A/C Blows Fuse On High Fan Speed - Team Camaro Tech
Electrical & Wiring Troubleshooting electrical.

 
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post #1 of 11 (permalink) Old Aug 12th, 04, 06:15 PM Thread Starter
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It'll run for a minute or so and then the fuse blows. Volt meter (pulling from the fuse panel) shows a huge draw when it's on high - sub eight volts, 13V-14V when on speeds 1-3. Problem may have been there forever; I just realized today that my fan switch has been sticking at med-high. The thought crossed my mind that "high" speed wasn't moving much air, but I didn't check it out until now.

I suspect a weak ground lead or bad/dirty contact from the blower motor to ??firewall??. I'm operating under the assumption that the motor grounds from any point of the case to the firewall. If this is the case, I may be able to access one of the mounting screws and run an auxillary ground - at least for testing.

Any ideas or troubleshooting suggestions?
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post #2 of 11 (permalink) Old Aug 13th, 04, 02:22 AM
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Fuse opening up is a cause of too much current. Either the blower motor is drawing too much current, motor case gets very warm, maybe too hot to the touch, and/or the resistor block giving the motor speeds has bit the dust.

High speed selection gives a full 12 volts to the blower motor. The resistor block determines the lower speeds, thus, giving a lower voltage to operate the motor.

Also, if A/C is selected, does this fuse also handle the current for the A/C clutch coil, or is the coil on a separate fuse?

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post #3 of 11 (permalink) Old Aug 13th, 04, 02:58 AM Thread Starter
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I'm getting all four speeds and the fuse doesn't blow on the first three, so I'm thinking that the resistor is ok. If the motor is suspect, as evinced by the case getting very hot (high speed), would this show up on med-high as a very hot case, just not enough current on med-high to blow the fuse?

I'll check to see if the AC clutch is on the same circuit when I get in this afternoon.
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post #4 of 11 (permalink) Old Aug 13th, 04, 04:27 AM
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No, hot case would not show up on lower speeds, just on high speed, because its getting the full 12 volts.

The lower speeds, the resistor block dissipates the heat, thus less current going to the motor.

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post #5 of 11 (permalink) Old Aug 13th, 04, 06:22 AM
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How big is your fuse? The 69 A/C fan high speed setup draws power right off a wire attached to the horn relay with a 25 amp fuse holder in the wiring gutter above the distributor. It takes all 25 amps to keep from blowing, a 20 will blow after a few minutes. This fuse feeds the high speed relay on the top of the suitcase. The other speeds go thru a fuse in the fuse block and the fan speed switch on the dash. The fan is grounded at the same point on the firewall that the ground wire from the high speed relay is grounded. The ground is located right below and to the left of the big wire grommet that brings all the A/C wiring thru the firewall, above the passenger side head.

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post #6 of 11 (permalink) Old Aug 13th, 04, 01:38 PM Thread Starter
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Everett - I believe the A/C clutch is on a separate circuit.

Mark - You're right, a 20A didn't hold up, so I replaced it with a 30A (I know...). I forgot about the inline fuse as it's been holding up ok. What's interesting is the fuse that is blowing is in the underdash fuse panel (Heater A/C).

I see fender removal in my future.
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post #7 of 11 (permalink) Old Aug 13th, 04, 02:53 PM
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Don't oversize the fuse, an underdash wiring harness costs about 400 bucks to replace after it catches on fire, and your car costs what, 15 to 20 grand, your asking for a fire. Fix whatever the problem is. Get a resetable glass fuse replacement circuit breaker. They look like a little aluminum box, with two 90 degree fuse end caps on them. CAP, NAPA, Autozone sell them for about 2 bucks apeice. They snap right into the fuse panel and come in all the normal fuse ratings. When they blow, all you have to do is pull one leg out of the panel, then stick it right back in. Never buy another fuse again. I've got one on my dash light circuit, because I've got an intermittant fault in my headlight switch that was blowing the fuse every once and awhile when I turned the lights on or off. Works great.


The clutch is fed from the underdash circuit whenever the left slide switch switch is in position other than vent or heat. There is a switch on top of the inner heater box that is operated by the left most lever that energizes the clutch thru the low temperature switch on the fender side of the suitcase. Shouldn't be an issue becasue it's also energized in the 3 lower speeds and the fuse doesn't blow then.

If it's your underdash fuse, and it's only blowing when the fan is run on High speed then most likely it's your relay on top of the suitcase that is blowing the fuse. Check all the terminals and make sure there are no big dents in the relay that could be shorting out the internals. Could also be a short in the fan switch itself, as those things start get dirty and corroded with time, I had to clean mine up inside to get it to work a couple of years ago.

The only thing powered thru the heater circuit when the switch is in high is the relay on top of the suitcase (and the clutch). The relay in turn feeds the fan motor thru the 25 amp fuse in the wiring gutter, so if it was the motor then the 25 amp fuse would blow. Check your relay and underdash switch (real pain to get at)

See this thread on finding the 69 A/C relay.

https://www.camaros.net/forum/ultimat...;f=10;t=003180

[ 08-13-2004, 05:25 PM: Message edited by: Mark C ]

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post #8 of 11 (permalink) Old Aug 13th, 04, 04:31 PM Thread Starter
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Thanks for the help guys! Good tip on the resettable fuse.

The relay was bad when I bought the car and has since been replaced. The replacement matches click's picture.

A previous owner didn't seem to think that ground straps were important and removed them. Replacing the ground straps solved several problems I was seeing.

In any event, if it's the switch, I should be able to unplug the harness from the fan switch and jump the connector to shoot straight 12V to the fan motor?

If the fuse blows, I'll take a closer look at the relay. Should be able to conduct the same troubleshooting by bypassing the relay.
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post #9 of 11 (permalink) Old Aug 13th, 04, 06:50 PM
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No. If you jump the high speed contacts in the switch all that is suppossed to happen is the relay should be picked up. Power to the fan comes from the 25 amp fuse in the inline holder in the wiring gutter. The relay picking up closes the contacts in the relay and power is sent to the fan motor. It doesn't come thru the 20 (now 30) amp fuse in the fuse panel. That's why I don't think it's your fan motor, if it was the 25 amp fuse would blow, not the 20 amp in the fuse panel.

Power goes from the fuse panel to the fan speed switch, from there it goes to one of three terminals on the blower motor resistor bank depending on which speed the switch is in. Power then comes out the single output terminal on the heater bank and goes out thru the firewall where it lands on the output terminal of the high speed relay. This is just a common terminal point so that you only need one wire to connect everything to the fan motor. The relay contact is normally open so nothing will backfeed into the rest of the circuitry. From there power runs to the motor and then back to ground. None of that circuit is used when you run the fan in high speed. Power runs directly from the switch to the relay coil, which closes the relay contacts which connects the 25 amp fused feed from the horn relay to the fan motor.

To eliminate the motor completely from your trouble shooting, remove the 25 amp fuse in the engine compartment and place the switch in high speed. The motor won't get power then. If it still blows the fuse then it has to be the switch, the relay or the wiring in between. If it does blow, replace the fuse and then do it again with the relay disconnected. If it blows then you have a problem with the switch or wiring.

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post #10 of 11 (permalink) Old Aug 29th, 04, 03:53 PM Thread Starter
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Believe this one is solved. I took the heater control assembly out of the dash today (clearly, these cars were not meant to be worked on by full-sized humans ). I bypassed the fan speed switch by jumping the harness tan to orange (pin #1 to #5 - high speed). The fan ran on high speed for several minutes without tripping the 25Amp breaker, which was different, so I'm looking at a suspect fan speed switch.

I pulled the switch out of the control assembly and took it apart. Inside the switch, there is a plastic carrier with a spring on top. On top of the spring is a copper conductor that completes the appropriate circuit, dependent upon switch position. The spring keeps tension on the conductor.

Close inspection of the copper conductor revealed that there are tabs on each end that should slide into grooves on each side of the plastic carrier. One of the tabs was bent up and would make contact with the switch case on high speed if you pressed it just right. This voltage to ground contact was causing the fuse to blow.

When I first noticed this happening, it would run for a while but eventually blow the fuse. I presume this was occurring as vibrations from driving created contact between the conductor and switch case. I bent the tabs down, cleaned up all contacts with a Dremel, loaded it up with dielectric grease, and reassembled. Everything is working great so far.

Thanks for all the tips!

[ 09-04-2004, 06:25 AM: Message edited by: Randy S ]
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post #11 of 11 (permalink) Old Aug 30th, 04, 02:32 PM
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Right on Randy. Good job figuring it out. Thanks for the report.

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