Dear Jim,

My 1998 Isuzu Trooper (3.5L, V6) is giving me some transmission trouble (it's a 4L30E model automatic). Before the engine has warmed up, everything's fine. When the engine gets hot, though, I completely lose the reverse gear.

Also, the lockup mechanism kicks in when I shift up into second gear. It's not slipping gears before that, so why is this only happening once the engine is warmed up?

-- Nathan Chao

That's a fairly technical question, bro, so I'll need to first tell our readers that the "lockup mechanism" you mentioned is usually a good thing, not a malfunction.

It's a fuel-efficiency feature that locks up the torque converter and sends engine output straight to the transmission. Whenever you feel a four-speed automatic transmission kick into that "fifth" overdrive gear, that's the lockup feature in action.

Lockup is controlled by the car computer.

We've talked about computer problems in this column before, but the trouble you're having probably isn't the computer's fault. There's a part called the lockup solenoid, and it's just a little electrical valve on the side of the transmission.

When your car wants overdrive, the computer tells the solenoid valve to turn on and inhibit the torque converter. Once you slow down, the computer tells the valve to turn off and let the torque converter do its thing. On your Isuzu, it sounds like the hotter driving temperature is making that lockup solenoid stick in the "on" position.

That's no good. It means the lockup feature is staying on when the vehicle doesn't need it and, yes, it also means the transmission is prevented from going into reverse.

Next time this happens and you lose the reverse gear, have someone check the electrical signal going from the computer to the solenoid. If that signal is there, then the computer is fine but the valve is sticking.

You'll need a new one. Replacement should cost around $70 -- a little bit better than, oh, two grand for a new transmission. And be warned, brother, because some shops will be glad to do that full transmission replacement for customers with the exact same problem you're having.

Tip of the Week:

Drivers, get your transmission serviced every 20,000 miles or so, but check the transmission fluid level more often. If you're having to top it off frequently, you most likely have a leaky seal or gasket. Get it fixed fast, because excess heat is murder on the whole unit, especially that torque converter we've been talking about.

Buffalo Jim, owner of Allstate Auto & Marine Repair in Las Vegas, writes a weekly column. Send your automotive questions to him at buffalojim@lasvegasniftynickel.com. If Jim picks your question to answer in Nuts & Bolts, you'll also be the proud recipient of a deck of official Buffalo Jim playing cards, presented in a classy, tin box bearing his hairy image.