Vibration in 1st and 2nd

Hey guys,

A couple months ago this forum helped me out when my 96 wouldn't turn over. After a lot of searching I finally found a new (to me) xj in my price range. The new one is a 90 limited with a reman remix 4.0 in it with the original AW4. For the most part it is great, especially for the $1000 I paid, but there is one issue that is evading me. I have checked other posts, but none of them seem to be quite the same problem that I am having.

Sometimes when it is in 1st and 2nd gear the entire Jeep vibrates. As soon as it hits 3rd it is smooth as can be. It also doesn't happen every time. On cool, humid mornings it either doesn't do it at all or only a little. Hot, dry mornings are the worst.

I have checked the u-joints and they are good. I also replaced the transmission and engine mounts. They needed to be done anyway, but did little to nothing for this issue.

I suspect it is something worn out in the transmission (it does have 260k miles on it after all), but I was wondering if there was anything else that I should check first? A new transmission is in the plan, but it is something that I would rather put off as long as possible.

One more thing, it only does it when accelerating. When coasting there is no vibration.

Too simply declare that the transmission is faulty with no diagnostics is kinda rash.


Is the AW-4 full of Dexron-III/Mercon and not ATF ? Yes it does matter, there a plenty of reports of ATF causing AW-4 shifting issues. Are all of the tune-up parts top quality parts and fresh ? Tested the ignition coil ( that can be affected by moisture ) ? Consider swapping a junkyard Transmission Control Module before you pull the transmission.

What diagnostics have you performed ? : engine sensors, wiring, transmission solenoids, fuel pressure, etc. ? There is is a whole separate service manual for AW-4 diagnostics you can find with some searching on-line. Have you checkced the setting on the Throttle valve cable ? Be especially suspicious of the workmanship of whoever replaced the engine. Look for misplaced wires that are chafing on sharp metal or melting on hot parts. Did the mechanic mistreat the wire harness and separate any of the dozens of wire splices under the hood ? Are there any wire plugs with corrosion or bent/pushed back wire pins ? Are all the engine sensors genuine Jeep parts or crappy out of tolerance Chinese clone parts ? It is well known that auto parts store engine sensors are often faulty right out of the box. You should start with the basics and go thru all the systems, testing and inspecting wires, wire plugs, ground points, fuses, relays, sensors, fluid pressures. I would also look at the leaf springs and bushings to be sure there is no spring wrap or excessive drive line angles from saggy leaf springs.

When I bought it I did drain the old fluid and replace it with dex/Merc since I didn't know what was in it and I don't trust POs in general.

I have combed through the wiring harness while chasing other issues that I have since fixed and it seemed good.

I know that it is bleeding off fuel pressure somewhere because it has a long crank after sitting that can be somewhat mitigated by turning on the ignition for a few seconds first to prime the fuel pump. I suspect that is the drain back valve in the fuel pump since there is no fuel in the engine bay and I don't smell fuel in my oil, but I haven't gotten to troubleshooting that yet. Once it is running though it seems fine.

The plug wires are ac Delco and the plugs look good. When I change the filter and gasket for the transmission I will swap the solenoids too. I have an almost new set in my 96, just haven't been able to get up to my brother's since the coronavirus started to pull any parts.

The main reasons that I suspect the transmission is that there is a slight slip sometimes, but it shifts on point every time. Shifting is smooth for the most part and the slipping tends to happen when it is already in gear. It is not a bad slip, but I can feel when it happens and the rpms go up a little. It idles as well as can be expected with stock fuel injectors (also gonna pull the neon injectors from my 96) and runs perfectly smooth as soon as it hits 3rd.

I will look into testing the coil this weekend and see how it looks.