Not switching gears after security system install

Hi,
Have a big problem, hoping someone may have some wisdom to share.
I had a security system installed yesterday with remote engine start (in a 91' XJ). After the system was installed, the vehicle would not automatically switch gears. It basically starts out in (Drive), and creeps along until it gets up to speed, then runs fine, except it won't switch down gears again or pop into passing gear. Manually gears work fine.
Installers (Best Buy), disconnected the system but problem is still happening. Best Buy said it could brought into a Jeep dealer, and if they find the problem was caused by the install they would take care of it, other wise it would be my problem. I wasn't having this problem before bringing it in, it seems strange that it would just start happening while sitting in a garage while the system was being installed without it being related to the install.
Any clues what may cause this problem? Electronically, is there something they could have effected the would tell the XJ not to switch gears. I would appreciate any feedback before the dealer makes their deciesion.
Thanks

Did you try moving the manual lever to a position other than Drive? If they cut a power feed to the TCU, which is mounted on the pass. side lower dash, the trans goes into limp and all you will have is D(third gear). If that is the case you should still be able to get other gears by manually selecting them.

POOF!!! and then there was smoke! Sounds to me like they either let the "magic smoke" out of one of the control modules, or maybe just blew a fuse. I'd test the fuses first. That's the problem with going to Hack shops like best buy, tweeter, etc...(95% of shops out there I consider hacks, but im a install freak) Hope that you can get them to fess up to their mistake(may be dificult) But like I said- start with all the fuses, many times the shops will still use testlights instead of volt meters and might have tripped a fuse, or popped something in one of the modules. Im not particularly familiar with the older XJ's, but this would be my best guess. Good Luck. --Chad--

Yes, I have reverse (D), and gears 1 & 2 manually. Basically once in drive, it limps along until it builds up speed, then runs nornally at nay spped...until I have to stop again. Is this the signs you are talking about? What do you mean by "TCU"?

explorer said:

Did you try moving the manual lever to a position other than Drive? If they cut a power feed to the TCU, which is mounted on the pass. side lower dash, the trans goes into limp and all you will have is D(third gear). If that is the case you should still be able to get other gears by manually selecting them.

Click to expand...

Trans control unit. Its the module that controls the shifting of your trans. If it loses power, It goes into limp mode. which gives you exactly what your experiencing. They either cut a wire, or more likely, as the audio guru said popped a fuse. The TCU has a fuse in the harness near the module or on the module itself. It's been a while since I worked on one so I can't remember. But it could also lose power if they blew another fuse in the main block that feeds that circuit.

The TCU is the transmission control unit, a seperate computer, to control the tranny. It´s behind the curved bottom part of the dash on the passengers side. Mine is black and about the size of a pack of cigarettes. The power for the TCU on my 88, is an inline fuse, in the harness for the TCU, near the TCU. I´ve never had to replace the fuse, but did notice it, when I had my glovebox out. Don´t know for sure, if the locations are the same for your 91 or not.
There is another fuse for the TCU in the fuse block, your 91 is probably different than my 88, but on my 88, the TCU fuse in the normal fuse block, shares a circuit with the back-up lights.