"Looks like +$100 will get a rebuilt amp. Bizarre behaviour is right!!
Even on "DEF" the fan dies off to nothing. I wonder if trouble shooting might help, like put a V-meter on the power supply to the fan when it's doing this weird stuff. 12v means the amp is OK, and the fan
is the problem, right? 1v means the amp is screwed (or possibly the "new" servo). Will source an amp first thing Monday. AJ "
You can easily rule out the fan motor without testing it. As TG pointed out, erratic behavior is often the amp or servo which drives the whole system. When you move the temp wheel from min to max with the cabin at about 70, the system should cycle from max cooling to max heat. Among the things that change while doing this are the flaps to divert air and the blower speeds. The air should be all coming out the dash vents for max cooling and from the footwells, with some bleed to the two side dash vents, for max heat. When you move the wheel from min to max, the flaps/air should change over and at the same time the blower should go from max cooling speed to max heating speed. And of course, hot air should show up at the footwells too. It does this changeover in steps over a min or so.T he max heat blower speed is one speed less than max cooling speed.
If it's the servo/amp, you will typically not get the correct flap/air movement, nor the blower speed changes, heat, etc. If it's the blower motor, then just the blower will be erattic and will warrant further investigation of the blower/circuitry.
If you are convinced it's the servo, before you buy a new one, there is one last test you can do. That is to drive the servo directly. There is a small motor and gear train in the servo that in one direction winds over to max heat, in the other to max cooling. You can disconnect one harness from it that has the wires that the amp uses to drive this motor. With jumpers, you can momentarily connect a 9V household battery to the two terminals on the servo that drive the motor. With one polarity it goes to max heat, the other maz cool. You can hear it run. The terminals are shown in the wiring diagram in the service manual. If you do this make sure you don't leave the battery connected for more than a couple seconds without hearing the servo motor running There is no cutoff, so when it reaches the end of travel, if left on too long, you will burn the motor out.
By moving it to max cooling, then starting the car with the one harness still off, the system should be at full cooling. Vice/versa for heating. If that works, it's most likely not the main part of the servo that's a problem. However, there is one part inside the servo that could still be a problem. That's the feedback potentiometer. This is part of the temp chain of resistors that together with the feedback amp drive the servo. The chain is an outside temp sensor, the dash thumbwheel, and the feedback resistor in the servo. If any one of those is intermittent, you get erratic performance. The pot in the servo, like all pots, is subject to getting bad spots, like the old pot type volume control on radios.
If you take the top of the servo off, the pot is sitting there, driven by the gear train. Some electronic contact cleaner sprayed into it has been known to rejuvinate them. It's also a good idea to always cycle the system from max heat to max cool at least once a month. That moves the gear train, pot etc through its full range and keeps it lubricated.