Replace fan motor causes mph sensor to fail

I have a 96 Cadillac Deville Sedan. Last year, I had problems with the heat and A/C working intermittently. Then both crapped out. I went the summer without A/C…waiting for winter. Have gone all winter with no heat, which is not a huge problem, since I live in Florida. However, I’m planning on taking this car on a 1600 mile journey to Maine. So on Thursday, my bf installed a new HVAC fan motor. Or, yeah, whatever you call them. It worked and the car produced heat. I had a parade to go to Thursday night, so I drove the car there and it was fine. On my way home, the car was revving, like I was driving a lawn mower. Then, while I was still driving at a low speed, the mph went to 0. During the 10 mile drive home, which included interstate, the mph returned and the car acted normal. Then the mph went to 0 again. The speed sensor failure light came on, along with the check engine light.

I thought maybe while replacing the fan motor, the bf might have unhooked the speed sensor. This makes sense to me. But the bf checks and can’t find anything obvious.

Today, when I first start driving the car, it again sounds like a lawnmower. The rmp is into the 3000 and 4000 range, while I am driving (approx) 15 mph. If I step on the gas, the car lurches forward, then returns to lawnmower status. The mph is showing 0. The instant mpg is also remaining at 0. The fuel range is showing numbers, but they are not correct, since there are 14 gals of gas in the car and the range is at like 85 miles and declining. I get around 20 mpg. So that’s not an accurate range. After a few miles, the car stops over-revving and sounds normal. On the interstate entrance ramp, the mph started working, and worked for several minutes, then reverted back to 0.

So, to summarize. The check engine light is on all the time. The speed sensor failure light comes on in the beginning. The struggling lawnmower act lasts for the first few miles. Lurching if I step on the gas. Then, it might drive normally. The mph stays at 0. Every once in a while, the mph will start working, then stop again. What could have happened during this fan motor repair to have caused this? Or is this just a huge coincidence?

I don’t think that replacing the blower motor has anything to do with the problem you’re still having. It would seem from the information you’ve provided that there’s a problem with the Vehicle Speed Sensor. This sensor not only sends a signal to the speedometer to indicate the vehicle speed, but it also sends a signal to the computer. The computer then takes this information and determines such things as when the transmission should shift among other things. If the computer loses the signal from the VSS, and from all indications it is because the speedometer registers 0 MPH, and the speed sensor light comes on, the computer doesn’t know how fast the vehicle is going. The computer will then go into a limp mode and this can cause the engine/transmission to operate erratically.

If you want to confirm if this is the problem, take the vehicle to a parts store such as AutoZone. Some will hook a code reader to vehicle and pull any codes for free. If a code comes up related to the Vehicle Speed Sensor, then that would be the circuit to check out.


Thank you Tester. I appreciate your response and info on what’s happening. The car is having trouble knowing when to shift, so what you’ve said all makes sense.