When I first bought my 82 300SD the cruise would work occasionally, now it will only accelerate, but not hold the speed. What is usually the cause for this? It would be nice to have the cruise working, but I will not pay the prices for the new parts

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1971 R600 Mack toterhome (SuperDawg)
99 NU-WA Snowbird 34+2 SE 102 5er
If it's not diesel, it's not in my stable!
53 F100 617.951/auto
95 Dodge 3500 Cummins/5 speed service truck