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Discussion Starter #1
Over the weekend I replaced the heater core in my car, now the cruise isn't working as it should. If you set the cruise, it immediately speeds up about 5 miles an hour faster than where I set it and then it will slow down and speed back up, repeatedly. I have looked at everything and all I can come up with is either there is a vaccuum leak or the amplifier is crapping out. Any thoughts on this?
 

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Over the weekend I replaced the heater core in my car, now the cruise isn't working as it should. If you set the cruise, it immediately speeds up about 5 miles an hour faster than where I set it and then it will slow down and speed back up, repeatedly. I have looked at everything and all I can come up with is either there is a vaccuum leak or the amplifier is crapping out. Any thoughts on this?
I thought these cars used an electric servo on the cruise control instead of the older vacuum controlled? I'm not positive, but I'm pretty sure.

However, a vac leak could still cause the surging. Also, I had a TPS not functioning correctly on my Mark VIII that caused that kind of behavior though not quite as drastic as what you're describing!
 

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Discussion Starter #3
I replaced the TPS when I bought the car, so its less than a month old. It doesn't surge like that unless you use the cruise. My buddy has a parts car. I may grab the box off of it too, among the other pieces I want.
 

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I replaced the TPS when I bought the car, so its less than a month old. It doesn't surge like that unless you use the cruise. My buddy has a parts car. I may grab the box off of it too, among the other pieces I want.
It's under the driver's side fender - have to pull out the plastic wheel well to get it. I already sold the one off my parts car...
 

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I thought these cars used an electric servo on the cruise control instead of the older vacuum controlled? I'm not positive, but I'm pretty sure.

However, a vac leak could still cause the surging. Also, I had a TPS not functioning correctly on my Mark VIII that caused that kind of behavior though not quite as drastic as what you're describing!
Only the 96/7 MN12s were electronic, all other years use a vacuum servo.
 

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Only the 96/7 MN12s were electronic, all other years use a vacuum servo.
Hey, I learned something today! I knew the one I took out of the 97 parts car wasn't vacuum, just didn't know about earlier years.

Thanks, Matt!
 

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Only the 96/7 MN12s were electronic, all other years use a vacuum servo.
Yah, the 1991 at least sucks ... :diablo:

RwP
 

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Discussion Starter #9
I got it figured out. I checked and rechecked all of the vaccuum lines, and nothing. That led me to recheck the amplifier box under the hvac housing. There was one wire that had gotten pulled out of the harness connector that I hadn't seen. I pushed that back in, and the cruise works.
 
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