator is good.
The battery voltage is low so it needs a charge, or the meter is off.
What does the battery voltage do when the engine is above idle?

The battery voltage didn't change when I increased the revs.

The low voltage at the battery may be because I've also been adjusting the high-idle settings on the carb, which involved starting and stopping the engine a few times but not giving it much time to run and charge.

Chances are you have bad grounds and poor connections all over the place, which is typical of an old vehicle. Go through and clean connections wherever you can--on the + and - sides. If grounds are missing, put them back.

Definitely good advice, I'll trace through what I can next weekend and clean them up. Is there a good reference for where there should be grounds (eg, should there be a cab ground somewhere?) The shop manual isn't much use.

[craiginaustin]

My alternator also doesn't seem to have an explicit ground wire attached. Should there be one, or is it grounded through the housing via the mounting bracket and engine block?

[vince1]

A ground wire goes from the regulator case to the alternator housing but my experience is if the connection is bad the charging rate is too high. If internally regulated I don't think it needs a ground other than through its mounting brackets.

The original Delco external regulator may have a removable cover and be somewhat adjustable.

At the alternator output shouldn't it be the same reading as at the battery? Clean up the connections between the two. What are the readings with the lights and everything on?

I like it when the readings at the battery are between 14.3 and 14.5 but that means nothing because I'm no expert, it's just based on my observations over the years.

[craiginaustin]

I just repeated the at-battery readings with no extra load, then headlights on, then headlights and electric fan on. I'm getting really inconsistent results.

At battery, no load: around 13 V, but fluctuates.
At battery, headlights: usually the same fluctuating but around mid-13s reading, but occasional spikes to 15 - 16 V. At one point it was reading high, then dropped back lower with what seemed like a change in idle speed (but may have been imagining it, or perhaps something changed in the fan noise of the alternator)?
At battery, headlights + fan: mid 13s, didn't observe the spike to 15 - 16 V.

If it helps, the alternator is an Ultima 61 amp, externally regulated unit. I'm running a HEI as well.

[63burban]

Do you have a good ground wire from the battery or engine block to the body?

[craiginaustin]

Originally Posted by 63burban

Do you have a good ground wire from the battery or engine block to the body?

I have a connection from the battery to the block, then on the same mount on the block I've added a strap to the frame.

I think it's a solid connection to the frame but I'm definitely willing to grind back the metal with the dremel and make sure.

[craiginaustin]

It also bears mentioning that the lights in my gauge cluster are remarkably dim; but the lights in my aftermarket gauges are fine. The aftermarket gauges have a direct ground to the battery.

[63burban]

The body needs to be grounded, the frame is fine but does not provide grounding to the cab itself. Install a heavy ground strap from the battery to the firewall or engine block to the firewall.

[craiginaustin]

Originally Posted by 63burban

The body needs to be grounded, the frame is fine but does not provide grounding to the cab itself. Install a heavy ground strap from the battery to the firewall or engine block to the firewall.

Thanks, I've ordered a second ground strap from Amazon and will report back the results.

Out of curiosity, do you know where are it's mounted on the firewall from the factory?

[65 C20]

http://67-72chevytrucks.com/vboard/s...t=ground+strap
Check out this thread.
Also go over to the electrical forum for other threads.

[craiginaustin]

Originally Posted by 65 C20

http://67-72chevytrucks.com/vboard/s...t=ground+strap
Check out this thread.
Also go over to the electrical forum for other threads.

That's an excellent reference, thanks!