Heater control valve and cable service.

[cnoe]

Being based on the Gulf Coast my heater rarely gets used but it's always functioned well. In fact I can't imagine how anybody could ever need more hot air than this thing pumps out when fully open; it is overwhelming at any temperature above freezing. Even with the "cabin vent" fully open it only takes a small amount of "cabin heat" to warm things up when south of the Mason-Dixon line. And then...

On my recent trip into Colorado I penetrated a cold-air mass enroute that had me grasping for the "cabin heat" knob only to find that it was stuck firmly shut. I pulled harder and harder but didn't want to break anything. Particularly I didn't want to pull it wide-open and be unable to close it again. Eventually I managed to feel a little "give" in the knob/cable and started feeling some warm air flowing from the vent. After getting just enough air to stay comfortable I left it alone for the duration of my trip modulating the temperature by opening and closing the "cabin vent" instead (and stayed plenty warm). So you ask "where's this going CNOE?" Here...

I finally got a chance to investigate the issue and wanted to share my findings/observations to aid others who may need to service their own plane with this issue. On my J ('78) the heater control valve is a very simple flapper-type valve located on the right firewall. It resides inside the oddly-shaped box where the scat-hose connects between the exhaust muff and the firewall. Servicing it was quite straightforward but I'll remove a bit of the mystery about what's inside this box. I apologize for not taking more pictures.

In order to lubricate the flapper-valve assembly, spring (yes, spring), and heater-control-cable you should remove the outer box (shroud) for complete access to the inner workings. It's also advantageous to have the cable completely free in order to properly lubricate it.

To do this first remove the scat hose from the box and tuck it out of the way. Next, loosen the control cable nuts and slide both the nuts and washers from the threaded sleeve (this will allow you to clean up the threads completely before re-installation). You'll also need to remove the retainer screw that clamps onto the cable. If your cable is curled into a "U" at the end as mine was don't fret; there's an easy way to remove it from the retainer nut. To do this simply position the retainer/are as far to the end of the cable as you can and then wiggle-and-rotate the retainer so that the hole slowly makes its way around the cable's U-shaped end. It's not obvious upon first-look but as you spin/turn the retainer it will allow you to remove the cable entirely. Once that's free you can slide the cable out of the attachment and move it out of the way. Now all you have to do is remove the shroud from the assembly (which will remain fixed to the firewall).

The shroud is connected to the valve-assembly by four machine screws (essentially one at each corner). These are not the four screws attached to the firewall (see photo). Oddly 3 of the 4 screws were secured into fixed nut-plates on the back of the valve's outer flange, but the remaining screw (on the lower right side) passed through the flange and was secured with a free-floating locknut. This nut required a 11/32" backup wrench for removal and this was complicated in my application by the adjacent Oil-Separator (which thankfully I did not have to remove). Also note that this lower-right screw is slightly up/under the angled deflector (see photo) so you cannot get directly inline with your #2 phillips screwdriver. Make sure that your screwdriver tip is in good shape or you may have trouble removing this screw at the small angle required. The other 3 screws are fairly easy to remove. All 4 screws have a flat-washer under their phillips heads. Once the screws are out the shroud slides straight forward and off of the flapper-valve.

The valve is a simple square flat plate attached with a long shaft through 2 pivot points. You'll also find a small pivot spring on the shaft that is designed to keep the valve fully closed when not held open by the control cable. The plate seats against a robust tubular structure passing through the firewall. There isn't a rubber seal or similar but the flapper (plate) has a circular bead of hardened sealant on its surface that mates with the adjacent tube (do not remove/clean this material). Lubricate this entire assembly generously while swinging the flapper plate open and closed repeatedly. I would also recommend brushing/moving the spring independently of the flapper to clean it up as well. You may also want to use a brass brush or similar to gently clean the entire assembly. Oh yeah, don't forget to put a rag under the area before doing all this, and a shop vac comes in handy as well. Once the valve is cleaned up and thoroughly lubricated it should easily swing open and closed with the spring capable of closing it on its own.

Next is the cable lubrication step. I don't have any fancy methods of doing this so I simply wrap a rag around the end of the cable sheath and start pumping oil into the end while an assistant repeatedly moves the cable knob in and out from the cockpit. Also have the assistant put some oil on the control rod in the cabin. Once you get some lubricant inside the cable sheath it should start moving easily.

Now all there's left to do is re-assemble everything just the opposite of how you removed it. Putting the screws back in place was easier than I expected though starting the one locknut took a bit of patience. Also, don't forget to put the cable-nuts in place before you re-install the retainer at the end. When it's all back together make sure that everything is moving smoothly and adjust the nuts in/out so the the valve is fully closed with the control knob not-quite all the way in (you want to leave just a little "push") like on the other controls.

That's it! There's nothing too difficult about it and those of you with good mechanical skills will scoff at the need for this write-up. But if it helps anybody de-mystify things then my time writing it wasn't wasted.