Heater switch advice

[Streetglide2015]

So during my lengthy process of elimination as to why fuses continue to blow on my '64 Chevy Thriftair heater, I have done the following, with no success:
- replaced blower motor with new unit.
- replaced resistor.

But now it seems that with both of those items replaced, the fuse only seems to blow when fan is turned to "High." It runs as long as I want on the first two lower fan speed settings, but when switched to high, the fuse will blow after a few seconds.
I'm now thinking that the switch itself could be causing the problem... It's also difficult to pull out and push in (for heat control) and is probably original.

Thoughts, advice, and comments are welcomed!
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[Isaiah92260]

Originally Posted by Streetglide2015

So during my lengthy process of elimination as to why fuses continue to blow on my '64 Chevy Thriftair heater, I have done the following, with no success:
- replaced blower motor with new unit.
- replaced resistor.

But now it seems that with both of those items replaced, the fuse only seems to blow when fan is turned to "High." It runs as long as I want on the first two lower fan speed settings, but when switched to high, the fuse will blow after a few seconds.
I'm now thinking that the switch itself could be causing the problem... It's also difficult to pull out and push in (for heat control) and is probably original.

Thoughts, advice, and comments are welcomed!
Streetglide2015 is online now Report Post

Try checking the wire harness. You could have a short somewhere

[Streetglide2015]

Thanks!

[nxtruck]

I just went through this with my '65. Make sure that the 12v hot wire (the wire that is by itself on the back of the switch) isn't rubbing on the cable when the knob is turned. Mine connector was a little loose and when you would turn the knob to turn the blower on, the connector would move just enough to barely touch the cable and would immediately blow the fuse. I tweaked that spade on the switch enough to tighten it a little and made sure the connector was firmly attached to the spade. That, and wrapped the end of the connector with electrical tape.

As far as the knob being hard to turn, you could try shooting a little WD40 in the cable and working it in there by pushing and pulling the cable in and out. I had to do this to mine, also. It did help it some. My original switch does still work, though.

[Streetglide2015]

sensible advice! I'll crawl under the dash and have a look.
Thanks!

[damnyankee36]

Of the two components you changed, the motor is what I would suspect that partially fixed your problem. With the old motor, did the fuse blow immediately or after a period of motor operation? If immediately, I would suspect a direct short to ground. If the latter, I would say the motor was drawing too many amps.

Now, with the new motor, I would still say it's pulling too many amps for the fuse to handle. Since it's not blowing immediately, I wouldn't think it's a direct short to ground. What size fuse are you using? I believe a 25A is the typical rating.

If you have a multimeter, try getting a current reading across the fuse terminals with the fuse out. See what the current is for all speeds.

[Streetglide2015]

Originally Posted by damnyankee36

Of the two components you changed, the motor is what I would suspect that partially fixed your problem. With the old motor, did the fuse blow immediately or after a period of motor operation? If immediately, I would suspect a direct short to ground. If the latter, I would say the motor was drawing too many amps.

Now, with the new motor, I would still say it's pulling too many amps for the fuse to handle. Since it's not blowing immediately, I wouldn't think it's a direct short to ground. What size fuse are you using? I believe a 25A is the typical rating.

If you have a multimeter, try getting a current reading across the fuse terminals with the fuse out. See what the current is for all speeds.

The fuse blows after the fan motor runs for a brief period. But since motor replacement, it only blows the fuse on the highest speed. The two lower speeds now seem to work fine, for any length of time.
As specified, there is a 10 amp fuse for the heater. I would be very apprehensive about using anything larger. Thanks!

[damnyankee36]

Wow, 10A sure is small, but that's correct according to my 66 owner's manual. If it only blew after a time with both the old and new motor, then it's definitely an overload rather than a short. I'm wondering if the replacement motors today are more powerful. It might be too "big".

I can't find a truck heater diagram at the moment but all the chevy's of the 60's I'm aware of use 16 ga wire for that circuit. 16 ga can handle 15, even 20 amps for short runs of a few feet. Even the tail and stop light circuits uses a 15A!

I guess a larger fuse can't be recommended but I feel that's your only choice other than finding a motor that draws less power.

Were you able to measure the actual current when the blower is in operation?