Fogging of the Instrument Glass

[awolff280sl]

A search on this forum topic will reveal that this is an intermittent problem for a few of us. It has yet to be adequately explained or resolved. At one point I had thought that I had it fixed but it's back depending on the the weather. In my my case it's only the speedometer while others have had up to all three instrument glasses fog up. When the fog dissipates there is no residue and all three instrument glasses are perfectly clean and clear.

[71Beige280SL]

Moisture getting through the firewall or other ventilation issue?

[awolff280sl]

I assume that moisture is getting through somewhere. I have checked my firewall for gaps in a dark garage with light shined on the firewall, and I have sealed all visible gaps. Yet, there are surely Pagoda owner whose firewalls are not totally sealed or who have some ventilation gaps here and there.
Regardless, moisture has to get into the speedo itself. Other than around the glass bezel or less likely somehow up the cable, I am not aware of any other potential ingress points.
I am thinking about trying to seal around glass-housing interface.

[reggie]

Condensation traveling up the speedo cable 

[mdsalemi]

Andy,

This is a pretty simple explanation, but there really isn't much of a solution I'm afraid.  Our instruments are not (and cannot be) hermetically sealed, and therefore, ambient air does eventually move into the space between the glass and the gauge.  That air will certainly contain moisture unless you live in a desert.  Oh, you are in Florida...   Not exactly a rain forest, but humidity is something you know about?

The glass gets condensation or fog, when the temperature of the glass falls below the dew point of the air that is in contact with it.  So, warm, moist air gets into the instruments through no fault of anyone other than your locale.  Then your temperature drops...and faster then you know it: condensation.  Happens in headlamps, too.  Not much to be done about it.

[awolff280sl]

thanks Mike, I'll try to fogetaboutit. It does get pretty bad at times.

[ja17]

Hello Andy,

The Chrome Bezels do have seals on them I believe. Moisture traveling through the cables should be minimal. This is a rare problem for most of us. Possibly the plastic cover on the speedometer cable is broke or cracked and letting moisture in? One of the two light sockets left out of the back of the gauge?

[getsmart]

Hi Andy,

It may well be a combination of many factors....as others have noted...

1. Do you have the heater on or conversely cool air or leak from the heating/air system that could be causing a change of temp around there, or perhaps when you have the demister on...
2. Sealing the bezel might improve it...
3. There could be a thin film of oil or chemical residue on the inner face similar to the thin film one sees on the inside of a windscreen that hasn't been cleaned for a long time. Cleaning it might help, at the same time sealing the bezel...if you're going to go to that trouble.
4. Whats the humidity factor there ? Does it always happen when you first get in the car ? Or later ? or when travelling at higher speed...Does it happen with the hardtop on or off ? or both ? When you say dependant on weather what kind of weather ?
5. Also I think there's products available to apply to glass to inhibit or minimize fogging...
6. The light sockets are mentioned and if one not in, definitely could be an entry point for air but also the heat from the extra light could be enough heat to keep the air dry

All things to consider...that will help you work out the cause.
My bit, hope this helps...

Rgds, Joe

[awolff280sl]

Thanks to all for the suggestions. I did a little web search research.
From what I gathered, fogging occurs in older instruments with glass faces and if not fully sealed. Plastic lenses conduct temperature much less and are not prone to fogging. Assuming the humidity inside the gauge is identical to cabin humidity (i.e. even in the absence of excessive moisture or water inside of the gauge), fogging can occur when the heat from the engine warms moist air in the gauge. When the cabin temperature keeps the outside surface of the glass relatively cooler than the inside surface of the glass, the relatively warmer moist air in the gauge will condense. (I suppose that moving parts inside the gauge may contribute to heat generation, but the center gauge has also been reported to fog up as well).
There are "interesting" suggestions out there from (Porsche and Mustang owners, among others).
1. Cleaning the inside of the glass, especially with alcohol - did not work for me
2. Drilling a hole in the instrument case - people have reported to have tried this without success, good luck with that
3. Sealing up the instrument better - likely won't work unless all moisture removed from the air inside the gauge first.
4. Silica moisture absorbing packets - if you ever wondered what to do with them, now you know

It was theorized both here and elsewhere that with the dash lights on, bulb heat in the gauge might "burn off" the fog. This appears to make my fogging worse, probable by increasing the temp differential across the glass, but never getting warm enough to burn off the moisture, at least not for a good while, My worst fogging occurs in humid night air with the lights on within a few minutes of driving as the engine heats up. Eventually, the fogging goes away, but only after at least 30-60 min of steady driving.

If I get around to it, I might do the only thing that make senses to me: placing a small computer-type fan on the shelf behind the instruments to speed up air flow. Based on the windshield defogging mechanism. Maybe a separate thread for Pagoda owners in Florida, Texas, etc.

[mdsalemi]

It's really akin to double-pane insulated window glass, which has been building code in northern climates for many years.  My home is 20 years old, and over the past 10 years, one by one, the windows fail; they fail by having the seals break. (We replace the panes as they fail.)  The breaking of the seal is the breaking of the adhesive bond between the glass and the aluminum frame.  You cannot see the break--usually it is microscopic.  When new the glass was hermetically (impervious to gas or air motion) sealed to the frame.

How do you discover the break?  Well, moisture laden air migrates through this microscopic seal break, and when it gets cold out, you get condensation on the inside of the glass.  So, when ever it gets well below freezing in February, I'm on the hunt around the house for the broken seals, mark them, and replace the panes in spring time.

Yes, we may have small seals between the glass, bezel, and instrument, and we may even have a rubber seal on the back of the instrument where the cable gets in.  But it is not a hermetic seal, and sure as shooting, moist air will get in.

The condensation will rear its ugly head in places like Florida, with vast amounts of humidity, and some wild temperature swings with dew points during certain seasons.  Not likely to see this in Arizona. I don't have the issue at all in Michigan on the instruments, but then again this is not likely to occur just in summer here!

[al_lieffring]

The double pane windows that Michael mentioned also have desiccant crystals inside the aluminum frame rails. I was wondering if putting a desiccant envelope in the bottom of the gauges might help.

[TheEngineer]

Have the same problem with my older cockpit instruments on the boat: When the boat is not in use there is a cover over the instruments. The instruments are installed in an enclosed pod. After removing the cover the glass is clear, but then it fogs up. But when the sun shines on the instrument, the fogging soon clears up. Try a heat lamp to project some GENTLE WARMTH on to the instruments! You may notice that newer vehicles have no glass over the individual instruments but an acetal cover well away from the instruments. I wonder why that is?

[don p]

I only have the problem with the speedometer. I always thought it was because the trip odometer did not have a oring gasket between the glass and the reset knob. I guess i will see if sealing it off makes a difference.

[450sl]

There are several antifog fluids available , often used by motorbikers on their Goggles And Helmetlenses . a cheap alternative is to use some soapdetergent.

[awolff280sl]

Fogging of the speedo not cured but remedied: I just had my speedo refurbed and replaced the cable. While in there, I put a high-speed computer fan on the ledge next to the speedo wired to a hidden switch. I also placed a few silica dessicant packs around the area. The other night the speedo fogged big time and I flipped on the fan. It took about 20 minutes but the fog receded inward from around the circumference to eventually only a small spot in the center of the glass. Next time I won't wait, and I will turn on the fan at the first sign of fogging. Very gratifying (and surprisingly effective) because this has annoyed this Florida driver for a long time.
(Unfortunately does not change the fact that the speedo is dimmer than the tach)