Running light/head light issue causing car to lose all power.

[pigboy's71]

The Problem, i can drive the car all day long with no issues but when I put the running lights or head lights on the car will drive fine for awhile and then suddenly lose all power and dies.No power to anything. But if i wait a minute or two I can fire right back up and drive away.Its happened a number of times the last being a couple days ago when were out for a cruise. It was dusk so i had the running lights on and we drove for about a half hour and pulled into a drive through were it died. After a couple minutes of sitting it fired right back up. Im at a loss to what it might be. If it was battrey, alternator, shorts wouldnt the car or the head lights not be running or working? Heres are some details
year 1971 with an AAW classic update Kit.Dakota digita dash . New high output (105a-150a) alternator. Battrey is a group 78 with 985 cranking amps
My readings i took off the battery 12.56 off 14.66 running in park 14.60 in drive.
running lights 14.59 and 14.52 in drive and head lights 14.50 and 14.45 in drive.
Do these numbers look reasonable?
May be the battery cant handle a load put on it ?
The idle is at 600 rpm in park is this to low?
Thanks
Jeff

 

[COPO]

If you still have the horn relay and voltage regulator on the drivers front rad support near the washer bottle, check those for loose or worn out wires.

 

[larrylarry]

Like Copo said, you have a bad connection somewhere. Kinda had the same problem once, if I turned on the bright lights the car would die along the sudden then turn back on quickly. Checked the dimmer switch and all the usuals. I have a one wire alternator and it was that female connector plugged into the alternator. I’ve found that when trying to figure out a electrical problem check the obvious first.

 

[pigboy's71]

I was thinking along the lines of a bad connection and will have to recheck but Im confused why I can drive the car with no lights and have no problem but when I put the running lights or headlights on I can still drive for half hour to 45 minutes as it doesnt die immediately.Then suddenly die even if idling in the driveway like may be the alternator is not keeping up to keep the car running.If it is a bad connection wouldnt it show as soon as I turned on the lights or is it different than a short?
Sorry If this sounds like dumb questions,pretty new to this electrical stuff.

 

[Knucklehead]

Agree with the above recommendations. First, it is possible that the connection with the lights being on is simply random/coincidental. Or maybe having the lights on draws just enough extra amps that it heats up the loose connection, wherever that is.

 

[Bills69]

load test the battery

 

[mallard]

The fact that it restarts shortly after dying makes me think it’s not the battery. However, it’s definitely worth at least giving it a load test anyway.

Has it required a boost after immediately stopping?

I think COPOs most likely right in that it’s a connection issue.

 

[Jim Streib]

Most GM cars only had one terminal spot in the firewall bulkhead connector that allowed power to get into the car from the battery and the alternator. If this connection is poor it might work at a low amperage draw but then when the headlights are kicked on the amperage gets too much for the connection to handle and then power is either reduced badly or stopped. Maybe too this connection initially is fine but then after use, heat is created from a poor connection and things go wrong.
Most ignition systems will work down to around 8V and from what you have with tests at the battery under the hood, maybe do some tests under the dash. Be aware too that if you test for power and ground at the battery posts and then check for power and ground at the fuse block and having the meter grounded to some metal under the dash, you will never know if you have an issue with just the power or just the ground and there might be issues with both sides.

Jim

 

[pigboy's71]

Thanks all for the replys,Bad connection is what Im thinking to as the charging system was working keeping the car running fine without the lights on and even when they are on the volts dont drop that much but wnated to confirm. Never had to have it boosted so its not like the battery is drained but will load test it to rule it out.

 

[72BIGBLOCK]

What kind of headlights do you have ? Are they stock or aftermarket high intensity lights ?