blower motor speed

[chrismoore701]

I have lost the first two (2) speeds on my 79 C10 blower motor. Just replaced the resistor and to my surprise it did not work. Thoughts? Still only have high fan speed

[truckin 79]

It has to be the switch on control panel then.

[chrismoore701]

Originally Posted by truckin 79

It has to be the switch on control panel then.

was thinking that, can I measure the voltage at the plug on the AC box to confirm?

[truckin 79]

I have only dealt with non-ac trucks when it comes to fan motor. If it is a AC truck then it could be at the relay itself or a bad ground if AC stuff has been altered.

[87chevy.com]

From a few wiring diagrams I have, 84, 87 and 91 they seem to be the same, 4 wires:

Tan, Orange, LT Blue and Brown/White

Orange wire - High Speed Signal
Tan - Low Speed signal
LT Blue - Medium Speed Signal
Brown/White - Power in from Selector Switch

Orange, Tan and Light Blue wires go to the Blower Resistor on the Evaporator box case.

You can check voltage, ground to each of the wires and verify you are getting approx the same voltage from each wire. It is s switch, not a resistor so the voltage should be about the same.

If switch reads fine:
DK Blue wire at the Blower Motor Relay should be the POWER out from the resistor. This wire will have the different voltages that goes through the relay and to the Purple wire that connects to the Blower Motor... Low speed should have lower voltage, medium speed should have higher voltage and High should be even higher voltage, but not battery voltage

Max Speed signal goes from Switch to Blower Motor relay. It activates the relay and gets power from the 10 Red gauge wire (that should be Hot at all times) and goes through relay and out the Purple wire for Blower Motor. This should be same as battery voltage. If the reading is more than 1 volt difference, I would check the wiring and if ok, replace the relay as the contacts are worn.


clint

[chrismoore701]

Thank you, she is covered up in storage, so she may have to wait awhile.

[hatzie]

The high speed blower relay is kicking over so the power is being routed from the J studs through the relay and directly to the blower
This means power in to the switch is OK and the high speed wire from the switch to the relay coil is OK.

Check your Low and Medium control wires from the switch to the blower resistor with a headlamp bulb when they are selected.
If it lights up bright then the resistor is screwed up.
If it's dim or doesn't light up then the switch or wires or both are not happy.

I use a 9006 headlamp bulb in a spray paint lid with a connector pigtail I liberated from a vehicle headed to the crusher. Kinda Ghetto but it works. 55Watts should be enough power draw to test this circuit.
A meter can read 12v or 14v when the switch or wiring is dodgy since it doesn't pull a lot of current. A test lamp only has a tiny little low wattage bulb so again it will not pull enough current to tell you a lot. A headlamp bulb can stress a dodgy switch connector or wire.

IIRC the switch terminals are Pak Con II and the resistor terminals are Packard 56. The connector part numbers are in the wiring diagrams so you can look em up to find out what family they are.