Alternator circuit in 91 740: no charging, no light even!

[elijahs]

This should be a simple one for someone that knows these cars...

91 740 has been sitting for a while in the humid tropics - usually
this means a myriad of electrical connection problems, and this has
been no different. I had to hotwire the fuel pump relay because it
was sticky; the car starts and runs but doesn't charge the battery.

In addition, the alternator light doesn't ever come on - before car
start (when all the other idiots lights show) or when it's running.

So, what should I check? there is a big wire and a little wire
running to the alternator, but they disappear into wiring bundles.
I'm assuming that one is for the light, and the other for the actual
charging. Is there a ground cable as well?

I'm good with a voltmeter but I'm not sure what to test exactly. I
could just throw another alternator in there, but I hate to spend $100
when it's just a corroded connection.

thanks in advance!
elijah

 

[James Sweet]

This should be a simple one for someone that knows these cars...

91 740 has been sitting for a while in the humid tropics - usually
this means a myriad of electrical connection problems, and this has
been no different. I had to hotwire the fuel pump relay because it
was sticky; the car starts and runs but doesn't charge the battery.

In addition, the alternator light doesn't ever come on - before car
start (when all the other idiots lights show) or when it's running.

So, what should I check? there is a big wire and a little wire
running to the alternator, but they disappear into wiring bundles.
I'm assuming that one is for the light, and the other for the actual
charging. Is there a ground cable as well?

I'm good with a voltmeter but I'm not sure what to test exactly. I
could just throw another alternator in there, but I hate to spend $100
when it's just a corroded connection.

thanks in advance!
elijah

These are internally regulated alternators so the system is pretty
simple. The big wire is the output of the alternator and connects
straight to the battery. The actual route in these cars is over to the
power terminal on the starter which is then connected straight to the
battery. The small wire is field excitation and receives power from the
accessory circuit whenever the ignition key is turned on. Ground is both
through the frame, and an additional wire between the alternator and the
engine block.

If the indicator never comes on, you probably have a burned out bulb or
a dirty/corroded socket.

 

[Baz]

These are internally regulated alternators so the system is pretty simple.
The big wire is the output of the alternator and connects straight to the
battery. The actual route in these cars is over to the power terminal on
the starter which is then connected straight to the battery. The small
wire is field excitation and receives power from the accessory circuit
whenever the ignition key is turned on. Ground is both through the frame,
and an additional wire between the alternator and the engine block.

If the indicator never comes on, you probably have a burned out bulb or a
dirty/corroded socket.

Also, in some cars, the indicator lamp is required to provide "exciter"
current to "start" the alternator. If the lamp is blown, the alternator
will not start charging.

Ragards
Barry

 

[ransley]

This should be a simple one for someone that knows these cars...

91 740 has been sitting for a while in the humid tropics - usually
this means a myriad of electrical connection problems, and this has
been no different.  I had to hotwire the fuel pump relay because it
was sticky; the car starts and runs but doesn't charge the battery.

In addition, the alternator light doesn't ever come on - before car
start (when all the other idiots lights show) or when it's running.

So, what should I check?  there is a big wire and a little wire
running to the alternator, but they disappear into wiring bundles.
I'm assuming that one is for the light, and the other for the actual
charging.  Is there a ground cable as well?

I'm good with a voltmeter but I'm not sure what to test exactly.  I
could just throw another alternator in there, but I hate to spend $100
when it's just a corroded connection.

thanks in advance!
   elijah

You should check the battery, leaving a battery go low ruins it from
sulfation fast, get it tested