• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Charging woes

Cwazywazy

Single jingle
Joined
Aug 20, 2016
Location
Granby CT
1988 245 turbo with electric fan. Dave Barton wiring harness, about a year old. Basically, I've got a brand new battery and a Denso 100a from Philski. With the old 80a Bosch it'd go up to ~13.5v after starting and then immediately drop to ~12v and go down from there if I used headlights or anything. Same voltage at the back of the alt and the battery terminals. Replacing the brushes/regulator made zero difference. No resistance to ground on the alternator body. Belt tension is fine.

Now with the Denso it's almost exactly the same, except I was getting a steady 13.5+ when it was idling with nothing on, but driving with AC it got down to about 11v. I'm pretty stumped at this point, and a bit mad that I replaced what was probably a good alternator..
 
To check the alternator ground wire. Put the clip end of a test light to the positive B+ on the alternator and touch the probe of the test light to the alternator case. A resistance check is good but be sure some current will pass through the circuit as well. A light bulb test light not a led one.

13.5v is too low without accessories on. Is this a rebuilt or a good used Denso? I've experienced bad regulators giving low output on the Denso alternators. Plus with the newer batteries a slightly higher voltage set point is good. For example a 14.4 set point or there is even a 14.6v one available on ebay.
 
Maybe check for a circuit that is drawing a bit too much current like a window switch stuck or something like that? Stereo amp on? Could be a bad regulator?
 
Maybe check for a circuit that is drawing a bit too much current like a window switch stuck or something like that? Stereo amp on? Could be a bad regulator?

I kinda doubt that I'd have two bad alternators behaving the same way, especially since I replaced the regulator on the first one with no change.
 
.3 ohms between the battery and alternator positive terminals, exciter wire is at battery voltage with key on, battery light works as normal, and 0 ohms between alternator body and negative terminal. I replaced the ground wire cause the old one was looking kinda crappy, but nothing changed.
 
<iframe width="560" height="315" src="https://www.youtube.com/embed/LlrSNeYFOe4" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
 
IF belts do not slip....
IF you have supplied an additional path to ground for the alternator.... Say from alternator case to the battery via jumper cable AND you have supplied an additional path to B+ at the battery via another nice big lead.... Perhaps the other wire in that set of jumper cables.... And you still get lower then expected voltage with accessories on...

What does that leave for a fault OP?
 
Okay, so I took my extra long crowbar and tightened the crap out of the belts, got a definite improvement. 12.6v with everything on, rises to 13 if I rev it. 13.5v everything off. (Battery was under 12v before starting.) Marked my crank pulley with paint and it doesn't appear to be slipping. A jumper wire from the battery to the alt made no difference.

I think I replaced the belts when I did my engine rebuild, but I'm not sure..
 
Brand new group 65 battery. Fully charged it and threw on new Dayco belts since the old ones were looking a bit glazed. Still can't get 14v out of it with no load.
 
How is the battery in the multi meter? Have you tried a different meter with a known good battery just to be sure it's not an inaccurate reading?

I'm using the volt gauge and a nice multimeter, they both give the same readings.

I've got a solid 13v with the AC and lights on, more like 13.6v with no load, which is good enough for now. I may try replacing the regulator.
 
Last edited:
Those Densos always go bad for some reason. I stick with the Bosch 100 amp alternators now and haven't looked back.
 
Back
Top