98 4.0 Sport, shifting in and out of overdrive/5th

[redfusion]

Posted a response in a different section but it probably belongs here.

Currently on my 98 sport (4wd/ControlTrac) the OD is engaging then quickly disengaging at highway speeds (around 60mph is when it will start presenting itself.) I believe the "speed mismatch" is what is causing my issues with the I recently had a tire that was replaced with a brand new one (same tires all around) and I believe it is the tread depth that is causing the speed mismatch between the axles, engaging the transfer case causing it to hop out of OD. I do not recall this issue ever arising before the new tire, no codes were reported to me after a local trans guy hooked up his scanner and monitored the shifting.

I'm wondering if putting another new tire on the same axle will solve this problem, but I don't know how the speed sensors in the axles are compared (right to left, back to front, both, etc) which signal the computer to activate 4wd. Can the BWM solve this riddle?

 

[PopRichie77]

I have had to do the same thing, also I have ran my spare for thousands of miles which is a different brand tire and neither caused any problems. My tires are stock size, if you are running tires that are not stock size and there is a huge difference between the old and new tire then it might cause the 4X4 to engage, but the trans should still shift ok as the PCM uses a rear output shaft speed sensor for shifting, it does not use the ABS sensors for shifting.

 

[redfusion]

Thanks for the response Rich! The tires are oversized (265/70/16) but I don't believe there is a huge difference between the new tires tread and the "older" ones. The transmission shifts ok. There is also apparently a correlation with the OD issue and going down hills and the amount of throttle applied, Still working on that theory.

 

[PopRichie77]

Why do you think it is going out of OD? Does the OD off light come? If you are just watching the tack then this is whats probably happening. The Torque Converter unlocks when going down hills, if you let up some on the gas and it will lock again when you apply more gas, this is normal, also the trans contains a coast clutch which also lets the tack drop to almost idle when going down hills with no gas applied. There is no engine braking when in drive with OD on going down hills.

 

[redfusion]

Scenario: 50mph letting off the gas completely, rpm's shoot up (as if it downshifts) and will continue to hold the higher rpm until enough throttle is applied to trigger a downshift and then will shift again into high gear, rpm's will then drop to 'normal' for current speed.

Also Rich, the OD light does not flash on indicating it's off, nor does the 4x4 light when experiencing the symptoms mentioned previously.

 

[PopRichie77]

In my opinion, I don't see how the RPMs could go up with your foot off the gas, the first thing that happens is the torque converter unlocks, then if more gas is applied the RPM will go up. It sounds like an engine problem to me. The idle solenoid takes over when you take your foot off the gas and maybe surging open to much, which would cause the engine to rev up. Some kind of vacuum leak might cause this also. I have nothing else to suggest.

 

[redfusion]

I do believe this is more of a transmission/converter, possibly even computer issue, than it is related to the engine. Maybe a solenoid in the transmission? However, I'm not an expert in the least.