Naykers

That could be what is happening. When I get the car on the road I will thermocouple that location and outside air. Thanks for the ideas.

I agree that warm air is desired but page 526 of the Motorbooks restoration guide has the PDM showing where the tap is for the tach vent line. The location is the underhood side of the heater plenum which is air from the blower that will be going through the heater core and into the other half of the HVAC assembly for distribution through the in-car ducting. At this pick up point per the shop manual and of course we've all pulled those cases over the years is just outside air pulled in by the blower. No heating involved unless some backwash from the heater core is happening. I didn't think that was likely because it would generate noise at higher blower speeds and reduce heater performance/efficiency. That's why I figured I'm missing something. Thanks for responding.

Thanks Frosty and 360 Rocket for your replies. Not sure why the vent line works yet but I'll definitely go with it. Certainly see why cold water car wash would condense moisture on the inside lens surface. This question originated with the hood tach on my son's '68 which would fog on a regular basis. When that tach failed (not due to moisture) I replaced the original d'arsonval meter movement with an air core movement for increased durability and the illumination lamps with surface mount LEDs for improved night time viewing. I also added the vent tube fitting as shown in the '72 Factory PDM. I have not reassembled the unit yet and since I couldn't understand why the vent line worked. I asked this question to see if I was missing something I should include in the repair. This tach is not an original AC and was bonded together with RTV. As 360 Rocketman pointed out it leaked at the bottom edge of the polycarbonate lens so water intrusion was easy. This design does get credit for putting in a small drain hole although it did plug up. Looking for design improvement ideas I examined an original, never installed, AC hood tach for a '69 or later model. It has a glass lens that appears to have a full perimeter seal around it. (This tach also has one of the water intrusion factory fixes applied by AC. It looks like the light yellow/tan body seam sealer and is applied around part of the bottom joint.) It also has 2 holes towards near the rear of the unit that I thought might be for a vent exhaust but can find no documentation for their function. The tach to hood gasket would have somewhat sealed them. I blocked the holes, applied 10 inches of water vacuum, and found it to be nearly perfectly sealed, i.e. dead headed. If this unit is not representative and air could flow in somehow I expected the line to supply "defrost" air. However the air supplied from the fitting on the heater plenum is not conditioned air. It is outside air from the cowl intake grill that exits the blower squirrel cage before the heater core. On humid days it would introduce outside moisture laden air into the housing. And since the pressure is always positive downstream of the blower gravity would allow water to settle in the low spots of the hose. The Illumination lamps help if they are on but since the tach circuit draws only a few milliamps only engine heat and the sun would speed up defogging. Still puzzled but the vent line will be used.

I've read posts on the hood tach fogging problem and also have seen the factory vent/purge line fix from the heater plenum. It must work but I would like to understand how outside air in the plenum at a slight positive pressure directed to the tach removes the condensation in a dead headed tachometer case. Warm air flowing in and out would make sense like defrost but this isn't like that.