Charging System Problem

STYLEMANMD

MEMBER
1992 DODGE CARAVAN
6 CYL
FWD
AUTOMATIC
330,000 MILES

Greetings
I have a84 Dodge Ram 150 with a 318 engine in it. The trouble is the charging system. It's got a 78 Amp alternator that's putting out 13.5 Volts at the alternator post before it even gets to the voltage regulator. I get the same reading at the battery after the voltage regulator. The alternator guage shows a slight discharge at all times and even moreso when the lights are turned on. In a matter of a week, the battery will run down and eventually leave me on the road with no power.
The battery was replaced with a brand new one not a month ago along with two new voltage regulators (thinking it might be a faulty one) but to no avail.
If the alternator is putting out 13.5 volts at the terminal and the same at the battery, why would the alternator guage show a slight discharge. If the battery runs low, wouldn't the guage show that it's charging instead? And why does the battery (a new one) go dead over a period of a week? I'm confused unless there's a short somewhere in the wiring but if that were the case, then I'd see a spark, even a small spark if something was draining it, correct?
I'd sure appreciate some expert advice on this one because I'm at my wits end. LL
Sincerely
Ron

3 Replies

FIXITMR

MECHANIC
9,990 POSTS

13.5 sounds low I would try another alternator?

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Sunday, January 23rd, 2011 AT 4:34 AM

CARADIODOC

MECHANIC
33,916 POSTS

Sounds like there's a little confusion on how this system works. The voltage regulator has nothing to do with the output wire. That wire goes straight back to the battery so both ends should have the same voltage. 13.5 volts is a little low and will result in the symptoms you described. Normal is between 13.75 and 14.75 volts.

First, be sure the regulator is bolted solidly to the firewall. That is one terminal of the regulator and is where field current flows through.

Next, feel if the alternator is getting unusually hot. That's one sign of a shorted diode.

Measure the voltages on the two small wires on the back of the alternator while the engine is running. The blue one should have full battery voltage. The green one will be lower. The lower it is, the bigger the magnetic field it is trying to produce and the more output it is trying to produce. At wide-open, that voltage won't go much below 4 volts. Monitor battery voltage, then ground that green wire either at the alternator or unplug the regulator and ground the green wire there. If voltage doesn't go up a bunch, suspect a shorted diode. A load test will verify that. You will lose two thirds of the rated output when one diode is shorted so 25 to 30 amps is about the best you'll get.

Caradiodoc