MX83 Cressida AC Fan Speed on constant high, Help needed

[mamba]

My 92 Grande's ac fan speed is constantly on max speed regardless of whether or not I use auto fan control or the [ lo 2 3 hi] settings on the control panel.

I have tried another fan speed resistor - no change still only max speed
tried other relays - no change
tried another fan unit - no change
tried another hvac control unit - no change
tried with ac on and off and heater on and off - no change

This has left me confused. What else controls the fan speed. With the fan speed resistor unplugged the fan stays on. This morning when I tried changing the fan speeds on the panel it worked, with the resistor unplugged?? I thought the resistor controlled the varying speeds, but I was able to change from low to high settings without a problem. After awhile it reverts back to max speed again.
So plugged in or not, the fan speed is maxed out all the time...

Anything else I could check?

[TRD-12A]

Are you sure you found the fan speed resistor cause that defanetely sounds like it's your problem..

Did you get a secondhand one or a new one??

[toyoda]

On the Auto systems there is usually 2 controls.

Low Speed = Fixed resistor.

Other Speeds = Power Transistor

if the fixed resistor is blown you will only get high speeds or if the Power Transistor is blown you will only get high speeds as the current is usually split between them.

The Power transistor has a thermal fuse built in and I have replaced a few of these succesfully.

They are similar in appearance externally and both bolt into the air ducting for cooling.


Hope this help

Toyoda

[mamba]

I know where the fixed resistor is located. It's the one with the ceramic looking top on it and is near the actual fan under the glove box.
Where exactly can I find the power transistor cause that sounds like my problem.

[toyoda]

The one with the ceramic top sounds like the power transistor.

the fixed resistor is a coil of wire .

they should be pretty close to each other.

Toyoda

[cambelt]

The power transistor is the heat sink looking thing , look on top of the fan box , they are in the same area as the wire/ceramic resistor .
Dave

[mamba]

The power transistor is actually located on the ac box looking thing behind the glove box, where as the fan speed resistor is underneath on the fan unit. It has the heatsink on it like you said with what appears to be a Toshiba transistor on it. I changed it with another unit and it works now.

Thanks for the help.

Strange how even the manual controls reverted to max speed whereas only the transistor was faulty which controls the auto speed. Seems that if either one is open circuit both manual and auto fan speed control reverts to max.

[toyoda]

Yep, thats because the fixed resistor is always in line so shares some of the load regardless.

Glad it all worked in the end.

Toyoda