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Water temp gauge not working.

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Water temp gauge not working.
#1
  This topic is about my 1980 MG MGB MkIV
ricornc Avatar
ricornc Silver Member Chaz Ricorn
Petworth, Surrey, UK   GBR
My water temp gauge is not working.
I've checked the wiring and all looks ok, I have continuity from the gauge to the sender via th Green/Blue wire and from the voltage stabiliser on the other side.

I'm assuming that the temp sender unit is a thermistor, so what resistance should I expect across it when cold & hot ?

I believe that I should see 10v out of the voltage stabiliser, is that correct ?

And finally how can I check the actual action of the gauge, if I put a 5k resistor on teh downstream side to earth should it deflect ?

Sorry don't know how this double posted



Edited 1 time(s). Last edit at 2018-05-24 04:02 PM by ricornc.

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MGB567 Avatar
MGB567 Gold Member Barrie Braxton
Sunshine Coast, Queensland, Australia   AUS
1966 MG MGB MkI "Money Guzzler"
1979 MG MGB GT V8 Conversion "Darkside"
"I believe that I should see 10v out of the voltage stabiliser, is that correct ? "

Yes

"And finally how can I check the actual action of the gauge, if I put a 5k resistor on teh downstream side to earth should it deflect ? "

Does an electrical gauge sender work the same way as the mechanical? if that's the case pop the sender in a container of boiling water and watch.

That's all I got for you.



Convertible: CKD 11/66 first registered 8/5/67. Owned since 3/77. 90% original sheet metal. 18GB +40 balanced with almost all new internals. Peter Burgess big valve fast road head. Piper 285. Fidanza FW. Basil's followers and pushrods. TR7clutch. TT exhaust. ARP everywhere. 123 ign. Needham 4synchro c/r box.. Stock rebuilt/replaced suspension. Superpro bushes. New brakes all round including all pipes in SS flex. Interior redone. CAMS approved roll bar and side bars. Lots more. Hybrid of o/e and show/fast road car. Not for sale - it's my toy!

GT: UK car built/sold December '78. Stripped back to bare shell (with extensive bodywork to come). Powered by 'worked' Rover 5 litre V8 (ex TVR Chimera) with efi. T5 box. FC IFS. CCE rear attached to Salisbury axle with Quaife. And a whole lot more to yet to come. Stealth is the word.

jjonns74 Avatar
jjonns74 Gold Member John Dailey
Indianapolis, IN, USA   USA
1974 MG MGB "Athena"
1974 MG MGB "Apollo"
Accuracy of the Temperature Gauge can be verified by providing 10vdc to one of the terminals (doesn't matter which, not polarity sensitive) and jumpering the other terminal to Ground via a 222 Ohm resistor = Gauge should read "C".
Change to a 65 Ohm resistor = Gauge should read "N".
Change to a 20 Ohm resistor = Gauge should read "H".

Coarse test:
Key Off = gauge should read full scale Left (no current flow thru gauge)
Key On, disconnect wire from Sender & short to Ground = gauge should read full scale Right (Maximum current flow thru gauge)

Do not short wire for more that 20-30 seconds to prevent damage to gauge mechanism !!!!


The same applies to testing the Fuel Gauge, as they are identical internally (just different "faces"winking smiley

John

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Fairfield, CA, USA   USA
the sender needs to be properly grounded.

In a pot of water it won't be, and won't work at all.

Did you use some thread sealant around the threads when you screwed it in? That might be why sh no work. Clean threads, both male and female, and try again.



1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

ricornc Avatar
ricornc Silver Member Chaz Ricorn
Petworth, Surrey, UK   GBR
Thanks guys and John particularly for the tech detail.

Mac, that is a good point, I probably did use sealant, I'll check for continuity between block and body of the sender.

I checked the temp of the sender unit with the engine running with infrared & contact thermometers.

Someone mentioned that the VS doesn't output a constant voltage (unless its a solid state one) so may not show on a digital multimeter, but I guess the old bulb and wire would light up wouldn't it ?

lil.red.roadster Avatar
lil.red.roadster Bernie Anderson
Marshfield, Gloucestershire, UK   GBR
1970 MG MGB MkII "The Beasty"
If your fuel gauge works then the V Stabiliser must be working



Bernie

"discere mutari est"

Wiring Diagrams: http://www.advanceautowire.com/mgb.pdf
Paul Hunts very useful site: http://www.mgb-stuff.org.uk
Manual for download: http://www.geomatique-liege.be/MGJP/DocumentsPDF/MGB_Workshop_Manual.pdf
UK MoT guide: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/518634/mot-inspection-manual-for-class-3-4-5-and-7-vehicles.pdf
Some basic guides: http://www.howacarworks.com/

1970 Roadster (First Reg July 10 1970). Std exhaust (Bell SS) AFAIK rest is bog standard

ex-tyke Avatar
ex-tyke Graham Creswick
Chatham, ON, Canada   CAN
1976 MG MGB
Quote: the sender needs to be properly grounded.

In a pot of water it won't be, and won't work at all.

Did you use some thread sealant around the threads when you screwed it in? That might be why sh no work. Clean threads, both male and female, and try again.

X2 thumbs up

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ricornc Avatar
ricornc Silver Member Chaz Ricorn
Petworth, Surrey, UK   GBR
So the latest is,
1. Got a good working gauge, test by sticking 10v across it
2. Changed the voltage stabiliser for a new solid state one, getting 10v from it.
3. Tested that the body of the sender is earthed.

Temp gauge still didn't work.

4. Checked that 10V is showing on 1 side of the gauge
5. Earthed the other side and it works.

Finally checked the cable between its reading 240ohms, not a short as it should.

Its the section highlighted in the picture, I really can't what the heck is going on.

Before I just run another cable any one got any ideas.


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2018-06-03_23-42-14.jpg

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