2nd Gen Non-Engine/Transmission Sporatic heater control

[AWadsley]

I have a 99 3500 cab-chassis with 178k miles. While going down road with either vent or air conditioner on the heat will kick in on it's own for 5-10 seconds and then turn off again, this will do this about once a mile. It does not seem to matter if there is a light or heavy load, fast speed or slow. A problem was first starting to apear when running the ac and there was not good control over temperature of the output, the heat was either all the way on or all the way off. I was suspecting a faulty vac pump but the local dealer said that the cruise control also works from vacum and if the vac pump was not working reliably he did not think that the cruise would work reliably either and the cruise works flaulesly. Any ideas?

Al

 

[DFerverda]

Check the vacuum lines. One big on from the pump and goes to the firewall. From there one will go to the cruise control(under drivers side battery box. ) The other run on the firewall will go to the cad, if 4x4 and the end will be on the passenger side firewall and go inside the cab about the middle of the glove box.
Check it there for vacuum, if it is working it is most likely a door problem inside the heat/ac box behind the glove box.
If the air will switch from the defrost to the middle, vacuum is probably working. Without vacuum it defaults to defrost and on the floor with no air coming out in the middle.
Also check the level of the coolant, it will cause weird stuff if it is low.
Hope this helps.
Dirk

 

[SSolderitch]

mine does the ac cycle in vent high/low - 3rd position.

note my controls has 2 ac snowflakes and older controls have 3 ac snowflakes.



since I bought the wrong control uses I swaped out the vacuum switch, didn't help.