Broken auto heat dial part - 79 SC

DrK · 02-08-2015, 05:21 PM

I just posted a want ad hoping to find a replacement, but maybe I better have Plan B. The little arm that attaches to the shaft of the potentiometer (rheostat?) broke. I could only find the part number for the entire unit, which is available from our host for $405. If I can't find a cost-effective replacement, what are my best options for repair? Would something like JB Weld work? Obviously, nothing will hold up to abuse, but I'd like to get the heat functioning again.

Thanks!

Rich

Bill Douglas · 05:41 PM

I threw the whole unit away and now have a coffee cup holder there.

sugarwood · sugarwood's Garage

What's the part number?

cmonref · 02-09-2015, 08:38 AM

There is no part number for Drk's broken piece. Mine broke 25 years ago and I have never found another. I tried to fabricate a sheet metal piece to recreate the geometry of the broken piece but was unsuccessful. (My car is now in for paint and I am going to give another crack at the fabrication during the downtime.)

Started out with a pack of cigarettes to hold the lever up, but it quickly got crushed by the heat lever. Made used a small piece of wood with its three dimensions including full ON and two lower values at which I would most of the driving. I gave up on that after the first winter.

I switched to manual heat for a while by obtaining the necessary pieces to do the conversion. It involves removing and replacing the bolt that mounts the emergency brake lever and re-doing the pieces on the bolt to incorporate the friction discs necessary to hold the heat lever still when you reposition it. Print a copy of the PET diagram for the bolt and its accessories, run an inventory, and make a list of parts that you need to get from your local dealer.

The heat won't work exactly right after the broken piece is removed. With the piece intact, when the dial is turned full CCW, it flips a switch that disables the system and returns the heat lever to full down. When the dial is turned full CW, it flips another switch that bypasses the controls and moves the heat lever to full up (for defrost.) In between, the system works correctly, providing a balancing of resistances between the resistor in the supply airstream (the piece the OP posted above connects to the engine side of the driver's side heat valve), the resistor in the interior sensor (between the sun visors), and a built in resistor in the control unit. If the two resistors are correctly sized, the dial MAY control the heat system from very low heat (essentially turning it off) to high heat (essentially turning on the defrost) if you are lucky. Without the resistors in place and functioning, all bets are off. (You definitely need to reinstall the supply sensor in the driver's heat control box.)

My car is an '80 911SC. At some point later, the broken piece that we both have was replaced with a more robust unit that won't break like our weak piece.

SilverWT · 02-09-2015, 02:30 PM

I fixed my one with some "Araldite" two part epoxy. I held the parts in the correct position with tape and pins on a piece of flat wood with some pieces of scrap plastic positioned at the sides to keep the epoxy in place while it set. Been working file for some time now.

Mark

DrK · 02-09-2015, 05:44 PM

Thanks Mark. It's good to know I'm not way off base. I figure I've got nothing to lose at this point. I noticed the heat lever was always up so I finally took it apart and started testing resistances as cmonref alludes to. That is when I found that the dial spun around almost 360. It used to work fine so I'm hoping this is the only fix it needs.

Rich