2nd Gen Non-Engine/Transmission Vacuum Supply A/C Heater Controls

[BMelhuish]

I have a weird issue. Last week all of the AC started coming out of the defroster vents. I was on a trip pulling my 5th wheel so I was limited in what I could do. Sometimes it would work and sometimes it wouldn't. Sometimes it would not work on start up and then start working. When I parked the trailer at the campground, it was not working. That lasted the remainder of the day. I checked all lines, and they seemed ok. My intent was to check the speed control on the way to the my hunting area the next morning. But, the next morning the AC controls worked properly and have worked properly ever since. It has been 2 days now. I seems to me that the lines either leak or they don't, the actuator either works or it doesn't, the control switch either works or it doesn't, and the pump is either providing vacuum or not. Sticking check valve? I can start checking all, but I'd rather have a better idea of where to start. Ideas.

 

[fest3er]

Defrost is the default when there is no vacuum. Check the connection on the vacuum pump (down by the P/S pump). That 'elbow' can rot over the years. I found a replacement at Advance Auto and mine's been OK for a few years now.

 

[BMelhuish]

I plan on double checking all the vacuum lines this weekend. But again the issue is intermittent. Thanks

 

[BigPapa]

Defrost is the default when there is no vacuum. Check the connection on the vacuum pump (down by the P/S pump). That 'elbow' can rot over the years. I found a replacement at Advance Auto and mine's been OK for a few years now.

I agree with this.

 

[Missouri Mule]

Sometimes the line under the dash can get pinched or may be somthing is plugging it up. If it is like or is 94 thru 98 12 valve the vaccum line is very close to the exhaust manifold it then goes into firewall near heater hoses.

 

[BMelhuish]

All the lines look just fine. It's been running normally for 5 days now. Speed control works as it should, and all the AC modes work too, dash, floor and defrost. Control switch? No idea. I guess I'll need to wait until it becomes a problem again. Of course it will happen on a 95 degree day and in a place where I can do little to fix it. Thanks for your help.

 

[CCCRAM]

Hello your post helped me. I just had my truck in for service on a different issue. That problem was resolved, but then I noticed I only had air blowing from the defroster vent. Got on to TDR and ran a search and came across your post and several others. Went back out to the truck and found the vacuum line running from one side of engine compartment to other broken at midpoint. I suspect the diesel shop did it when the removed the valve cover on the engine. I taped it together temporarily with duct tape and it works. Will get new line from dealership to replace.

Am grateful we have great folks like you all to refer when we have a problem. TDR fixes my AGAIN (for the eighth time!)

 

[GAmes]

. I taped it together temporarily with duct tape and it works.

I use electrical heat shrink tubing, a couple layers for strength. My "temp" fixes have lasted several years.

 

[petersonj]

No idea. I guess I'll need to wait until it becomes a problem again.

If you have access to a hand operated vacuum pump, you can probably find the leak. Follow the main line from the vacuum pump to tee connections (typically on the firewall) where lines branch off to other circuits (HVAC, front axle lock, cruise, etc). Test individual circuits with the hand operated vacuum pump. When you stop pumping, the vacuum gauge should hold steady, if not you have a leak in that circuit.

The engine driven vacuum pump is a fairly strong pump. The vacuum system can have a significant leak and the vacuum pump can easily keep up leading you to think that everything is fine.

- John

 

[CCCRAM]

I use electrical heat shrink tubing, a couple layers for strength. My "temp" fixes have lasted several years.

Thanks GAmes! I was thinking that might work, but your post confirms it, so I'll give it a try this weekend.

 

[BigPapa]

A piece of vacuum hose or even windshield washer hose would work, also. You could even put shrink tubing over that for added reinforcement if you like.