What is the procedure for troubleshooting a charging problem on an externally regulated system? (1970 GM, not Oldsmobile, but close ) I hadn't noticed any problems until recently, when I returned from a cruise, and the car wouldn't start again. Upon getting it started, it seems to jump up to around 14 volts, and then stabilizes to around 13. When I turn on the lights, it drops briefly, but then comes back up. But after driving a little bit, sitting at low idle, I turned on the high beams and the heater blower to high, and voltage drops below 12, and then continues to fall to under 8. I turned off the lights and blower, tapped the gas, and it goes back up to ~13-14. Let idle return, turn on the accessories again, and it drops to ~8 again.

I ask because it's a little difficult to have the alternator tested, as they don't test externally-regulated ones at the parts store. The alternator appears to be working though, it's capable of decent voltage. It seems like the cheapo regulator might be failing. Any thoughts? Thanks.

 

Sounds like your regulator is on the fritz, however it could also be the battery has a bad cell and loading the circuit.

 

Why not follow the troubleshooting procedure in the Chassis Service Manual?

- Eric

 

Both Erics have given good advise. My 1st guess from info given would be a loose belt or a bad stator or diodes in the alternator. I find it hard to believe a parts store can't test it, but I guess that is a sign of the times.

 

The manual calls for a "VAT-20" tester to test the system, and I'm not sure what this is, how it works, or where to get one. It looks like an integrated dual multi-meter, but I'm under the impression that it's an old, proprietary GM tool, which might be hard to find. Also, because I have a modern solid-state regulator in place of the original mechanical one, there's a whole aspect of testing and adjusting that I'm not able to do according to the manual. (I put the modern one in originally when I was having a different problem, because the original mechanical one required testing and adjustment with the VAT-20, whereas the solid-state one is supposed to be self-regulating. So from this aspect, I really am looking for people's experiences in dealing with an old system that incorporates a new modern component, something that wouldn't be covered in the factory manual.) Finally, since this is a pretty specific scenario, I thought maybe someone would have seen it before and it might ring a bell. But I will try to go through the manual and find tests that I can do without the VAT-20, or scenarios that are similar. As for testing of the alternator, the places I've been to are not even familiar (as of five years ago) with externally-regulated alternators. They're used to just plugging internally-regulated ones in to their tester. Thanks guys.

 

I'm sorry, but in my 1970 CSM, the only special tools that are mentioned in the Charging System section are the BT-7002-3 Belt Tension Gauge (can be replaced by "your index finger") and the 421 Battery Tester / Charger.

In the Charging Circuit section (pp 12-12 to 12-30), the manual contains complete schematics, as well as cut-away and functional diagrams, and several troubleshooting decision trees.
It tells you (fig. 12-22, p 12-14) how to "full field" the alternator to check maximum output and discusses voltage modifications for different types of batteries (the adjustment procedure would be different, but many electronic regulators are also adjustable).

Honestly, my favorite troubleshooting procedure for external regulators is:
1. Reach into old toolbox under workbench full of old regulators
2. Withdraw one
3. Replace suspect regulator with regulator from toolbox
4. See what happens.

Takes less than five minutes, but someone once told me that everybody doesn't have an old toolbox full of regulators under the workbench (which is hard for me to fathom), so I didn't suggest that.

- Eric

 

I don't have an old toolbox full of old regulators... However I know a guy who knows a guy.

A Sun Vat-20 from my recollection was for generators. Basically it had a voltmeter and an amp meter and could test load and voltages.

 

As I mentioned, this is not Olds, but 1970 GM, so the procedure in my manual is different, and I guess this is where the difference in brands becomes apparent. My manual wants both the VAT-20, and also some specific temperature testing tool (J-8529) for adjusting the regulator. However, I do have ONE old regulator in my toolbox, and that was my thought, to throw it in and see what happens, since it WAS adjusted by the factory once, and may actually still be good. I'll give it a try and see what happens. Thanks guys.

 

Oh. I've got one like that made by Snap-On. Reads up to something like 600A.

Got it at a flea market for $20. Works good.

- Eric

 

Yes, but that is for adjusting a mechanical regulator, which you don't need to do.

You just want to troubleshoot the system to see which part is bad.

And all of the GM systems (and wire colors) are the same, with the exception of different sizes and capacities for different models through the range.

- Eric

 

Looked like this:



Instruction manual:
http://home.earthlink.net/~rchaskell...un/vat20um.pdf

 

I am in awe.

An EarthLink link that works.

That may well be the last one.



Mine is a newer version of the same tool, a Snap-On MT552, looks like this:




Okay, hijack over...

- Eric

 

Ok back to our scheduled program.

Test your voltage, should be 14.2 - 14.7 volts at 1500 rpm and engine running. with the engine off battery voltage should be 12.6-13volts fully charged.

 

It's within spec, except for that one scenario, where I'm idling and I put a big load on it, when it drops to 8 volts. Still have to try the other regulator, will also check grounds at that time.

 

Also, if you can put an ammeter on it, try to full-field it and see if its output drops off.

An occasional drop that low, as Stellar said, can be a rectifier diode as well (easy to replace inside the alternator).

- Eric

 

It could be as simple as corroded posts and cable ends also.