tranny acting funny

[tda13]

This is the second time I have tried to post this, so if you see two simular, I appologize.

My tranny in my 97 is acting a bit strange. At highway speeds, sometimes you can hear the tranny making a noise as if it is trying to shift from over drive to Drive. You can hear and see a change in the RPM of about 200 RPM. It ONLY does this when the AC/Defrost is on.

I will probably flush the tranny this weekend. Kinda feels and sounds like a vacuum problem in the tranny

Any ideas.
Tim

 

[unity]

A vacuum problem in the tranny? Oh my, I highly recommend a shop manual .... and some basic knowledge of cars!

It only does it when the AC is on? Then its just he AC engaging, the extra strain would lower RPM, so the car compensates. Even my 73 did this. Its VERY noticeable when the AC clutch engages when parked and idle. The AC does not runs continuous, but fast on-off cycles of the clutch can be indicative of low refrigerant.

 

[poniesviii]

Unity's stating that you don't have a problem.

But suggests you should make sure your AC is recharged.

 

[tda13]

Unity is correct... I do not do transmissions. I have changed two in my Nissan truck, but rebuild we done by someone else.

I never thougt to check the ac refrig. The change in RPM is very noticeable to the ear, but that may be because of the loud exhaust.

 

[tda13]

I do have one question. If it is because the AC, would'nt it do it when it was setting at idle? Would'nt you hear the engine compensate for the compressor? The only time you notice it is when you are driving down the highway at 55.

 

[unity]

Yes you would notice it at idle, but the car is not under load there for less noticeable. You can try it by just turning on MAX AC, then off, then on, etc. If it does it at almost the same time you press buttons, its the AC. It could be a sign that you are low on refrigerant, or a dying compressor. My guess is that its normal. 200 RPM is not a flare up of the trans. Usually is the trans flares, its a lot more than 200 RPM, like if it drops into neutral for a second, etc.

Try playing with the switch and see what results you get. If nothing then there is other things it may be.

 

[segxr7]

I really don't think that drag from the compressor cycling is causing the problem. The serpentine belt would shred before it would ever make enough drag to slow the engine down by 200 rpm at highway speed.

The symptoms sound more like a bad MLPS to me. Fortunately replacing it is a DIY job involving about $30 and 20 minutes. You should be able to find a lot more info on replacing it by searching these forums; it's a common problem.

 

[poniesviii]

I really don't think that drag from the compressor cycling is causing the problem. The serpentine belt would shred before it would ever make enough drag to slow the engine down by 200 rpm at highway speed.

The symptoms sound more like a bad MLPS to me. Fortunately replacing it is a DIY job involving about $30 and 20 minutes. You should be able to find a lot more info on replacing it by searching these forums; it's a common problem.

That's a good point.

Unity sounds to be on the right track since it only happens when the AC is on, that 200 RPM serge is probably a bit of exaggerated speech, but you can see it with the eye, even with a new compressor, if you're staring at the tach as it kicks in. More noticeable on defrost. At least in my car.

If it's not the compressor, the MLPS sensor is a good possibility!