Possible charging issue?

[Mr_Mudney]

I have a 1979 Datsun 620 that I’ve recently got back on the road. During the rebuild process I’ve had it running in the driveway idling for a long time, and i have never had an issue with overheating. But now that I’ve taken it around town I’ve noticed the temp gauge will rise to just below the hot level, but as soon as I stop and get back to an idle it drops back to the halfway point. I’ve also noticed the fuel gauge will do the same. If I fill it up and get on the road it sits at the “full” level, but when I get on the road it will peg the gauge way past full, until I’m at an idle when it will drop down along with the temp. If the fuel is at a quarter tank, it’ll rise the gauge to the 3/4 mark and sit there stead until I reach idle again and then it will drop back down. My charge light has come on, does this sound like a bad regulator and the over voltage is giving the gauges a false reading?

[datzenmike]

Sounds like an over charge. I wouldn't drive it that way as it will overheat and destroy the battery. Get a volt meter and check across the battery posts and also across the cables that are clamped onto the posts.  (they should read the same) Look for +- 14.5 volts when revved up. If so try cleaning the battery posts and the cable ends and try again.

 

The '78 and up Datsun/Nissan had internal voltage regulators inside the alternator so if something is wrong the whole thing has to be replaced. I will say that the replacement alternator will almost for sure be an extremely poor quality. So bad that there is a chance that it is 'bad in the box'. In fact the one you have on there now may be one of these $60 pieces of crap. If you do have an original Hitachi alternator you might consider not turning it in as a core and keep and have it professionally rebuilt. You might talk to the shop that does this and have the power output increased. 

[Mr_Mudney]

Thanks for the reply mike, the regulator for the alternator is located on the brushes of the alternator, can this be replaced easily or is it just better to take it and get a shop to take care of it? 

[datzenmike]

The Diode pack in unique to the 620 alternator but the regulator is common to anything between '78-'80 620, 720, B-210, 280z, 280zx, S10, A10, 310 and 810.

 

Disassemble and solder it in?

[Mr_Mudney]

I’m taking your advice and bringing my stock alternator in to be repaired. You mentioned increasing the output?

[datzenmike]

Some shops can provide a higher output. This may be higher capacity diodes or re-winding or both. The stock 620 alternators were 35 amp and an optional 38 amp and that was it. The car alternators are 50/60 amp but are larger diameter and they do not fit well in the tight space between frame and block just above the idler arm and so close to the lower radiator hose..