Author Topic: Wierd Issue  (Read 3605 times)

Offline VeeTwin60

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Wierd Issue
« on: February 10, 2008, 11:18:45 am »
I just noticed the other day that whenever I have my AC/Heat fan on high in the cab, the engine idles higher.  I guess there are three or four fan speeds for the heat, and it only does this on the top one.

Any ideas?
1986 C-10 1/2 TON 2WD SWB

Offline 77c15

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Re: Wierd Issue
« Reply #1 on: February 10, 2008, 11:38:25 am »
The "high" speed bypasses the blower resistor and gives direct 12v to the blower motor. The resistor just knocks the voltage down to the other fan speeds, for example med.high is 9v, med. is 6v, and low is 3v. I would check the wires going to the resistor and the main power wire that bypasses the resistor in "high". Also, check your ground wire on the blower motor and harness. It sounds like an electrical issue, possibly in the harness, battery, or alternator. Hope this helps!
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Offline VileZambonie

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Re: Wierd Issue
« Reply #2 on: February 10, 2008, 11:54:08 am »
You have a fast idle solenoid on the carb I take it?
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline VeeTwin60

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Re: Wierd Issue
« Reply #3 on: February 10, 2008, 03:49:02 pm »
I'm not sure... what do I look for?
1986 C-10 1/2 TON 2WD SWB

Offline 77c15

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Re: Wierd Issue
« Reply #4 on: February 10, 2008, 06:12:18 pm »
A fast idle solenoid is on or near your factory rochester carb and keeps it idled up until the engine gets warm and then takes it "off" of fast idle once it warms up. I spliced into this wire on my 74's new harness to hook up the new edelbrock carb's electric choke hot wire, and it worked exactly as it should.
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Offline VeeTwin60

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Re: Wierd Issue
« Reply #5 on: February 10, 2008, 06:24:52 pm »
I feel dumb - after your explanation, I figured it out. :-\

Anyway, I have a Holley, and it does not have a fast idle solenoid... it doesn't even have a dad-gum electric choke or cable for the manual choke. :-[
1986 C-10 1/2 TON 2WD SWB

Offline VileZambonie

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Re: Wierd Issue
« Reply #6 on: February 10, 2008, 07:07:17 pm »
Check your ignition coil. When you put the heater fan on high the alternator output is increased, if your coil is weak the increased output can effect the firing voltage of the coil.
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10