A faulty amp could cause this, yes.

But....

I can't remember the details at the moment but, like your garage door, there IS some sort of limit switch or position feedback potentiometer to keep the main servo axle from over-rotating in either direction. If there's a problem here, it's plausible that 'rocking back and forth in a loop' could occur....as though the system stops and then reverses itself almost as soon as movement begins.



The servo has a brown plastic cover but I'm not sure if t can be removed with the servo in place.....and even if removed you might not be able to get a full view of the innards of the servo to see if something is jammed up

You could disconnect the servo wiring and drive the servo motor by applying power and ground directly to the motor wires. If the jam still occurs then you know it isn't the amplifier

Others will chime in

Cheers
DD

 

further info: after applying current to the servo and running it to its extreme, I plugged it back in. I adjusted the temp to the hot extreme. I turned the key to position 3. The servo ran the gears all the way to the other extreme, then oscillated back and forth. I would like to test the in line diodes. Could any helpful soul tell me where they are, are they built into the amp?

 

I have removed my Servo for inspection and it is not that difficult.

The plastic cover that Doug mentions, mine is black plastic, very thin, is held by two push-down plastic plugs, carefully pry the plugs off the cover and maneuver the cover out.

then you need a small 10mm or 11mm wrench to remove two nuts. Then you disconnect the round plug with the 7 thin wires. Then disconnect the vacuum hose from the Servo, then you maneuver the Servo out.

In the case of my Servo, I found one of the what-do-you-call-em Switches was partially or intermittently disconnected. I tightened the spade connector and I've never had an issue again.

yes, replace that Heater valve if it is showing its age, start there.

A friend of David ( Everyday XJ ) is now manufacturing a replacement Amplifier, but the price is high and the warranty is little.

does this happen only in Heat mode?? what happens if you put the Temp switch in 60 ??

 

Thank you Doug, I did just that! The motor successfully turned from one extreme to the other, and the limit switch stopped the motor at either extreme without a problem. The odd thing is the behavior when I hooked the harness back up, and turn the temp to hot. The gearbox moves smoothly to a point then oscillates a tooth or two back and forth!

 

Thanks for the procedure, I was a little baffled, I removed one of the plastic fasteners and did not see the other. Good to know it is possible without too much madness. The weird back and forth only seems to happen when the dial is set above 70. ???

 

update: John's cars thinks it sounds like a bad potentiometer on the servo, ugh. I ordered the manual from him for testing procedures...I will report back.

 

do they sell replacement servos? if yes, how much are they charging?

 

I went through the testing procedure and it revealed that the position potentiometer was not in spec, not a linear value of resistance to 2k. So i went to pull a part and learned how to remove the servo on the junked xj6 there.
It was way less complicated than i thought. On the passenger's side of the tunnel after removing the panel, remove the angled air vent-4 tiny screws- this reveals a single silver nut. that is the only one. There is a locating pin to the right but that's all there is that connects the servo, other than a sticky foam pad sandwiched there. After disconnecting the round harness connector, the 2 actuating rods and 2 colored hard hoses and one soft black one, simply pry the servo down with a fat screwdriver to overcome the sticky pad and it pops down and out.
I tested the pot located on the forward side of the servo main shaft in the yard and it was good. I unsoldered it and replaced the one on mine with it. (my servo tested well in all other respects and it looked to be in better condition physically)
The system now operates as it should !
I believe John said that they have some servos, but i cannot recall how much they are. He is an affable fellow though, give him a call!

 

That is what I would do. A PO of my car swapped out the original for a generic plastic unit. It works just fine. I modified my car's hot water supply lines. I added a manual valve to the hose that did not have the vacuum controlled valve. I call it my summer/winter valve. Closed in summer and no hot water is in the heater matrix. So, as my AC isn't hooked up. The fans come on and I have a powered vent system. Nice in lower temp but not really cold or really hot days.


As to fan speeds. The selector switch seem to include resistors of various values to lower the fan motor speed. High means direct, no resistance.


My Jeep is similary configured. But, the resistors have all probably failed. High fan speed only. No big deal, in summer, that is usually what I want. In winter's cold, same answer. Too hot or too cold. Adjust the temp on the red/blue dial...


Carl

 

Yes, the local auto electrician who is a car nut spent ten hours figuring ours out and took both a faulty amplifier, stripped gear in servo unit plus faulty micro switches and burnt wiring to make our totally manual controlled.