Faulty Alternator Test

[duane m simmons]

Nate
The check of a GMC alternator is very simple since everything is
contained within the Alternator Housing. First pull the two wire
connector off the generator & verify that +12v dc is present at each
connector pin (cable side) with the Ignition On. Clean the contacts &
reinstall two pin connector. Now start the engine, any way that you can
& measure the DC Voltage present at the center terminal of the Battery
Isolator. There should be about 15 volts present if the alternator is
functional. Shut off the engine & the + 12 v DC should go to near zero
with a small test lite on the same center terminal. That's all there is
to it. If suspect faulty Alternator, take it to a Alternator Repair shop
(not a auto parts house) & have it tested & repaired. I find that the
rebuilt 80 amp alternator for a 75 Olds Toronado available at Pep Boys is
a reliable unit. If you go with a high output unit (ie 125 amps) then
the entire charging system wiring requires major work to prevent a fire
hazard condition. The existing wiring is marginal, at best, for the 80
amp OEM Configuration. 80 amps is a lot of power....why would you want
more ????

Duane of Orange CA

 

[dave1]

>The check of a GMC alternator is very simple ...... If suspect faulty=20
>Alternator, take it to a Alternator Repair shop ...

You may also want to check to see if its the alternator or the regulator.=20
There is a D shaped hole on the rear of the alternator. Inside of this hole=
=20
is the terminal the connects the output of the field winding to the=20
regulator. With the engine running (and being very careful) use a small=20
screwdriver to short this terminal to the case of the alternator. If it=20
starts working, chances are the regulator (or one of the tab connections)=20
is bad. Replacement regulators are only about $10.

>... 80 amps is a lot of power....why would you want
>more ????

I have found that, with the dash A/C on high, the headlights on, the refer=
=20
running and a couple of interior lights turned on, there is not enough=20
power left to charge the batteries. With everything turned on and the=20
engine at idle the batteries actually discharge.

Its usually not a problem if the Onan is running. I rarely run the dash air=
=20
on high and my Todd-75 charger supplements the alternator. But this spring=
=20
my Onan died in Florida and the 80A alternator was not large enough to run=
=20
everything. So upgrading the charging system just got a bit higher on my=20
list of things to do.

Dave
Ann Arbor, MI.
73 Sequoia (26'/455/HEI/TBI/160=B0/3.42)

 

[alan bredbury]

>Now start the engine, any way that you can & measure the
DC >Voltage present at the center terminal of the Battery
>Isolator. There should be about 15 volts present if the
>alternator is functional.

13.2 to 14.7 V actually

> 80 amps is a lot of power....why would you want more ????

80 amps is not a continuous rating.

=====
Alan Bredbury
Clinton, Ct.
1974 GMC 26' Classic Motorhome
http://www.finesttool.com/index.html/special.htm

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