92 Legacy Jerks When AC Is On

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ralphl
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92 Legacy Jerks When AC Is On

Post by ralphl »

About a month ago my 92 Legacy (manual transmission, non turbo) began to jerk, when the AC is on. When the AC is off, I never get the jerk. At first, when driving, the jerk made me think I had a bad spark plug wire or even a bad coil (causing 2 cylinders to malfunction), the jerk was so severe. However, then I noticed that the jerk happened only when the AC was on. Has anybody else had the AC cause the whole car to jerk?????????

Normally, when driving, I never noticed when the AC compressor cycled on and off, it happened so smoothly. If I was idling at a stop light, I could notice that the tac would show the engine speed up to about 1100 rpm,when the compressor came on, and drop to about 650, when the compressor turned off. At idle the compressor mostly STILL smoothly cycles that way with no jerk on the engine.

I have tried several things. First,I checked the sight glass on top of the filter drier. Saw no bubbles. Waited for the compressor to cycle off. Saw a few bubbles when the compressor was off, but none when it cycled on. AC still gets as cold as it always did, so I don't think I am low on R12.

I unplugged the connector on top of the compressor so the compressor would not turn on. I drove around and experienced no jerks, so I think the jerk must have to do with the compressor cycling. So I reconnected the compressor so it would run for the following tests.

I swapped the fuse box compressor relay (right most relay looking at car from driver's side) with the one next to it. Drove around; car still jerked. So I don't think its a bad fuse box relay.

I unplugged the bypass air valve (idle air valve) and drove around. Car ran rough, but still jerked. So I don't think the bypass air valve is involved in the jerk. I reconnected it.

I unplugged the throttle sensor and drove around. Car ran rough, but it still jerked. I also ran all the voltage/ohm checks specified in my old Chilton paying particular attention to throttle closed/open lead. The voltage turned off and on as it should, when I manually opened and closed the throttle. I also checked the rheostat wiper lead to see if the voltages were in spec (full open and full closed) and checked to see if the voltage changed smoothly with the opening and closing of the throttle. Everything checked out fine. So I don't think the the throttle position sensor is involved in the jerk. Reconnected throttle sensor.

I have considered the possibility that the compressor clutch could be grabbing and causing the jerk on the engine. However, I never get the jerk on the engine, when the engine is at idle and the compressor cycles. Is it possible that the clutch grabs only when the engine is at driving speed?

While I know that the compressor must be running for the jerk to occur, I don't know if the jerk occurs when the compressor turns on or when it turns off. Maybe the ECU, in trying to control engine speed, sends out a signal to some place (injectors are all I can think off) to cut engine speed and then decides it too slow and immediately sends more gas to the injectors causing a jerk.

I have been reading 3 threads on this website dealing with AC compressor cycling problems. These are:
viewtopic.php?t=36434
viewtopic.php?t=32066&highlight=thermostatic
viewtopic.php?t=40114
They discuss problems with the evaporator thermostatic switch and the evaporator cut relay switch both located above the glove compartment and on top of the evaporator box (note that I have not yet looked at them). As I understand it the thermostatic switch is attached to a thermocouple or thermistor attached to a fin in the evaporator box. This sensor switches on and off according to temperature and feeds its output to the ECU. The ECU uses this information (along with other information such as the pressure switch output and the throttle position sensor) to cause the cut relay to turn the compressor on/off. Does anybody think one of these switches could be the problem? Also, have I correctly described the workings of these 2 switches?? The only way I could see these switches causing a jerk is if they "chattered" causing the compressor to very quickly turn off, then on, then off again and the ECU to feed more, then less, then more gasoline to the injectors for engine speed control.

Also, I see only a single pressure switch (on top of the filter drying. I presume this is a high pressure switch. I looked for a low pressure switch that would be used to say when the R12 was low. However, I couldn't find one on the low pressure pipes. Is it possible there is a low pressure switch under the dash, inside the compressor, or some other place. Maybe there is no low pressure switch???? If there is one, maybe it could be causing the problem???
Legacy777
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Re: 92 Legacy Jerks When AC Is On

Post by Legacy777 »

Welcome to the BBS.

I moved your post to the engine forum.

What AC system do you have, Zexel or Calsonic?

Even if your AC is still blowing cold it's possible you could be low on refrigerant and as the compressor rpm's increase the suction pressure lowers quite a bit putting more load on the engine, and that could be the jerking you're feeling.

Could you describe the jerking a little more? If you accelerate lightly in 3rd or 4th gear do you experience the jerking or is it more prevalent when the engine rpm's are higher?
Josh

surrealmirage.com/subaru
1990 Legacy (AWD, 6MT, & EJ22T Swap)
2020 Outback Limted XT

If you need to get a hold of me please email me rather then pm
ralphl
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Re: 92 Legacy Jerks When AC Is On

Post by ralphl »

Josh, Thank you for moving my post to the correct spot. I had some difficulty learning the web site.
I have a Calsonic ac system.
The car will jerk in 3rd or 4th gear even if accelerating only lightly. Sometimes I can even feel a slight jerk, when going mildly downhill.
I discovered some more about the jerk. First it happens when the compressor disengages, not when it engages... Surprise!
Second, I found that if I pull the leftmost of the 4 ac relays (looking at fusebox from driver's side) and drive, the car no longer jerks. The wiring diagram I have says that is the sub-fan relay and upon closure of the relay, it appears to drive the passenger side fan. The passenger side fan never seems to turn on, even when the engine is hot. So I unplugged the connector to the passenger side fan, put the relay back in, and the car still jerked. What the heck could cause that.
I took the passenger side fan completely off the car. It is sitting on my workbench. The plug for the fan has 3 leads--one ground and two that should have 12 volts to drive the fan. I presume the fans are each 2 speed fans, because of the 2 leads. With the fan on my workbench I connected a battery (negative to ground lead, and positive to one or the other of the 12 volt leads). Fan did not turn on. I can easily turn the fan by hand so it isn't frozen. My conclusion is that I have a bad passenger side fan motor. However, because the jerk still happens, when the left most relay is installed, but the fan is disconnected, I conclude that the passenger fan has nothing to do with my jerking problem.
So now I am really puzzled and am grasping at straws for how to go forward.

Any help is appreciated........
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