Paul Brand: Remedy for a 'limping' transmission

Q I own a 1998 Ford Escort SE station wagon with an automatic transmission. While driving at freeway speeds, the transmission will drop out of overdrive, causing the engine to rev up, and the "check engine" light comes on. I have found that if I stop the car and shift from drive to park and back into drive, or turn the ignition off and then back on, the problem is resolved for a while.

I have talked with two dealers and two local transmission shops, and they told me to bring the car in when it acts up and they can diagnose the problem. The problem with that is when I shut the car off, or shift into park, it eliminates the problem. I have had the codes checked and was told the mechanics didn't find anything that would affect the transmission.

I think it is an electrical problem, but I don't know where to look. The car has 107,000 miles on it, and it runs great.

I tend to agree that the problem is electrical, not mechanical or hydraulic. You appear to be experiencing the transmission dropping into the "limp" mode of operation. If the transmission or engine management system senses an issue with transmission operation, it will eliminate overdrive, raise the hydraulic pressure in the transmission and turn on the "check engine" light.

My Alldata automotive database identified service bulletin #98-21-13 that deals with an intermittent shorting of the transmission fluid temperature sensor wire inside the transmission. During normal operation, if this sensor registers high transmission-fluid operating temperatures -- to the point of endangering the transmission -- the powertrain control module (PCM) will put the transmission into the "limp" mode as described above. That may be exactly what's happening if this wire is being pinched or shorted intermittently. This fault usually sets a DTC P0712 code, and the repair involves nothing more than dropping the transmission pan, checking the wire for evidence of damage, and insulating the wire with a plastic sleeve.

Ask the dealer or transmission shop to test-drive the vehicle -- with you along for the ride -- with a scan tool plugged into the diagnostic link. If the symptom appears at highway speed, the scan tool should be able to identify the fault -- or any fault code that would trigger the "limp" mode -- as it occurs. It's worth noting that other fault codes from the PCM -- for example, the loss of a speed-sensor signal -- would also put the transmission into the "limp" mode.

Q I have a motor home with a GM 4L80E automatic transmission. I have a temperature gauge that I believe has the sender in the line going to the oil cooler. The temp will often get to 250 degrees, and I don't usually let it get higher. Is this excessive heat? Is the length of time it stays at that temp critical? The cooler fan will bring it down right away, but I want the transmission to last a long time.

Ideally, I'd like to see transmission fluid and engine coolant operate at about the same temperature. That's one reason the transmission fluid on most vehicles with automatic transmissions is routed through a chamber in the radiator to cool the fluid.

I remember seeing a graph published by an oil company that showed transmission "life" as a function of operating temperature. With the fluid operating at roughly 180 degrees, the transmission "life" was more than 100,000 miles. If transmission-fluid temperatures operated at 300 degrees, the transmission life was something like 100 miles.

About 250 degrees is higher than I'd like to see the transmission fluid, particularly if it stays at that level for any significant length of time. You're moving a lot of weight with a lot of aerodynamic drag, so you're working the transmission quite hard. An additive such as "Lubegard" might be a good investment to improve the lubricity of the fluid. Even investing in synthetic transmission fluid would be likely to lower its operating temperature. You also might be able to add an additional transmission cooler to further reduce and control fluid temperatures to help prolong the life of the unit.

Q My wife and I do a good bit of driving on back roads up north. We see a lot of moose, but so far we have not hit one. My question is: If I'm driving along at about 40 mph -- pretty much the maximum on twisting dirt roads -- and I hit a moose, will the air bags deploy with enough strength and duration to keep the moose's body from coming through the windshield?

In the more than 20 years I've written this column, this is the most unusual question I think I've ever received. And it deserves a serious answer because an impact with a moose would certainly be a serious crash.

I don't believe that air bag deployment would prevent this massive animal from penetrating the vehicle. The air bags deploy and deflate in just more than one-tenth of a second. The impact with a moose would crush the front end of the vehicle, break the windshield and most likely crush the roof structure downward. All of this damage would take time to occur, probably more than the tenth of a second the air bags are functional. That means the air bags probably would be deflated by the time the animal began to seriously penetrate the passenger compartment.

Also, the driver's air bag is relatively small and is engineered to protect only the driver as the driver's body is thrown forward by the sudden deceleration of the vehicle. It is too small and too close to the driver to be of any value in stopping or slowing a massive animal from penetrating the windshield.

The passenger air bag is considerably larger in order to protect an out-of-position passenger or a middle-seat passenger and is designed to use the inside of the windshield as a "backboard" to rebound off of. But it is not designed to stop a large mass from penetrating the windshield.

All of this is not to say that the air bags would not help reduce injury to you and your wife in a crash, including an impact with a moose, but I do not believe the air bags would prevent penetration of the windshield, the crushing of the roof structure or intrusion into the passenger compartment.

The message? Avoid hitting moose at all costs.

Send questions to Paul Brand, Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488 or paulbrand@startribune.com. Please explain the problem in as much detail as possible and include a daytime phone number in case I need more information. Because of the volume of mail, it isn't always possible to send a personal reply.