Mysterious Tranny gremlins

[calicojax]

First, 99 supercharged 3.4 automatic

At highway speeds, it feels as though the transmission is slipping. The engine randomly jumps up three hundred rpm. It won't slip under any other conditions. And it won't slip more than three hundred rpm. I started to think the torque converter. Then I turned off the ECT power and it also turned off the overdrive. The overdrive button won't function. Turned on the ECT and overdrive stayed off until I restarted the engine. The overdrive button works unless you mess around with ect.
So now I'm leaning towards the PCM. Has anyone else had this happen. Or have I been hacked by the Russians?

 

[alumarine]

If it's jumping up and down by 300 rpm it sounds like your torque converter is unlocking and locking.

They will do that if there's inclines. If it does it too much it will heat up the fluid. What are your trans fluid temps? If you have an OBDII reader you can check.

Is this a new thing?
Any gear changes or bigger tires?

 

[gamefreakgc]

Highway speeds as in you're driving steady and it slips around? Or you go on the gas and it'll float up 300 RPM and not produce any power?

Sounds like your transmission is on its way out. For us supercharged folks, the transmission likes to stay in very low RPM's since we don't need a ton of throttle to get power.

 

[Bad Luck]

I second torque converter locking/unlocking. OD switch and ect switch positions may be normal operation. If you describe what you have turned on and won't work a little more clearly for me then I would be able to compare with mine.

 

[calicojax]

Cruising at 65mph, flat ground. Rpm is 2300 then randomly jumps to 2600 for a second or so. The trans temp is 155-160
With ect off... overdrive switches on and off with o/d button
With ect on.... overdrive switches on/off w/button ...until you switch off ect. Switching off ect also turns off o/d, regardless of overdrive button position. Switch ect back on and overdrive stays off regardless of o/d button position until I restart.
Today my elocker light came on by itself for a few minutes and then went out.
I'm leaning towards an electric short, bad PCM maybe

 

[gamefreakgc]

Cruising at 65mph, flat ground. Rpm is 2300 then randomly jumps to 2600 for a second or so. The trans temp is 155-160
With ect off... overdrive switches on and off with o/d button
With ect on.... overdrive switches on/off w/button ...until you switch off ect. Switching off ect also turns off o/d, regardless of overdrive button position. Switch ect back on and overdrive stays off regardless of o/d button position until I restart.
Today my elocker light came on by itself for a few minutes and then went out.
I'm leaning towards an electric short, bad PCM maybe

Yeah that's not normal operation. I can turn O/D on or off regardless of ECT being on or not. ECT should not turn off overdrive.

Is your e-locker original or was it spliced in? I'd look at wiring first and then see if swapping out the ECU solves the issue. I agree though, now that you describe it a bit more it sounds more like an electrical issue than a mechanical transmission issue. I thought at first you meant you'd go on the gas and the RPM's would rise but nothing would happen.

 

[Bad Luck]

I understand now. Thanks for clarifying, and that's not normal operation for me.

 

[calicojax]

Okay a little follow up. The surging at highway speed turned out to be a bad tps. How ever the ect button interfering with overdrive still has me stumped besides being trans. Related what does one have to do with the other? :redbrick:

 

[gamefreakgc]

Okay a little follow up. The surging at highway speed turned out to be a bad tps. How ever the ect button interfering with overdrive still has me stumped besides being trans. Related what does one have to do with the other? :redbrick:

The ECT Power button raises the RPM shift point by a few hundred under moderate throttle. Even higher under heavy throttle, so it very much affects the transmission operation. Now HOW it raises the shift points I'm not sure, I'm not very well versed in automatic transmission repairs. They are made of unicorns and pixie dust.