DJS70cutlass

So I come out to the garage this morning and the radio in my ‘70 S is on. Upon closer inspection the GEN light is also on with the key out of the ignition. Car has been sitting since late Saturday night. Grabbed the keys and after cycling them a few times GEN light wouldn’t go off. Didn’t have time to start it as I was on the way to work. The GEN light did this a couple times right after I put the alternator in a couple years ago but had been fine since. Is the alternator now failing? They seem to be really hit or miss from parts stores!

Olds64

You can test your alternator at home with a multimeter. If you don't have one you can get it tested at the auto parts store.

The problem your describing sounds like an issue with the ignition cylinder, not an alternator problem.

DJS70cutlass

The alternator charges great when it’s running, right at 14.2. It’s the random GEN light illumination that’s weird. And only when the key is totally out of the lock cylinder. Basically sometime last night the GEN light and radio turned themselves on with no key input

oldcutlass

Do you have an internal or external regulator? It may be the regulator.

Sugar Bear

The next time the GEN light is on unplug the alternator. If the light goes out it is most likely a leaky diode in the alternator.

Is the radio aftermarket?
​​​​

DJS70cutlass

It’s external on the firewall, and is one of the solid state ones with no moving parts.

DJS70cutlass

Will do. Yes it’s aftermarket, plugged into the ign terminal at the fuse block. It shouldn’t even have power with the key out!

DJS70cutlass

Got a chance to look at the car, re-connected the battery and the gen light was back on. Pulled the alternator plug......light is off. Plug it back in.....light is back on. Will report back if I can find a reman that works. Where would one source the parts to rebuild it myself? I’m considering that option as well.

Charlie Jones

If you want to learn how to rebuild an alternator the 1970 Oldsmobile Chassis Service Manual will show you how.
1970 oldsmobile chassis service manual for sale | eBay
You will need a multimeter to test diodes and windings.
You may also need a small press to press in diodes and bearings.
Cleaning and bead blasting equipment may be needed if you want the unit to look like new.
Most of the parts you will need can be had from Rock Auto.
1970 OLDSMOBILE CUTLASS 5.7L 350cid V8 Electrical | RockAuto

Fun71

It’s been a while since I rebuilt mine but at that time the parts were available from the local auto parts store. NAPA is most likely to have them.

FYI, your symptom points to a leaky or shorted diode.

no1oldsfan

Just my two cents but I would follow your grounds. Sounds like a grounding problem to me.

stellar

x2

DJS70cutlass

I tried an alternator from Advance, charging voltage was very low with the lights on. Topped out around 13.1. The Gen light was off with the key out however. Voltage was marginal with the lights off. It also made so much noise the car sounded like it had a gear drive. Will be trying one from O’ Reilly and report back.....

Charlie Jones

Seems to me like an open phase in the stator.

Sugar Bear

Whenever possible I use a local independent rebuilder that does the rebuild personally on the item brought in, no exchange. Unfortunately they are getting hard to find. Consider looking for an automotive electrical shop in your area. It's usually a same day service.

stellar

The noise could be from having the wrong fan on the alt. I've seen ford alt fans on delco units and the ford fan is a bit bigger causing the fan blades to nick the mounting adjustment bracket. It doesn'r really matter now. Lets hope the next one works for you.

PatL

^^2

I had a grounding issue light up a unrelated bulb. Electricity tries to find the shortest path. Easy enough to check, pull the gauges and check to be sure the metal grounds are in place. the gauge mounting screws also need to be tight. Be sure your firewall ground strap is in place.

oldcutlass

The failure rate for some solid state VR's is pretty high and a lot are bad right out of the box. 13.2v at idle with all the lights on is not out of the ordinary.