93 breakaway 33'.Need to find replacement ideas for the dash control without having to replace the entire system.

Using the search page, I knew it was here somewhere. I suspect it may be in this posting by our host Bill NY.

This presents an interesting challenge, one I began to tackle a while back on my 86.

It appears that the HVAC system operates mostly by vacuum control. I say mostly because the heat control is a wire cable running to the hot water valve in the heater loop. Also the fan control is a multipole resistor switch and there is a switch to turn on the AC.

I was faced with vacuum leaks and the system vent controls did not work. Decided to troubleshoot.

This much I know:

There are 4 vacuum dashpots which attach to flap valves in the HVAC system. One dashpot/flap for DEF vents, one for HEAT, one for DASH vents and one for fresh air vs recirculating. Each dashpot has a vacuum line from the dash panel to operate that vent.

The dash panel buttons determine which of the four controls gets a vacuum signal.

With no applied vacuum, all vents are open. This is how it defaults when the vacuum source is lost.

As soon as the system builds vacuum, with the control buttons in the OFF position, the ports are open to the vent dashpots. The vacuum will cause all the vents to be pulled closed. (OFF)

When the panel buttons are pushed in to select a vent, they block the vacuum to the corresponding vent dashpot and the vent returns to the open resting position. In the case of the fresh air flap, when fresh air is selected, the port allows the flap to open.

Sooo...

The control buttons are ported vacuum valves which are supposed to selectively open or close specific vents. Problem was with mine they were leaking all the time and there was not enough vacuum to operate the dashpots to close any vents.

While troubleshooting, I removed vacuum lines from the dash control, one at a time. Applied suction to determine which vent it was and if that leg was leaking. All dashpots still worked but one leg was leaking through a bad vacuum line. Replaced that line, figured good to go.

Unfortunately, the dash control valves were the real problem. They are prone to leakage. Never did figure out a repair, the unit seems to be sealed.

Replacements are rare, pricey and prone to similar failure.

Began to consider a home made "manual control."

Start with a main vacuum line with four teed legs. Each leg has has a port valve and connects to an HVAC vent dashpot. The valves allow independent control of all four dashpots.

I was considering aquarium tubing and valves but that was as far as I got.

You would still need the temp cable, fan switch, and a switch for AC, but the vents could then be used.

As for a vacuum source in a diesel rig, that is another story.

Hope this helps, it is far from a complete answer.

The system is very similar to the one used in our 1978 Mercedes 450SL. Chrysler sold the design to Mercedes in the mid 70s--can't think of any good reason the German's bought it. It is a nightmare. Ours works well since we have a source for dash pots that still work. If I ever had to replace it, I would do so with a unit from Vintage Air. There's a reason for the statement "nature abhors a vacuum"
J

Vintage air (and others) will sell the electronic control part of their systems separately. The control operates the various "doors" in the distribution plenum by 12v electric motors (one for each door. It can be connected to the ears or post on the doors with a little work. Once the "pots" are connected to the motors you replace the hot water valve with their electric one. No more vacuum requirements and hoses behind the dash. They have different styles of head units and you can keep your in-place system. Applying new technology to ancient contraptions solved all my problems.

Excellent solution. No need to reinvent the wheel!

With electric motors you could just wire a simple switch to each motor to select the desired vents.

My Monarch still has vacuum HVAC. The venturi vacuum generator is primitive. I will either install a dedicated vacuum pump or go with the electrical route. Way more reliable.

No need for switches if you buy the head unit. It is just connect the wires and you are done.

Vacuum Pump www.accuratediesel.com
Ford Super Duty/Dodge 5.9L Electric Vacuum Pump Part# L95602
(selected model for heavy duty trucks)
$116.99 plus shipping.

For the original systems to replaced the venture pump.
Mount pump on firewall(somewhere above generator), and wire to 12v and switch on dash.
Plumb pump to vacuum reservoir(black plastic ball), and other side to "pot" that connects to a lever accessible at the bottom of the heater core box(firewall access).

Was previously discussed on an earlier post. I appreciated the info as my vents were "inop".

8K miles on my Breakaway XL this year and the system has performed flawlessly.

25 years with the vacuum system and still working. Not certain the electric system will last that long. The vacuum generator is still working and I have a spare one now that I no longer need vacuum for the cruise control.

Some day I might add an on/off switch for the ac compressor.

My only problem has been the cable going to the hot water line. Gave up and put another shutoff valve in the system. Not very handy but workable, just have to thing ahead.

Thanks for this great info.I have installed an electric vacuum pump and added an extra reservoir.I have one bad dashspot for the recirculation circut.I found some on ebay but this one requires an extra long throw and am still looking for a replacement.It would appear that vacuum systems are becoming more difficult to come by.The vintage air solution sounds like a good way to go.

quote:

Originally posted by Gary Carter:

My only problem has been the cable going to the hot water line. Gave up and put another shutoff valve in the system. Not very handy but workable, just have to think ahead.

I was having trouble with the heating system on another vehicle. I just used a long throttle cable. The cable happens to be for a lawnmower choke control. I put a heat sticker over the "C" for choke.

---

The cable is attached to a shutoff valve I installed in the heater core line.

---

---

It is easy to control the amount of heat I need to from the driver seat. No vacuum is needed to regulate the heat. The cable operated valve is easily obtained at any auto parts store. If your counter person needs a vehicle application, just tell them it is on a 1963 Ford truck heater core valve.

Why is it that sometimes, the simplest solutions are the best solutions? Great idea, Kevin. Think I'll use it on my '78 Mercedes that has a $450 computer that decides how much heat I want.