I have no low speed fan on my '78. I checked the 25 amp fuse - its good. I checked the switch using Willcox's video as a guide. Switch is good.
When I switch from low to high or high to low I hear an audible click and feel it in the relay.

I pulled the resistor out and it looks good. Not sure if I tested the resistor correctly - I put the meter on OHMS and touched the different terminals and the OHM meter pegs out at all three settings. Being maybe out of my league here, is this good or bad?

I tested the leads on the relay and resistor (I put the lead on the wires coming out of the top of the resistor). When switch is in high and fan blowing I have 12 volts on the orange, Purple, Dark Blue and Red wires. On the Resistor I have 12 volts on all the wires (Dark Blue, Brown, Light Blue and Tan).

When I put the switch into speed two, on the relay I have voltage on the Red and Dark Blue wires and nothing on Orange or purple. On the Resistor I have 12 volts on all wires.

Should the voltage on the resistor wires be different for each one?

AS the air conditioner does not work I don't have the terminals to the compressor connected.

While I had the engine out last year I installed a new blower motor and added a second ground wire - read where that makes the blower motor run better. So, I have two grounds.

Any advice where I should check next or direction to what I am missing?

Thanks in advance.

 

Have you bypassed the wirng and made sure the fan actually works? In your write-up....you actually never wrote that the fan works in any speed.

I would bet you have an issue in your connection at the relay or the relay itself. About 95% of the time...it is at the relay where I have found the problem. AND this id due to this relay carries a lot of current and this area is not protected against dirt and moisture and the terminals can get loose or corroded and not want to grip well and work. By it making sound and clicking it is getting the signal from the switch...but it may not be able get the current to go out the correct terminal to make the blower motor run.

Remove the purple wire that goes to your blower motor and do your testing and see if you get any voltage on the purple wire.

DUB

 

Going back to your test of the resistor.
What scale did you have the ohm meter set for?
You say the ohm meter pegged out, do you realize you dont want infinite resistance which would mean the resistor is burnt out?

 

Thanks DUB. the blower runs great on high. Lots of flow from heat, vent or defrost as directed. No noise no binding.

I also forgot to state this is a new fan. The one with the larger sqirlecage. I have a 3/4"spacer IIRC to mount the larger assembly. When I installed it I read a lot of advice to add a second ground. I added a 12awg wire from ground to the frame. Black ground wire on relay pegs out on OHM meter.

When I switch from high to low, medium 1 or medium 2, I get nothing. I have 12 volts at the red and dark blue terminals. Orange and purple are 0. That I have nothing at the purple wire feeding the blower motor tells me it's a bad relay. But

On the top of the resistor I have 12 volts on brown, light blue, tan, and dark blue. Regardless of where the switch is set except high. Shouldn't I have voltage to only one of the three inputs that will give me a different voltage input to the relay and then the Blower? Or could a bad relay backfired voltage to the resistor giving me voltage on all three lines.

Hope I'm asking good questions here but electric circuits to me are confusing.

Jim

 

The one good thing about that design of resistor is that you can remove it and visually inspect it and see if any of the coils are broken or loose also.

I can say that it is not wise to have the system ON and the fan not working due to the air from the fan keeps these coils cool. If not...then the coils can glow red like that of a home dryer and possibly snap. Which I have seen this first hand.

DUB

 

The honest answer is no. I don't really understand ohms.

I have the meter set at X1. I put the leads on any 2 terminals and it read about 2 to 4. On the higher scale it reads less.

As the meter goes way right, I think I originally mis spoke when I said pegged out.

So, help me out. With a reading of. 2 ohms between any and all terminals is my resistors good or bad?

 

there is a common point where the all the coils attach. Put one probe at that point and move the other probe to the opposite end of each of the coils. The meter should be set to something higher than x1. You should get a number reading across all of the coils, not zero or pegged out.

 

GOT IT FIXED

With the help of my brother, his digital meter and skillful ability to use it we / he did determine that the relay is bad. We took it apart and the points are moving but the normally open contact for the low speed fans was not actually making contact. We sanded the contact points slightly and bent the arm a little bit and now everything works.

It is still a mystery to me how that circuit "knows" which speed to use but I'm sure its no surprise to anyone that I probably have a new issue to resolve.

Later

Jim

 

***WINNER WINNER CHICKEN DINNER***

Glad that is got repaired and I figured it had to be as I mentioned.

The speed is determined in the resisitor/rheostat that goes in the evaporator core case that has the coil/spring element of different sizes...THAT is what is controlling how fast the fan spins UNTIL you go to HIGH fan speed where the really is bypassed.

Kinda like the spring on your headlight switch that when you turn the know...your gauge lights can dim or go bright. same principle but different method.

DUB