Sethers775

Hey guys, I have an issue and maybe one of you jeep geniuses can diagnose it for me since my mechanic and I cannot.

When driving around 40-60mph, occasionally i will get a huge thunk from my jeep causing it to stop slow down for a second and then keep going. When this happens my tac and spedometer fall about 10 mph and then pick up, as well as I get a CEL and CHECK GAUGES light for 2 seconds.

This issue seems to only happen at high speeds and on inclines, when im in the 2-3k range on the tac. I suspect its my AW4 tranny going out. I just had it serviced and the guys looking at it said it looked fine, but was extremely low on fluid. After getting serviced theres less instances of the issue but its still occasional.

The jeep has 194k on the odo so this is not unexpected, but after hearing that it looks fine I am curious to know what the heck is causing it.

peep

You may not believe me.
It's not the trans.
It's the CPS, crank position sensor. Failing.
Some times I would just get a good thunk, and of course it carries on. Used to think I ran over something. Eventually longer shut downs. Fuel pump was a major suspect as symptoms are same. Sometimes I thought it was the brake drums, or the rear diff. It was puzzling????
I chased it on the XJ for years, then again the symptom on my ZJ.
My XJ is over 755K km and my ZJ just under 400K km. Change your tranny fluid yearly and your trans filter say every other year for decent service.
For my money, and yours, it's the CPS. Napa's Belden brand CPS's have worked for me.

nujeepguy

that scares me, do you check it occasionally

peep

Check Cruisers Tips for the details of the inspection method.
All the other tips are a great set of procedures too!

Turbo X_J

What is the exact number(s) of the code(s)?

tjwalker

This, this this! The codes must be retrieved as they are invaluable in helping to determine root cause.

Even if the CEL is on only when you have that issue, you will have a "stored code" in the computer which can be retrieved. You'll need a code reader or a scantool to retrieve the code.

Post the exact code number here for comment.

67 GMC

If you go a steady speed for say 20-30 minutes, does it cycle on and off or is it just randomly doing the stumbling/thunking? I had a bad 02 sensor that would cause this problem. I guess it would fool the computer into thinking the fuel/air ratio needing adjusting. Once it hit the maximum adjustment, it would reset to default. It would do that every 3-5 minutes but you could set your watch to it depending on the temp etc. Once I changed the sensor, it never came back.

dave1123

The CPS will set a code that can only be read with a scan tool, not by the key code method. I had this exact same scenario with my 2000 WJ. It would die while driving slowly and fire right back up by itself. I had no CEL, but a scan tool showed "loss of signal from CPS"

Sethers775

Thanks Dave!

jpk

I don't know about a 2000 XJ but my 96 had a bad cps and I diagnosed it with the key on/off method. I got an 11 code through my CEL. I know the 97+ have the digital readout vs analogue for the earlier XJ's. It seems if you could do it on the early XJ's certainly you could on the later ones, I would think. However I've never had any dealings with the later ones.

BlueRidgeMark

Sounds like the engine briefly dying, not a trans problem.

CPS is a prime suspect.