If your blower swith keeps burning out....

[mark93]

Hi all.



I run the fan on High all the time and burn out a fan speed switch about every six months.



There is a kit to fix the switch burning out. It is p/n 4723982.



It was made for '86-'92 vans but the wiring is perfect for a pick-up.



The kit is designed to power the blower through a relay when the speed switch is set to High.



Going by the instructions the kit is a little complicated to install.



If installed per the instructions, the following would result:



1) The fan switch would carry only relay current on the High setting.



2) The ignition switch and fuse block would be no longer carry the load current of the blower when the fan switch is set to High. The fan would still stop when the ignition swith is off, but circuit protection would be up to a fusible link protecting the ignition switch.



3) When set to High, the blower would run at the same speed whether the system was operating in the A/C/Vent or Heat/Defrost modes.



Item 3 above might need some explanation. The blower in our truck really has a total of 8 speeds. When the system is set to MAX A/C, A/C or VENT, battery current goes through resistors on the three lower speeds but on High speed there is no resistors in the circuit. When the system is set to HEAT or DEFROST there is one resitor placed in the circuit before the fan switch and all speeds are reduced. The kit bypasses that resistor when the switch is set to High. Thus you would get higher blower speed on the High setting in the HEAT and DEFROST modes than stock.



You would think that for the $100. 00 list price of the kit and switch that some switch could be built to handle the High speed blower current and eliminate the need for a whole rewiring kit.

 

[Tim1]

Hey Neil, the electrical wizard, what do you think of this? Tim

 

[ETOBICOKE]

Blower Switch

I certanly think that adding a relay to the fan cicuit for high speed would be feasible. The only problem that I have is trying to understand why the OEM switch fails so quickly. The fan speeds are derived by putting resistors in series with the fan motor, for the lower speeds and direct for high. If you have AC there are 4 speeds and without 3 speeds. The current the motor draws should remain relatively constant for all the speeds so the switch should be just as vulnerable in the lower speeds as high. This would lead me to believe that the motor has a problem and is drawing excessively high current. I don't use the high position for very long periods, only to defrost or defog the windows, so like most others have never encountered this problem. Given that a factory fix is available for the vans would indicate that the problem could occur on our trucks, as they share many of the same electrical components.

The parts kit for the van sounds like a good solution for your problem, just seems a bit exspensive. At that price I would hope everthing plugs in to the existing connectors. Don't know what the OEM replacement switch costs, but adding a relay to the existing switch yourself, should only cost about $10-$15 in parts.



Neil

 

[mark93]

Hi Neil and Tim1.



The Blower draws more current as the speeds are increased.



Placing resistors in series with the blower motor decreases the current because the total circuit resistance is higher with same line voltage applied (12V). Although the resistors get hot because of power dissipation the total power is less than when the circuit has no resistors in it (High Speed). Using resistors to reduce voltage to a motor so it will run slower is inefficient but cheap.



The switch High contacts carry full current and the other settings a reduced current. The size of the wires shows this: The switch is fed by a 10 GA wire. High speed uses a 12 GA wire, Medium High speed uses a 14 GA wire and Medium Low uses a 16 GA wire. Interestingly, the lowest speed uses a 12 GA wire because it is possible to run the blower on that circuit in all switch settings when in the HEAT/DEFROST modes. The current through the lowest speed circuit when the switch is set to High would be only a little lower than the AC/VENT modes.



If this sounds a little strange I'm with you. I had to study the wiring diagram for a while to understand why there are 4 resistors and not 3 as one would expect with a 4 speed fan. The extra resistor gives 8 speeds total, like how a 10 speed bike can be described as twin range 5 speed.



The switch is under-engineered just like the headlight switch/harness. The switch and not the harness fails in this case. I see black charred grease through the "vent" hole in the switch where there was nice petroleum jelly looking stuff originally. Also the backing of the switch (Phenolic?) is heat cracked. I do have to admit that I put the switch through extreme service but it should be designed to indefinitely carry the full load current.