A true story from someone who should KNOW better
There's this 1999 Mercury Sable. When the car shifts from 2nd to 3rd gear at around 30MPH, the headlights dim and the blower motor slows down to half speed. If you take your foot off the gas at highway speed (60MPH), it feels like the car is shifting out of overdrive--car immediately starts slowing down.
The owner (Rick) cleans the battery terminals and all the ground connections to eliminate electrical as an issue. Convinced that the 2-3 shift solenoid in the transmission is drawing too much current, he calls the transmission shop to set up an appointment. To get the side pan off the transmission, the shop will have to lower the engine cradle. The labor and new solenoids are gonna cost Rick about $500.
At the last minute, Rick decides to spend the $35 and call Ford tech support. The Ford expert says it doesn't sound like a shift solenoid because the computer would have caught that and set a trouble code (Ford computers excel at detecting transmission faults he says). He thinks it could be a slipping serpentine belt.
Huh? Why would it do that only at 30MPH?
Well, Rick, that's when the engine is revving up to about 2,300 RPM. Then, after the 2-3 shift, it drops about 600 RPM. So the belt is under the most stress (and likely to slip) right before that shift. The expert suggests I look at the belt and pulleys to check for glazing and or worn pulleys.
$97 dollars later, Rick has a new belt tensioner, idler pulley, and a new belt. The bearing in the idler pulley was so tight that the belt was slipping across it--it was even wearing grooves in the pulley as the belt slipped across it. So Rick pulled out his serpentine belt tool and rotated the tensioner. Then he slipped off the belt and let go of the tensioner. It took a few seconds for it to spring back.
Turns out, the belt tensioner was rusted and would stick when compressed. So the slow pulley caused the belt tensioner to rotate. Instead of springing back, it would stay there until the RPMs went down. So I wasn't really feeling it drop out of overdrive. It was just the tensioner springing back into place and putting the load back on the tight idler and alternator.
New parts installed and test drive shows the problem gone.
Morale of the story? If you're going to throw parts at the problem instead of diagnosing things properly, you should at least start with the least expensive parts first.
Transmission appointment canceled.
Rick embarrassed that he didn't check the belt first.
P.S. The back of the belt was glazed like a mirror, the ribs were fine. I changed that belt about 1 year ago.