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Here's a weird one: AC/Heat adjuster kills engine

BALTANAKT

NAXJA Forum User
Location
Berthoud, CO
Was driving the other day.

When I pushed the adjuster as far as it goes the cold side, the engine cut out momentarily, until I released the adjuster.. Very strange. Almost felt like spark disappeared or the engine stalled for a moment.

Anyone encountered this before?
 
No, the only two things I could think of to do something like that is a frozen/stiff ac compresser or a loose wire behind the ac controls. Does it have an aftermarket stereo and is it properly installed, meaning wires not just taped together?

I just rembered that years ago my 68 f250 would stall everything I turned on the wipers. Turned out to be a loose wire hitting the wiper drive every time it went by!
 
Check for loose/disintegrated vacuum lines as well. Could be getting a big gulp of unexpected air when its trying to move the blend door.
 
Check for loose/disintegrated vacuum lines as well. Could be getting a big gulp of unexpected air when its trying to move the blend door.

Very good point about vacuum.

You could confirm if this is it by plugging the vacuum source at the manifold, then run the motor and move the adustment control as before and see if the engine stumbles or not. if the stumble issue resolves, but then reappears once you restore the vacuum, then you will know with great confidence that you got a vacuum issue.

On a side note...



My friend had on old 1956 T Bird back in the day in the 80s, drove to Vegas from west coast in it. That T Bird had a vacuum wiper system, it leaked, engine ran poor when operating the wiper. AND just when you needed the wipers to run fastest, at accelerating, or even cruising at freeway speed, the vacuum is really low in those engine operational regimes , so the wipers slow down, let off the gas and coast, and the wipers speed up as engine vacuum spikes when the throttle is suddenly let up. We had non functional wipers on one road trip do to leaks, raining hard, late night, highway, three of us crammed on the seat. no wiper, but my friend tells me, the driver, that at over 90 mph the wind will sheet the rain off the glass. And sure enough, the windshield did clear up as the needle passed 90.
Crazy how I am still alive no? Oh and the car had a drip from the dash in the rain, dripped directly on the gas pedal foot, no way to avoid it. You drove in the rain, you had a wet right foot! That was some car, collectable nowdays, back then you could still get old beater cars like that cheap. Golly, the 1956 two seater convertible Alfas a friend of mine had in highschool, he bought two of them with money working at a carwash, and a pizza joint, I bet they are high five figure cars now, back then, they were just old cars rather than classics
 
I am not sure what year your XJ is but on pre 1996 models, there are two electrical connections (and one set of vacuum connections) on the heater control. The first is the fan control and it is pretty easy to spot. This switch is not connected (physically) to the movement of the levers so it isn't likely related to your issue. The second electrical connection sits on the back of the control and does seem to be affected by movement of the levers. Is it possible that this connector is loose or is somehow causing a short?
 
I installed a fuel pump kill switch last week... Maybe it has something to do with that, somehow...

The vacuum sounds plausible as well. Guess I'll look at it first since that's fairly easy.

This is a '91 by the way.
 
The computer will raise the idle speed when the AC compressor is engaged but if you are driving down the highway you are probably going faster than 40mph and the IAC is not being engaged.

Its possible that the load from the compressor is overloading the engine.

Another possibility raised above is vacuum, however that only changes when the vent selector is moved.
 
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