91 LTC Sig ATC High Fan SPD Only

To a Lincoln tech,

87,000 miles/car is driven only 500 miles on avg per year

How could I test the blower speed control module? With the module off the car is the resistor in/or on the module accessable so it can be be solder repaired with high temp solder?

What else does that module control, or does it only control the blower motor speed?

Can the module be bypassed so that the fan will still work on high, after I take it off to inspect/try to repair it/or replace it, etc and how could I do that, with some improv, but safe wiring?

I found a site that lists alot of used auto parts yards that have these; used speed control modules, for less than $75, but how can I be sure that they are ok, or that they will last (keep working)?

What other Ford cars and range of years and LTC range of years are the modules all the same,
so I can pick a used one from a later model car with less mileage?

Can you recommend a certain year Ford car or LTC that the modules were made better?

Recently, the blower motor started to only work on
the very high setting on the ATC vertical slide
variable speed switch.

Also with the car just started the ATC setting at the
bottom Auto position on the vertical moving variable speed switch does not get the fan to go on at all, with the temp control selector in any of it's settings.

After the heat is let to run a short time by setting the variable speed switch to manual high speed,
(which is the only fan speed that works) with the temp control set to 75 or more,

only then will the ATC Auto setting respond, but only if the temp control setting is set at 70 or below only.

There is no low, med, or med/high fan. No response, just silence.

How can I do some kind of basic or not so basic test to check if the part that needs repair or replacing is either the ATC Control Switch itself located on the dash, or the Speed Control Module located under the hood?

Floor vent setting has no response at all (no fan), but def/floor works fine, full flow at both vents???

The floor vent setting has not worked for a long time or has never worked (no response, silence) since I've had the car 3 years, but the max ac, ac, vent, def/floor, and def all work good.

Does that mean that the A/C/Heat Control switch itself has a short at the floor vent setting? Can that part of the switch be solder repaired, with the whole switch out of the dash?

This car has the Automatic Climate Control System/non digital display. It has a verticle moving variable speed switch.

At the very top position it goes on very high, and at the very bottom the switch clicks into the Auto Temp Mode.

At an auto parts site it says that if the 91 LTC has the ATC system then the alternate part instead of a standard delimiter type resistor is this part:

Heater Control Valve
VALVE, HTR WAT TEMPERATURE SENSING / LOCKOUT, ATC -Located in heater hose

What does the heater control valve do, and does it need to be checked for any of the problems mentioned above?

I did find one forum question that mentioned the ATC system and the blower motor speeds for this car. It was about the ATC and the A/C system not working as good as it should.

While responding to the question it was mentioned that when the blower motor goes on high, it has/uses its own seperate ground wire and the module has a relay for this.

Sometimes a problem with the other speeds could be related to the seperate ground wiring, for those other speeds and not to a faulty part.

Or the problem is the blower motor speed control module itself.

One of the forums mentioned that they sometimes have to be fixed or replaced because of solder problems with those units.

What is your diagnosis and/or suggestion on how
to test and fix the low-med/high blower speeds and the floor vent setting having no response (fan shuts off at this setting)?

Where/how do I check for proper ground for the different speeds that involve the speed control module.

There's alot of harnesses, wires and locations.
Which harness/location/what color wires will I be
checking/testing?

How/where do I check about the grounds?

I have a digital multimeter.

To test for ground the ohm meter is used to check for minimal resistance on full high speed, right?

I've never checked the fuses in this car, which ones should I check?

Side note: With engine running the lights inside the car are flickering and upon acceleration the flickering is not interupted that much it continues even with acceleration. What causes that?

With engine off the lights don't flicker at all.

What causes that?
Is that ok or is there something I can check/fix?

What does the Heater Control Valve located in the heater hose do as part of the ATC system?

How do I test this part? It is partly visible at the heater hose, right?

There is one more part to mention that I read about and that's the air temperature sensor (not the coolant temp sensor). Is that involved here in any way?

I read this on a forum

A common problem with the speed controller is that the master resistor that slows down the blower gets very hot, and melts the solder that holds it to the controller circuit board. In most cases, a higher temp solder fixes the problem.

I met a guy that had to replace a defective resistor too... He didn't want to put out the $160+ for the Speed Controller, so instead he found a supplier of just the resistor and made it work, that was two years back and still working.



The more reply the better, I want to do all of this testing and repair work on this part of the car myself even if I have to take apart the dash to test/fix the vent setting or the ATC part of the heater controll asembly.

Thanks