Author Topic: 79 trans am voltage drop only with fan on?  (Read 634 times)

chrislynn5

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79 trans am voltage drop only with fan on?
« on: October 14, 2017, 06:42:12 PM »
I have a 79 trans am, 403 olds engine, stock.
I decided to replace the stock 63 AMP alternator with a 100 AMP version since I noticed my voltage around 11 on the gauge while waiting at a light.

I thought that fixed my issue. But the voltage car gauge stays at 13V without moving for anything else except the fan.
I turn on the AC and it's 13V, I move up the fan switch (which by the way is a newly rebuilt AC control unit) and the voltage gauge goes to 11V????

I will add that my high fan position does not work, nor has it since I got the car 7 months ago. I've read posts about a wire needed off the alternator since the other settings go through the resistor. But I don't think that is my issue.

Ideas?

Brashanic_IM

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Re: 79 trans am voltage drop only with fan on?
« Reply #1 on: October 14, 2017, 08:58:50 PM »
You are picking up a ton of resistance somewhere

High speed fan  you should have full voltage to the fan.

If you are not getting it there is a loose or burnt wire somewhere

I just went through this on a 93 dodge.

The fuse block was melted and caused bad connection creating heat and resistance

I bypassed the fuse block with the right size inline fuse and wire to handle the load.

 
Fan speed increased and draw on system decreased.


I don't know where to start in the ta but I would check voltage at the fan first. If it's less than system voltage start tracking it with a volt meter until you find the voltage drop.
« Last Edit: October 14, 2017, 09:00:25 PM by Brashanic_IM »

via3d

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Re: 79 trans am voltage drop only with fan on?
« Reply #2 on: October 14, 2017, 09:50:57 PM »
Chrislynn5,  I think I may know the issue.

I have been troubleshooting all afternoon my blower HI not working. I have solved it. Had to trace out the wiring.
It was not the blower motor, blower resistor, or bloweritself.  It was the wiring that is fed from the alternator.

There is a red wire that runs from the alternator that feeds the ac/blower circuit in some manner.  I was getting a serious voltage drop to the blower when my fan switch was on HI.
..... instead of 12v it was dropping to 3.5-4V when HI was on and the blower would cease to work at all. 

I traced it to the connector.... the single red wire from alternator to a connector approx. behind the compressor to another red single  wire leading to wiring bundle....

Since you are
A. experiencing a noticeable voltage drop and
B. your HI setting stops the blower from working....
exact same issues I was having.... I would get draw down on guage but paid little attention to 
....I believe you will find the bad wiring, loose or corroded ground, or my exact cause - short across these connectors due to age and corrosion...


I tell you what, I have a new found appreciation for di-electric grease and paying attention to wiring & contact corrosion...


~ J

chrislynn5

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Re: 79 trans am voltage drop only with fan on?
« Reply #3 on: October 15, 2017, 07:11:32 PM »
thanks for the advice. I started reviewing the fans electrical, and started with what I thought had already been replaced (the fan resistor) but I was wrong.
Tomorrow I should have the replacement resistor and the connector (as it was just as rusted as the resistor).
I'm hoping that fixes my "load" issue. As the voltage delivered to the resistor was correct as 13.8V.

(I'd add a pic but I guess pics have to be external hosted as simple adding to the post doesn't exist?)




Brashanic_IM

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Re: 79 trans am voltage drop only with fan on?
« Reply #4 on: October 15, 2017, 07:26:55 PM »
thanks for the advice. I started reviewing the fans electrical, and started with what I thought had already been replaced (the fan resistor) but I was wrong.
Tomorrow I should have the replacement resistor and the connector (as it was just as rusted as the resistor).
I'm hoping that fixes my "load" issue. As the voltage delivered to the resistor was correct as 13.8V.

(I'd add a pic but I guess pics have to be external hosted as simple adding to the post doesn't exist?)

Hi speed typically does not run through the resistor.

The resistor primary function is to slow down the fan.

Very interesting thread. Keep us updated. Good luck

Re: 79 trans am voltage drop only with fan on?
« Reply #4 on: October 15, 2017, 07:26:55 PM »

chrislynn5

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Re: 79 trans am voltage drop only with fan on?
« Reply #5 on: October 16, 2017, 04:49:45 PM »
Here's one for the books.
I picked up the resistor and new connector and after replacing both I have the correct voltage on all fan speeds and I now have my HI fan setting. Go figure. I'd add pics but this site doesn't support image uploads it appears.

Thanx!

chrislynn5

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Re: 79 trans am voltage drop only with fan on?
« Reply #6 on: October 16, 2017, 04:52:37 PM »
now, one thing I've never had since I purchased the car 7 months ago is the MAX A/C setting. Normal blows cold. But MAX nothing. I've replaced the A/C control unit, still nothing. Maybe this is common?

firebirdparts

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Re: 79 trans am voltage drop only with fan on?
« Reply #7 on: October 16, 2017, 06:58:15 PM »
Well, there's been a discussion on it up in the sticky thread for the last 2 or 3 days.

Earlier models had a separate set of contacts to turn the fan on for MAX A/C, and that switch got carried over for a year or two.  On a 79 this circuit does not have any special function.  It's just jumpered from MAX A/C to NORM A/C.  However, it still has to work.  You'll have to find a problem in that circuit.  On a 79 the wiring we're talking about is only a half inch long.

On the older cars that had MAX A/C wired so that it always gives you MAX blower at the same time, then the wiring for that reaches over to the fan speed switch.  Still only 3 or 4 inches instead of a half inch.
I want to be like Paul Kenny when I grow up
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1978 Skybird 350
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1979 Redbird 301 4-speed
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Re: 79 trans am voltage drop only with fan on?
« Reply #7 on: October 16, 2017, 06:58:15 PM »
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