What could be causing my speedometer to fail and the "check engine light" to come on?

Dear Car Talk

Dear Car Talk | Jul 01, 1999

Dear Tom and Ray:

I have a 1991 Jeep Cherokee with four-wheel drive and the 4.0 liter straight six. I love it. However, occasionally the speedometer fails to register and the "check engine"
light comes on. This usually occurs on the first trip of the day and doesn't seem to affect the driveability of the car. The situation almost always corrects itself when the
car is started the second time. Do you have any idea what causes this? -- James

TOM: No. Next question?

RAY: He's so helpful, isn't he?

TOM: Actually, I'd put money on the vehicle speed sensor (VSS), James. The VSS is an electronic sensor that has two functions in this car. One is to tell the car's
computer how fast the vehicle is moving so the computer can factor vehicle speed into its engine management functions -- like the ignition timing and the fuel-air ratio.

RAY: The second VSS function is to tell the speedometer how fast the car is going. Instead of having a traditional speedometer cable, the speedometer in this car gets its
input directly from the VSS. The car needed a VSS anyway for the engine management system, so why not use the data to feed the speedometer, too?

TOM: If the VSS is faulty, your car's computer should be able to confirm that. When the "check engine" light comes on, the computer stores an error code. Your
mechanic can tap into the computer and see what caused the error. And if the computer says "VSS," you put in a new one.

RAY: And the answer to your next question is "50 bucks," James. That's what the part costs.

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