what causes auto tranny to shift? computer?

ok since my last question didn't generate any responses let's try again!

i pulled my steering column apart the other day and now my jeep won't shift when i put it in drive...it's like it's stuck in 4th?

is there a fuse or something in the computer/wiring that tells the thing to shift? i messed with the intermittent wiper/cruise control switch -could that affect transmission at all???

i have an 89 cherokee 4.0L...rebuilt trans that i just screwed up AGAIN!

Had a problem in the same general area as you did, found a solution to my problem, but remember surfing through sites related to diagnostics and remember a site where they did a step by step, and eventually traced it down to a blue wire off the cruise control shorting to ground in the colunm. been looking around for it, wish i saved it in favs. At least it does sorta answer your question- cruise control can affect shifting. Wish ya luck bro

thanks man! i'll give that a shot...

Ok...duh

When I pulled the column apart I also pulled the lower dash piece off...it had a box that looked like it was part of the A/C attached to it on the passenger side.

Well, after tracing my cruise control wiring harness out the fire wall I found it runs around the engine compartment and back in on the passenger side where it connects to that A/C box thingy! So I put the lower dash piece back on, plugged in the harness to the aforementioned box thingy and viola my transmission shifts again!

Weird...

Oh...and I found out I don't actually have cruise control...the guy at jeep thought my turn signal stalk had been added later and it had the cruise control stuff on it and a harness but I don't have the cruise module/servo whatever...hey but I'm learning a lot!

That little box thingie you found is the transmission control module. If you connect a SPST switch to the yellow wire going into the module you can switch the tranny from an automatic to a manual and then it will shift from the shifter and stay in the gear selected. This works good for off road stuff. Carefull staying in one gear to long can cause the trans to overheat. When done using one gear then hit the switch and its an auto again.

The yellow wire is the power wire for the module so switching this wire just turns off the module.

is that how the overdrive works? over-riding the TCM? maybe a dumb question - but i know next to nothing about the TCM :-)

it's fucking magic man!! the little elves ! do it

it's fucking magic man!! the little elves ! do it

an auspicious first post, dude

is that how the overdrive works? over-riding the TCM? maybe a dumb question - but i know next to nothing about the TCM :-)

The module is connected to the transmission (duuuu), the power/comfort switch, and the main computer (ECM). The ECM tells the module (TCM) what the engine is doing (RPM, Load, manifold pressure, etc.) and the module controls gear selection and torque converter lock-up to keep the engine RPM's at the lowest point while still remaining in the engines' torque curve according to a pre-set set of operating peramiters.

The power/comfort switch allows two differant sets of peramiters for the transmission to operate within.

The concept is to get the best fuel economy and the smothest ride.

The transmission is a four speed auto with a lock-up torque converter. Lock-up happens in third and fourth gear when manifold preasure falls below a certain threashold and engine RPM can still remain above 1500 RPM at the determined speed and throttle position. (when you let off on the gas and cruse)

When manifold preasure and throttle position rise beyound a certian threashold (when you punch it) the torque converter un-locks and torque converter stall speed is achieved and/or the transmission down-shifts untill the aformentioned condition presents itself again (you let back off on the gas).

When the throttle is floored (WOT) the shift cable overides the TCM and the transmission downshifts to the lowest possible gear.

When you apply the brake. The brake light switch also tells the TCM to un-lock the torque converter. This also happens when you let off on the gas and the throttle Position sensor (TPS) senses an idle condition. This allows the torque converter to become a normal converter and reverts to its stall speed.

What this amounts to is a trans that is always searching for the right spot (and generates a lot of heat) and you'r engine is always lugging.

45 MPH is the transmissions worst speed. It goes in and out of lock-up and just can't settle in.

45 and below, around town, and hilly conditions use the third gear position and you wont get all of that lock-up crap. Above 45 put it in over drive.

Personally I'll take an old fashion turbo 400 tranny with a vacume controlled shift module any day.

We can thank the EPA and CAFE regulations for these electronicly controlled transmissions that keep our engines lugging along at 1500 RPM while going down the freeway, cost a fortune to replace, and nobody can figure out how to fix when they screw up.

I hope this explains it.

I wish it was magic.

Thanks, BC - excellent post.

Thanks, BC - excellent post.


Any time... :smokin:

45 MPH is the transmissions worst speed. It goes in and out of lock-up and just can't settle in.

45 and below, around town, and hilly conditions use the third gear position and you wont get all of that lock-up crap. Above 45 put it in over drive.

I hope this explains it.

I wish it was magic.

WOW! Great Post. That explains a lot. My '88 Cherokee seems to hunt for the right gear around 45 all the time. I thought something was "broken". Too bad all of the town speed limits are 45.