Oil Temp Guage

[Cobra75]

I have a Smiths oil temp gauge that has a wire-wrapped tube from the oil pan to the guage. While attempting to fix a minor leak at the fitting where the line goes into the pan, I managed to render the guage inoperable. It now reads zero continuously. The tube does not leak and has no visible external damage.
The questions I have are:
Can this tube/line be repaired, and if so how?
If not, can the tube be replaced or must the entire gauge/tube assembly be replaced?
How does this gauge work anyway. I assume the tube is filled with some kind of fluid?

[Turbo II]

Scott,

I'm going to take a wild guess here as I'm not familiar with Smiths gauges but many temp gauges are based on thermocouples. Basically what you have within the tube (sheath) is two wires surrounded by a ceramic filler. The wires are of different alloy and where they connect at the tip, the dissimilar metals generate a small voltage (as temp changes so does the voltage..that's how the gauge works). What has likely happened is that one of the wires within the tube has broken. We used to repair our thermocouples at work on occaision but it's often impossible to find the break. You should get continuity between the two leads, if not I would replace the thermocouple (the tube).

What color are the wires going to the gauge? Thermocouples are usually coded by an international scheme according to type. The most common is "K" type which has red and yellow wires. If I remember correctly these are what Autometer uses. If your gauge supplier won't replace just the lead or is asking outrageous $$ you may want to check with an industrial supplier like Omega Engineering (www.omega.com), they can get any type of thermocouple in any length and sheath size.

Hope this is of help, (I'll soon be in the process of wiring my car up with a 10 channel digital gauge with "K" type thermocuples myself)

Regards,

Francis "Turbo II"

[wilf leek]

Scott - I suspect that what you have is a capillary-tube operated mechanical guage, I have the same in my car. The clincher is if the engine end terminates in a large brass coloured "bulb".
The guage works by expansion of the fluid within the bulb at the engine end.
The wire wrapped tube is a small bore (usually copper) tube which allows the expanded fluid to get up to the guage, where it operates a "bellows", thus converting the fluid expansion into mechanical action. These guages usally have a 270 degree sweep of the needle, and are known as "mechanical" type guages. The spiral wire wrapping "protects" the fine bore tube - but only to a point.

If you break the tube, or it's soldered connections at either end, the guage is effectively bust. You have two options - a/ buy a new one and be careful (very) with the capillary tube) or b/ find an instrument servicing outfit that can repair it. (and still be careful with it!)

Whilst the thermocouple theory above makes sense, it is way too modern for english "period" guages. You guys do know about english electrics etc???

[Cobra75]

Wilf,
Thanks for the info. The Smiths is a mechanical-not electrical-gauge. Your description exactly fits my situation and I obviously ruptured the tube when backing out the fitting at the connection. Your diagnosis and repair is what I didn't want to hear. Crawling back under the dash and re-running the tube etc. is something I don't look forward to.

[wilf leek]

Sorry Scott, but just think what a feeling of achievement you will have when you are finished!

[Turbo II]

Scott, sorry that I was totally off base! (never worked with a Smiths gauge)

With my last name, I DO have some familiarity with English electrics

Kindest,

Francis Lucas (no relation to the "Prince of Darkness")

[Mr.Fixit]

Sound like you let all the "magic gas" out of the capilary tube and bulb assembly. Just get a new gauge, it would probubly cost more to get the old one repaired, if this service is available.

[wilf leek]

Francis - if it makes you feel any better, I always use thermocouple type instruments to check out the real temperatures under the hood (and at the back axle too, which currently is getting far too hot on the track).
I would never trust a Smiths instrument (or most other car instruments for that matter) for an accurate, absolute reading.

Like the name - you sure you are, if not the prince, not one of his relations?

[Turbo II]

Wilf, actually I'm from the Dutch side of the Lucas'..unfortunately no relation to the famous lemon gin maker either

Would have made for some great family reunions!

Francis

P.S. Thanks for the tip on putting a T-couple on the rear, never thought of it and I have a couple of extra channels (10 seperate)

[SCOBRAC]

Scott,

e mail me I will send you my address. I have two of them, both never used. If you join the NRA for a year ($40) you can have one of them..

Regards,