Temperature gauge (again)

[hatman]

In trying to decide what (and when) to do about my misleading temperature gauge it has now occurred to me that it doesn't actually work like the set-ups on other cars I've owned in that they all had a cable from the sender to the gauge. This has the coiled, wire-bound connection that, I surmise, is a copper temperature conductor (which would explain why the temp gauge, when working correctly, takes ages to return to cold after switching the engine off).

If I've got this right, the gauge itself must have some kind of built-in temperature sensor and therefore it can only be the gauge that's faulty. Help! - I think I'm becoming (more) bewildered.

[rgh0]

The Elan mechanical temperature guage is actually a pressure guage. It measures the vapor pressure of a low boiling point fluid which I believe is ether. The bulb in the thermostat housing contains liquid ether. As it is heated its vapor pressure increases. That pressure is transmitted via a small bore copper tube to a pressure sensor element in the temperature section of the gauge in the dash. The needle is then calibrated in temperature units that corresponds to the equivalent vapor pressure being measured.


If the gauge reads high it is normally because of fatigue in the pressure sensor causing its spring resistance to go soft and thus respond more to any pressure than it should.

If the gauge reads low it is normally because you have lost ether out of the system and its running out of fluid to heat to cause the pressure rise needed to drive the pressure sensor.

hope this helps
Rohan

[hatman]

Thanks Rohan. All, as they say, is now revealed.

So, in my case, I would appear to have an ether shortage, meaning, I imagine, a whole new gubbins to include the gauge, copper tube and ether bulb.

[ElliottN]

As Rohan said, it's an ether filled bulb/tube which drives a mechanical gauge by pressure. If it behaves consistently but is misreading (simple test is to put the bulb in boiling water and look to see if you get sufficiently close to 121 F or 100 C) then it's a calibration issue. If the spring has lost tension then you can either recalibrate to compensate or put in a new spring (but you'll have to calibrate it again then anyway!).

If you are under reading it's not ether loss. If the unit develops a leak it's catastrophic (0 degrees no matter what) as all the ether will promptly vent from the smallest of holes. The ends of the tube are soldered onto the bulb and gauge respectively. The soldered connections are the weak links and it is almost always the connection to the bulb that goes as it is subject to engine vibration - though you can largely arrest the problem by properly connecting the tube to the heater valve just up from the bulb to isolate the bulb joint from most of the vibration.

I have only ever had mine fixed by an instrument company here in Sydney (Oz - sorry) - they re-"ethered" and recalibrated it - not overly expensive and a whole lot cheaper than a new gauge. As long as your ether circuit is complete I expect you can recalibrate it yourself (using the bulb in a pot of boiling water to get a reference point) provided you're prepared to venture into the internals of the gauge.

[Galwaylotus]

Another option, if you're not locked into "originality" is to replace the gauge with an electrical gauge and sender. It's cheaper than a mechanical temperature gauge.

[66S2]

The ether filled gauges are cheaply and relatively easily repaired. If you go to the Yahoo Lotus Elan group, look at message #24745. I posted some links to instructions on the web. Mine has worked OK for about 2 years.

66S2

[Galwaylotus]

. . . and how do I find message 24745?

[hatman]

Wow! Plenty of food for thought there.

Although not overly hindered by purism Galwaylotus, the existing gauge does of course also house the oil-pressure gauge and adding/replacing with an electrical set up would have a knock-on effect with the other gauge. ie leave existing gauge with non-functioning temp readout and add another dial or replace a perfectly good oil pressure gauge with a sole-function one, in addition to the new temp gauge.

Found the site you directed me to 66S2 and v. impressive it is too. Whether or not I'm confident enough to cut and fit a new tube/bulb to the existing dial, all the while keeping the ether in the bulb in a frozen state is however another question. Sounds more like one for the pros to me. Thanks anyway - very interesting.

I do see the logic of your contention that the ether cannot have escaped if the gauge reads above zero Elliott, which is indeed the fault displayed by mine (it rises t c40 degrees and goes no further). Think I'll pop the bulb in as well when I next boil my breakfast egg - see how far the needle rises for a known 100 deg C.

[frearther]

Hatman,
If you're in the US, you can try Nisonger to repair the gauge. Ray at RD told me that they can do that so that you'll still be original and also more accurate.
Art
26/4934

[ElliottN]

I had an experience with an electrical temp gauge in my S4.

When I bought my car the temp half of the dual gauge was u/s (no ether) and disconnected - and the PO had fitted a separate add-on electrical temp gauge.

I had myriad intermittent electrical problems with lights and gauges which checking, cleaning and re-earthing didn't fix.

I had my dual gauge repaired and on installing and removing the electrical temp gauge - all the electrical problems went away! I'm guessing the way it was wired in it had tipped the voltage reg over the edge and such problems could probably have been avoided but I'm a lot happier with the original gauges.

[hatman]

Thanks for the heads-up Art but I'm not in the US, I'm in the good 'ol UK (to coin a phrase). Still, as Elliott got his fixed down under in Oz and your guy does 'em over there in the other colonies it's a fair bet that there's someone beavering away quietly on them over here too. Just have to find out who and where - 'Speedy Cables' was mentioned on the earlier thread so I'll give 'em a ring and see if this sort of work is indeed their bag of chips.

[rgh0]

Hatman

If your gauge goes to 40 degrees and no higher it can be two reasons

1. Its almost out of ether and all the liquid in the bulb has evaporated by that 40 C point. This is the normal reason gauges read low. Ether has been very slowly leaking for years through a very small pin hole and its finally got to a point where no liquid is present over part of the operating temperature range. Once all the liquid evaporates you get no increase in vapor pressuer and no further movement in the gauge

2. There is something mechanical sticking in the gauge preventing the needle moving further than the 40 degree mark

Rohan

[hatman]

From the two scenarios you outline Rohan, logic tells me that it must be the latter, ie a sticking needle or whatever. What leads me to this conclusion is the fact that the malfunction happened without prior warning; the gauge had been operating perfectly until one day it just decided that anywhere north of about 40 was a no-go area. Had it been a gradual leakage of ether the highest indicated temperature would have been in decline over the whole period of the leakage, wouldn't it?

So, it's a gander inside the dial then; what a pity that I'll have to pull the whole connecting tube gubbins out through the facia to get at it though!

[rgh0]

With a slow leak you have no change in gauge behaviour until amost all the fluid had leaked out. So you would see no change in performance for a long time and then suddenly the gauge stops working over its full range when there is not enough fluid left to evaporate and increase the pressure.

The actual amount that evaporates over the operating range of the guage is very very very small so the gauge will appear to fail rapidly even with a very very small leak over a long time.

regards
Rohan

[hatman]

Ether shortage it is then. Game, set and match methinks.