Have owned an 83 300TD with 350,000 miles on it for about 14 years. The air quits about once a year. Recharging the A/C has fixed the problem in the past but this year the heat also failed when at road speed--it works fine below 30m.p.h. and some of the pushbuttons do not turn on until some time after being engaged. Is there a way of testing the controles one at a time to determine which to replace without buying an entire system?

Also, the A/C seems to be a weakness in this vintage deisel. Is there a conversion to a more dependable heating and A/C system? I once owned a 78 240D with the wheel temperature controle on the dash. It never failed nor did it ever need recharging. Could that be a solution? They look like a bold in job.
Thanks