Please visit the Club's website https://www.dloc.org.uk/ to join. Visit https://www.dloc.org.uk/adhoc to DONATE towards the cost of the forum.
Please don't post someone's email address to avoid it being harvested by spambots and it's against GDPR regulations.
Always look at "ACTIVE TOPICS" to see all posts in date & time order as they are sometimes moved; or look at "Your Posts".
Please add Reg. nrs. when posting a photo or anything about a car as this will help searches. Don't add punctuation next to nr. as this negates search.
CHANGED YOUR EMAIL ADDRESS since registering?, click your username and check your address in User Control Panel, Profile, Account Settings.
If you want help to register, use "contact us" at page bottom for help.

Temperature Gauge Problem

User avatar
Marc Mears
Posts: 84
Joined: Tue Feb 16, 2016 3:53 pm

Re: Temperature Gauge Problem

Post by Marc Mears »

Just to add my little bit in – when I restored my car I put in a new sender unit – only to find that my temp gauge was reading very low, and from memory (I’m getting old now) I was told there are different types – maybe something to do with the resistance of the sender not matching the gauge….
I can’t verify if this is true.
Anyhow – got a new one from BT and all is fine…

Hope you sort out your problem – regards

Marc
Daimler Dart. Healey 3000 Mk3. VW T2 Camper. BMW 2002tii. 996 Turbo. BMW Z3M. Hillman Tilly. Rolls Royce 20/25. XK 150 ots .BMW 3.0 CSL 73. Sunbeam Alpine Mk3.Morris 8 Tourer. 97 XK8 Convertible.Alfa Romeo Giulietta. Citroen Mehari

User avatar
heh101353
Posts: 157
Joined: Sat Sep 30, 2017 11:34 am
Location: Basel, Switzerland
Contact:

Re: Temperature Gauge Problem

Post by heh101353 »

Hi

Instead of opening a new thread I guess it makes more sense to expand on this one …

When I bought my SP it was equipped with a combined water temp. / oil pressure gauge probably the type used in Frog Eye Sprites.
Image


It is of the capillary type and the reading was always dead on. (cross check with a digital thermometer)
Image


To get the dash to look more SP250 like I decided to buy a gauge and a sender from Barry T.

As soon as the parts arrived I did some measurements to get an idea of the electrical parameters.

Image

To get a reading of 90°F the voltage required is around 6.5 Volts

Image

To get a reading of 185°F it takes around 10 Volts

iImage

And at 12 Volts it hits max.

These readings look very reasonable so I guess the gauge itself is fine.
Unfortunately I did not measure the characteristics of the transmitter at the time.

It took me more than a year to get around changing the thing as it was at the bottom of the priority list.

After installation, surprise, surprise the reading sits around 90°F! It will not move any further.
The gauge is properly wired with a solid earth connection. Shorting the wire to ground gets the gauge beyond 230°F

Comparing the transmitter of the capillary gauge and the electrical one the most obvious difference is the length of the feeler.

Image

The capillary one is about 1.5 inches and reaches deeper into the coolant whereas the electrical one is barely an inch long.
I searched the net far and wide but all the electrical ones are more or less the same (shorter) length.

I tried to get a few numbers from the installed electrical transmitter using a multimeter and a digital handheld thermometer.

Pointing the thermometer at the bulge where the transmitter is installed

Image

At 21.6°C (70.8°F) I get a resistance of 879 ohms

Image

At 81.6°C (178.8°F) the meter reads 63.5 ohms

Looks as if the resistance is way to high. The question is why?
Wrong transmitter, not matching the gauge?

Transmitter not getting in good contact (shorter length) with coolant?

I dug up a NOS SMITHS TT.3800/00A transmitter which I ordered on the spot.
Let’s see what comes out of it.

Thanks
Hans
Last edited by heh101353 on Sun Jan 26, 2020 2:49 pm, edited 5 times in total.

User avatar
heh101353
Posts: 157
Joined: Sat Sep 30, 2017 11:34 am
Location: Basel, Switzerland
Contact:

Re: Temperature Gauge Problem

Post by heh101353 »

Back again


I received the NOS temp. transmitter yesterday. Apparently it was manufactured August 1969
Image

It’s a genuine TT.3800/00A, exactly the model used in the SP250.
Image

Both transmitters side by side. They look pretty much the same and are the same length!

on the left = TT3800
on the right = who knows
The left one is clearly engraved as TT 3800 00A
The right one as 5602
Image
further markings 07.9 / 24F
Image
/ BMI
Image

The measurements of the TT.3800 transmitter
30°C / 86°F 214 Ohms
Image

40°C / 104°F 141 Ohms
50°C / 122°F 98 Ohms
60°C / 140°F 70 Ohms
70°C / 158°F 46 Ohms
80°C / 176°F 33 Ohms
Image
85°C / 185°F 28 Ohms
90°C / 194°F 24 Ohms
95°C / 203°F 20 Ohms
boiling Point 16 Ohms
Image

Note: The thermometer is not very accurate at temperatures above 90°C

The measurements of the transmitter I received together with the gauge
30°C / 86°F 535 Ohms
Image
40°C / 104°F 365 Ohms
50°C / 122°F 282 Ohms
60°C / 140°F 200 Ohms
70°C / 158°F 131 Ohms
80°C / 176°F 106 Ohms
Image
85°C / 185°F 90 Ohms
90°C / 194°F 78 Ohms
95°C / 203°F 66 Ohms
boiling Point 58 Ohms
Image


After installation in the car the readings from the gauge reflected the temperature accurately.

Case closed.

Thanks
Hans
Last edited by heh101353 on Sun Nov 17, 2019 2:04 pm, edited 1 time in total.
:o owner since 2015 :mrgreen:
https://www.wyhe.ch/

User avatar
Kbeal
Posts: 148
Joined: Mon Feb 15, 2016 9:01 pm

Re: Temperature Gauge Problem

Post by Kbeal »

There are plenty of temperature senders on the market that look alike, fit the Dart but give inaccurate readings. Barry Thorne and David Manners both stock the correct one for the Dart.

Kevin

Post Reply