Question: Heater Control Bypass Valve

[Alan Walker]

Hello All. I am new to this forum today and have posted a couple of photos of my C4 to the welcome forum this morning . I have an interesting question. The heater bypass valve located just in front of the heater blower under the hood has failed. I noticed when I started leaving a trail of green behind me coming home from town. The screw attaching the arm to the valve cylinder had come out and that's what the source of the leak was. I did some sleuthing to figure out what this component actually was and what it did and found a replacement for it. A direct replacement was quite expensive. As I live in Canada, with the US to Can exchange it would have been around $250 not incl shipping. I found an alternate one for 1/4 the cost that has the same guts inside but tubed with 4 tubes instead of 3. I can either block off the unused port or swap the guts out (which apparently requires a bit of filing on the plastic valve plug to fit). I am good either way but here is my question. The plug on the original has vanished. It was not there when I opened it up, and there were no debris pieces in the body. I was immediately concerned as I had replaced the heater core last summer and in no way desired to do that exercise again if the plastic crap plugged up the new one!!!! I backflushed the core to see if any pieces were lodged inside but none came out. Where could it have gone and is there anything I need to beware of in the future? This one has me puzzled as the valve plug would have had to disintegrate into pieces small enough to escape the body and get into the lines and I cant find anything! Any ideas?hnoes

 

[Hot Rod Roy]

I too, have an '84. I have replaced the heater control valve in my car with a p/n 74800. This may be the same one you're talking about, as it has four tubes. The '84 valve also has four tubes, with the fourth tube being a "T" on the aluminum tube that goes to the water pump! To use the 74800 valve, you eliminate the hose that goes to this "T" on the aluminum tube, as the bypass function is internal to the 74800 valve. There are only four pieces of hose required, rather than the six pieces in the standard '84 setup. (Eight hose clamps instead of twelve!)

I was pleasantly surprised to find that all of the tubes on the 74800 are the correct diameter for my car! You need a longer hose from the valve to the water pump, which replaces the aluminum tube in that location.



The lower right hose goes to the back of the intake manifold, and the upper right hose goes to the water pump. This is very similar to the plumbing in the '85.

 

[Alan Walker]

I too, have an '84. I have replaced the heater control valve in my car with a p/n 74800. This may be the same one you're talking about, as it has four tubes. The '84 valve also has four tubes, with the fourth tube being a "T" on the aluminum tube that goes to the water pump! To use the 74800 valve, you eliminate the hose that goes to this "T" on the aluminum tube, as the bypass function is internal to the 74800 valve. There are only four pieces of hose required, rather than the six pieces in the standard '84 setup. (Eight hose clamps instead of twelve!)

I was pleasantly surprised to find that all of the tubes on the 74800 are the correct diameter for my car! You need a longer hose from the valve to the water pump, which replaces the aluminum tube in that location.

View attachment 24524

The lower right hose goes to the back of the intake manifold, and the upper right hose goes to the water pump. This is very similar to the plumbing in the '85.

Nice install! I had not considered replacing the "Tee" Tube and using all 4 ports. I installed mine this weekend and simply plugged off the unused port on the valve body and retained the aluminum tee. I think I like your solution better and may change it in the future. For now, just going to drive and enjoy the spring!