Blower resistor wiring

[Pete Whitstone]

When I got my truck, the wires were all pulled off the blower motor resistor, I don't know why. The 3 of them were laying right there next to it, so I got out the EVTM and determined how the circuit was supposed to work. I plugged them in and the blower motor now works, but a couple of questions.

The EVTM seems to only refer to 3 wires going to the resistor, but there are 4 terminals on the resistor. The switch appears to have 4 positions in reality, from low to high, but the bottom position of the switch seems to be "off", not low. At least on my truck. So is the bottom position "off", and one of the terminals of the resistor unused? Or is something not wired up?

Second question - there seems to be very little difference between "low" fan position (1 click upwards on the switch) and "medium" (2 clicks upward). There is a small difference but not much. Is this just the way it is, or again, do I have something not wired properly?

If someone could post a pic of the wires that go to the resistor and where they plug in, that would be much appreciated. Thanks!

 

[ArdWrknTrk]

Pete,

First power goes through the blower motor.

Then the fan control is either off (open, not connected to any lead)

Through 1,2,3 resistors, or directly grounded (full, or max speed)

There are two available resistor boards.

AC or non-AC, usually identified by a red or yellow circuit board.

On the coil side of the board is also a thermal fuse (like you'd find in a coffee maker or iron)

You ask why 4 pins, and three resistors.

Power is coming in from the blower motor, and going out through one of the three resistors.

It's a known problem with these trucks that leaves get in through the slots in the cowl and catch fire if they pack up around the resistor coils.

So it's good practice to remove the blower motor periodically to reach in and check/clear any debris.

 

[Pete Whitstone]

Pete,

First power goes through the blower motor.

Then the fan control is either off (open, not connected to any lead)

Through 1,2,3 resistors, or directly grounded (full, or max speed)

There are two available resistor boards.

AC or non-AC, usually identified by a red or yellow circuit board.

On the coil side of the board is also a thermal fuse (like you'd find in a coffee maker or iron)

You ask why 4 pins, and three resistors.

Power is coming in from the blower motor, and going out through one of the three resistors.

It's a known problem with these trucks that leaves get in through the slots in the cowl and catch fire if they pack up around the resistor coils.

So it's good practice to remove the blower motor periodically to reach in and check/clear any debris.

I think I understand the basics of the circuit, and what you are saying. But I'm unclear on the 4 connectors/3 wires thing. You are saying that one of the connectors on the resistor is unused, correct? If so, which one? I also need verification that the 3 wires I have hooked up are in the right place. Thanks!

[ArdWrknTrk]

I think I understand the basics of the circuit, and what you are saying. But I'm unclear on the 4 connectors/3 wires thing. You are saying that one of the connectors on the resistor is unused, correct? If so, which one? I also need verification that the 3 wires I have hooked up are in the right place. Thanks!

Do you have factory AC?

What color was the old resistor board?

The only reason I can see someone pulling all the connections out of the plug is if they installed the wrong board, and needed it to work.

 

[Pete Whitstone]

Do you have factory AC?

What color was the old resistor board?

The only reason I can see someone pulling all the connections out of the plug is if they installed the wrong board, and needed it to work.

I do have factory AC, and the board is red. So I don't think it's a matter of having the wrong board, and it all seems to work, at least for the brief time I tested it last night.

[ArdWrknTrk]

I do have factory AC, and the board is red. So I don't think it's a matter of having the wrong board, and it all seems to work, at least for the brief time I tested it last night.

Did you look at page 83 of the EVTM where it shows the black ground wire connected to the thermal fuse???

[Pete Whitstone]

Did you look at page 83 of the EVTM where it shows the black ground wire connected to the thermal fuse???

No I did not see that. I found the diagram on the left side of page 80, and did not think to look for other diagrams, didn't figure there would be any. The one on page 80 makes no reference to that ground. But now I see that the one on page 80 has the word "Auxilliary". What the heck is that? Did some of these trucks come with 2 heaters or something?

So is there a way to tell what the "pinout" of the resistor pack is? I see that the 4th terminal should be grounded, but I still don't know which terminals are which. A photo of someone's correctly wired resistor pack would be really helpful.

Thanks!

[ArdWrknTrk]

No I did not see that. I found the diagram on the left side of page 80, and did not think to look for other diagrams, didn't figure there would be any. The one on page 80 makes no reference to that ground. But now I see that the one on page 80 has the word "Auxilliary". What the heck is that? Did some of these trucks come with 2 heaters or something?

So is there a way to tell what the "pinout" of the resistor pack is? I see that the 4th terminal should be grounded, but I still don't know which terminals are which. A photo of someone's correctly wired resistor pack would be really helpful.

Thanks!

That's the unfortunate thing about the EVTM.

They only show the circuit in question.

You might have three things going through one connector.

You only see three wires in 'C206', but the reality is, it is a 12 pin connector that handles Y & Z as well as X.... but you would never know that from looking at the circuit you're interested in.

I'm sorry... I don't have an '81 to photograph or show you the pinout of.

If the PO removed the socket does it matter?

You can get pigtail plugs for these resistor packs (at least I got one cheap for my '87)

Do you have a multimeter and are you able to ohm out the resistor coils?

The wire colors are identified per speed in the above diagram.

(I mean it's only a matter of choosing low or medium, right?

[Pete Whitstone]

That's the unfortunate thing about the EVTM.

They only show the circuit in question.

You might have three things going through one connector.

You only see three wires in 'C206', but the reality is, it is a 12 pin connector that handles Y & Z as well as X.... but you would never know that from looking at the circuit you're interested in.

I'm sorry... I don't have an '81 to photograph or show you the pinout of.

If the PO removed the socket does it matter?

You can get pigtail plugs for these resistor packs (at least I got one cheap for my '87)

Do you have a multimeter and are you able to ohm out the resistor coils?

The wire colors are identified per speed in the above diagram.

(I mean it's only a matter of choosing low or medium, right?

Well, this is a journey of discovery, and I think I may have just made the next one. You refer to a plug that goes on the resistor pack, but my truck is not so equipped. The three wires that go to the fan switch are all separate from each other and with a female "spade" or "quick disconnect" connector on them. I guess that's not stock? That's why I have confusion about where the connectors go.

I will sit down with the ohm meter and try to determine what goes to what. I must be close, as high, medium and low all seem to work (although medium is very close to low, as currently wired).

Thanks for all the help, by the way.

[ArdWrknTrk]

Well, this is a journey of discovery, and I think I may have just made the next one. You refer to a plug that goes on the resistor pack, but my truck is not so equipped. The three wires that go to the fan switch are all separate from each other and with a female "spade" or "quick disconnect" connector on them. I guess that's not stock? That's why I have confusion about where the connectors go.

I will sit down with the ohm meter and try to determine what goes to what. I must be close, as high, medium and low all seem to work (although medium is very close to low, as currently wired).

Thanks for all the help, by the way.

From looking at the illustration above we see (C602) for the three wires on the hot side and (C604) at the ground.

Connector 602 would have all three of those wires and definitely attach in only orientation, leaving ground as only one option.

And since the connector is at a discrete component we can be pretty sure it has only those wires.

When we see a connector in the midst of a wiring run the EVTM only shows the wires for X circuit.

Sometimes we see S??? and this means a splice in the wiring.

A ground point can be located at the front of the manual.

So you can look it up by number (G106 in this case) and it will say where it is attached.

There is even a highlighted note to see page 6 for grounds

Often there is an illustration, showing some spot on the block or up under the dash.

At the end of each section of the EVTM there is a description of how the circuit works and the drill down troubleshooting steps you need to take for each symptom.

This is all made a lot easier if you have the paper manual in hand instead of having to click back and forth through a bunch of tabs.

I got my '87 EVTM (used) on eBay for $16 delivered.