90 XJ shuddering under load?

Hi everyone!

New Cherokee owner here. I just picked up my new toy at the weekend and I am looking for advice please. When driving at between 35 and 45mph it shifts into overdrive, rpms drop to about 1000 and then immediately starts shuddering quite violently. This only occurs under load (even light throttle), it does not do it if I take my foot off the gas. Any ideas what the cause could be?

Oh yeah, the specs... its a 1990 XJ, 4.0L inline 6, AW4 automatic transmission, np242 transfer case and has 255k miles on it.

Thanks in advance for the help!

Rob.

First thing check your transmission and engine mounts.

When did it last have a full tune up including plugs, cap, rotor, and wires? Also, just for fun put the trans switch, located on dash to right of steering column in POWER mode.

Fouled sparkplug did the same on my '87 pioneer.

bad plugs and wires caused the same problem in my 93.

Thanks for all the suggestions!

Its due for new a tune up this weekend. I've only had the plugs out so far and there were 3 different types, one of which was completely fouled! I'll be checking the engine and trans mounts too.

Switching the trans switch over to POWER doesn't seem to make a difference, although I'm not even sure if the switch is working as yet.

I read my original post and thought I'd clarify that it shudders in overdrive no matter what the revs are. No shuddering at all in lower gears.

I'm not sure I understand how the engine running rough could only show up in the one gear whilst the rest are fine over the rev range? Can anyone educate me?

You need to determine IF the engine is running rough. Kind of sounds stupid, doesn't it? Chrysler FSM--first step in determining driveability issues is to ELIMINATE the AW4 as the source.

Unplug the TCU--under the dash, passenger's side, small box with a plug attached. There is also a yellow wire with a 10 amp in-line fuse running to the TCU, you can just pull that fuse instead of unplugging the box.

Drive it shifting manually. You will have 1st in 1-2; 3rd in 3; 4th in D; reverse, neutral, park. If you don't get that result, you have a mechanical/hydraulic problem inside the trans. Also, if the shuddering does NOT present, then the problem is electrical.

Post back your results.

Its kinda busy round here so my plans to look at the weekend turned into next month! Anyway, I pulled the TCU fuse as suggested and it finds all gears exactly as you described, Joe. When I shift into D there is no shuddering so I guess it must be electrical. Any suggestions as to what my next step should be? Would this most likely point to the solenoids?

Thanks!
Rob

Could be the brake switch, the TCU, the solenoids, the wiring--at least you know it is PROBABLY electrical in nature. I said probably because there is a chance the lockup in the torque converter is going out, but lets save that for last.

Here is a great link to a website for testing the AW4: http://www.transonline.com/transdigest/magazines/1997-10/Shift Pointers/index.html

Keep us updated.