weirdest thing

ok so, i just got my 87 Cherokee out of the shop with a new brake booster. the old booster was shot and had an air leak that you could hear while driving. anyways, my Jeep drives totally different now than before i took it in. the shift points seem to be off. i tested my theory on a big hill on the freeway. before the booster installation, i could pull the hill at 65mph at about 2000-2100 rpm. and when i pushed on the pedal, the tach only moved slightly (like the trans was locked in overdrive kinda) if i pressed on it hard, then it would shift in 3rd, but i could still pull the hill no problem in overdrive going 65. now, after the new booster, the air leak is gone and when i tried that same hill, my Jeep was running 2500-2800 rpm. this is the strangest thing because ive never heard of this before. my buddy's 90 Cherokee drives the same way. another friend's drives more like my Jeep used to, but his is a 91. i also noticed driving it home that it didn't shift into 3rd gear where it normally does (35mph). could there be something else wrong or is this just how Renix 4.0's drive? im going to check the tranny fluid right now.


by the way: 87 Cherokee Laredo, Renix 4.0/AW4/stock ride height.

could be the tps. my brother has a 91 limited and he had the same problem and it was the tps.

the TPS? interesting. so you think it could be bad. did your brother's Jeep's shift points change after replacement?

Ya it did. It shifted at really hi rpms. Tested the tps and it was bad. I grab xtra tps when I go to pull aprt.

thank you very much for the info. ill grab another TPS. im heading t othe junkyard soon anyways. :clap:

dont forget you had a vacuum leak before the booster was replaced. now you are getting air fuel mixture readings. probably a leaner mixture now

Maybe you want to readjust your transmission TV cable. Or maybe it was ould of adjustment before and they adjusted it at the shop?

I would also check the switches on the brake pedal. There's a switch there that tells the tranny computer to unlock the torque converter which would run the rpms a bit higher. If it's like the 89, it's a white switch way up near the top of the brake pedal arm. Could need adjusted or cleaned if its gunked up.

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jneary said:

dont forget you had a vacuum leak before the booster was replaced. now you are getting air fuel mixture readings. probably a leaner mixture now

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meaning? the vaccum leak could cause the trans to act differently?

Blaine B. said:

Maybe you want to readjust your transmission TV cable. Or maybe it was ould of adjustment before and they adjusted it at the shop?

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hmmm, good point. i thin ill call them and ask.

lawsoncl said:

I would also check the switches on the brake pedal. There's a switch there that tells the tranny computer to unlock the torque converter which would run the rpms a bit higher. If it's like the 89, it's a white switch way up near the top of the brake pedal arm. Could need adjusted or cleaned if its gunked up.

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ill be sure and check this out, it sounds like possibly the cure to my problem. thanks.

so for a little update, i was driving it today and normally it idles at about 900-1000 rpm in park and about 600-700 rpm at stop lights. however, im leaning towards changing TPS because at random after i got home, the idle jumped to about 1400 rpm in park. what do you guys think?

jneary said:

dont forget you had a vacuum leak before the booster was replaced. now you are getting air fuel mixture readings. probably a leaner mixture now

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I agree with this statement! UNhook the hose going to the booster while at idle and see if it goes back to running like it used to.

jneary said:

dont forget you had a vacuum leak before the booster was replaced. now you are getting air fuel mixture readings. probably a leaner mixture now

Click to expand...

I agree with this statement! UNhook the hose going to the booster while at idle and see if it goes back to running like it used to.

lowrange2 said:

I agree with this statement! UNhook the hose going to the booster while at idle and see if it goes back to running like it used to.

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i did this today. My Jeep's idle jumped from 900-1500rpm in park. i drove for about a mile. the shift points are the same before i disconnected the hose from the booster. so i gave it a vaccum leak and it seemed to drive the same except it had less power and the brakes felt like manual brakes. the next thing i gotta try is the TPS.

so, it didn't realy run like it used to. it had a higher idle after i disconnected the hose and the shift points remained the same. i'll check on top of the brake pedal next.

The shift points and t/c lockup are determined by the tranny computer. It's sensors are the output shaft speed , input shaft speed (only on 98 and up), TPS sensor, and the switch on the brake pedal. I can't see how a vacumn leak would cause unusual shifting.

lawsoncl said:

The shift points and t/c lockup are determined by the tranny computer. It's sensors are the output shaft speed , input shaft speed (only on 98 and up), TPS sensor, and the switch on the brake pedal. I can't see how a vacumn leak would cause unusual shifting.

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yeah i know, i can't either. today im going to take a look at the brake pedal switch and probably replace the TPS.

i can't seem to find the switch on the brake pedal. i tried a TPS off of a buddy's parts Jeep. is it up on top where the booster connects t othe pedal or higher or lower? im just trying to get this problem solved because its ruining my gas mileage. im going to buy a new TPS if i can't solve the problem with the switch on the brake pedal. any help is much appreciated. thanks.

It's about 6 inches above the brake pedal. A black box.