Ask the Mechanic

Transient Transmission Problem

(Steve Thompson)

READER: I know this sounds crazy, but I have a 1998 Dodge Cummins with an automatic transmission. I took it in to get the oil changed, and after that, the transmission wanted to shift up and down a gear very quickly all the time when cruising down the road.

If the engine was under any load, it would not do that trick. When it started shifting up and down, if I accelerated any at all, it would stop shifting until it leveled out again.

The truck has more than 300,000 miles on it, so I thought it was time for a transmission rebuild. But, I was told by a guy working on a truck under a shade tree that I should take a look at the big black wire running behind (not attached to) the alternator and move it back away from the back of the alternator. He told me that this would probably fix my problem.

Full of doubt and thinking that guy was making fun of a city slicker, I moved that wire away from the alternator, and I could not believe it. Problem solved. What is going on here?

STEVE: The big black wire behind the alternator goes directly back to the negative post on the battery. It gets pushed over near the alternator when the air filter hose from the air cleaner to the turbo is removed to make room to change the oil filter and check/change the air filter.

That happened to one of my Dodge trucks one time, and when I was told the "shade tree fix," I, like you, could not believe it fixed my shifting problem. I have included a photo of the "magic" black wire (above) that must be picking up transient voltage from the alternator.

The transmission controller doesn't like transient voltage -- it confuses it. The computer did it. On newer electronic tractors of today, I see a black solenoid component called a terminator. The terminator helps with transient voltage.

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-- Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email mechanic@progressivefarmer.com

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