car is a 1999 Mercury Sable- The blower motor runs constantly on ‘high’ as soon as the ignition key is turned on,whether the switch is on high or low - The relay and the blower motor have both been replaced.
Any suggestions as to the cause of this proble would be greatly appreciated.
Thank you.
If I remember correctly Ford controls the ground side of the blower motor so if there is a short to ground it will run all the time when the key is on. You will need to trace the ground circuit wiring to find out where the short is.
I will try to find a wiring diagram and research this further.
Does the blower run when the HVAC controls are turned to OFF. Confirm that this is manual controls and not electronic.
controls are electronic.
The AutoZone site does not show a schematic for this particular circuit (go figure) so this may get a bit complicated without a Helms electrical manual and the factory diagnostic manual.
The only diagram I have is one for my Lincoln Mark with EATC and the blower speed controller (which is the relay I assumed you replaced?) receives input from the EATC unit head.
IF this speed controller is what you replaced was the wire connector burned or charred in any way? This can really cause things to go stupid on them.
Instead of the normal resistors used for speed control you probably have an electronic speed control unit and it is shorted. This then supplies constant voltage to the motor. If the speed control is continuously variable rather than stepped, the controller is electronic. Follow the wires from the blower motor that will run too the controller module. Remove the wire connector and then remove the mounting screws for the module. The unit runs a little over 100 dollars normally.
I thank you all for the information- very helpful-
I think a “road map” would help a lot, don’t you? The wiring diagrams can help you find things that are otherwise not easily found. For help in understanding the A/C blower circuit controls, go to this web site: http://search.ebscohost.com. User ID = user; Password = password; click Login. Scroll to: Auto Repair Reference Center (click). Select: year; make; model; wiring diagrams; Air Conditioning; Automatic A/C circuit; drawing 1 (next, drawing 2). So, if you wonder, “Where does this wire go?”. You’ll be able to find out. Good hunting!
Here’s the blower motor circuit. The controls are on the ground side of the motor. It could be the pink/white wires is pinched and grounded or the module has failed.
This is my first atempt at attaching a file so I hope it works ok.
If you click and download you can see the image clearer. I also meant to say the contoller is probably defective not the module.