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1977 Trans Am - Fan Amp Draw?

Kalmek

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Hello there, I'm new here.

We (together with my father) have 77 Trans Am 6,6.

Everything works great, but now, in the fall, when I need to use the heater fan, I notice problems with charging, so I have a few questions.

Alternator charges alright until I switch on the heater fan (climate fan, inside fan, or what it is called).

It's not connected correctly, so the fan works only in the highest mode.

At that moment, the alternator is not giving enough amps, and the battery is not being charged.

It draws power from the battery so much that it won't start.

I have a 72ah battery installed.

I measured the fan amp draw with my cheapo multimeter, which showed 12amps while the fan ran at 3/4 speed and cables from the multimeter started to be hot, so the fan draw is, i suppose, about 15 amps.

I measured the draw lights, which were about 12 amps. My alternator has a written output of about 37 amps.

Is my alternator weak, so it's not powerful enough to charge when everything is on? Or is something wrong with the fan that it draws so much amps?

Or is the problem something completely different?

Thanks for the help.
 

transam1979v8

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Hi.

I have a 1979 Trans Am and am just rewiring the Heater wiring harness. Air Con was removed from my vehicle so I have Heater Fan/Blower only and it is functional. The following info may assist you regarding the wiring and function. The colours shown are those on my car which may differ slightly on other models.

When you set the blower to low, a voltage connects via the Brown Wire to the resistor pack ( this is usually mounted on the firewall under bonnet on the passenger side ). It passes through all 3 resistors providing low speed. The Black and White wire passes through only 2 resistors, providing a higher blower speed, and the Blue wire passes through only one resistor, providing even more speed ( BUT NOT FULL SPEED ). Only one of those 3 wires should have live voltage at any time. This voltage from the resistor pack runs to the normally closed terminal of the Blower Relay ( also mounted under bonnet ) where it provides power to the Blower Motor.

The big difference occurs when you put the Fan Control onto High. When switched to "high", the Grey and Black ( wire is sometimes Orange ) becomes live and operates the Blower Relay , killing the connection to the resistor pack, and connecting a Full System Voltage straight from the Alternator to the Blower. This red wire is usually connected directly to a terminal on the back of the alternator.

Regarding output from battery / alternator obviously check fan belt tension and then with the engine running, voltage from alternator usually about 14.2 Volts average. This would be my first starting point. After that I would fully charge battery and then check voltage after about 24 hours - should stay at about 12.6 Volts if good.

I hope this helps in some way.
Nigel
 
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