Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Inaccurate Water Temp. Gauge

On my 1984 4/4 -- with CVH engine -- the water temp. gauge reads high
I have checked it with a surface temp. gauge and have the following comparisons ----- all degrees cent.
Gauge -- 70 -- actual 54
Gauge -- 80 -- actual 62
Gauge -- 90 -- actual 68
Gauge -- 100 -- actual 77
Gauge -- 110 -- actual 81
Gauge -- 115 -- actual 85
Gauge -- 125 -- actual 90
The fan cuts in at actual 90 -- as it should.
I have fitted a new sender but this did not change the gauge reading
Is it possible to get a sender with the a different output in order to get a correct reading ?
Has anyone got a solution please ?
Greenmorgan

Is it a 10v derived gauge by any chance I wonder?

There are notes about 10v fuel sender & gauges needing 10v regulator

You could stick the sender in a cup with boiling water, put potentiometer (variable resistor) in series tune it to match at 100°C then measure resistance of pot and fit resistor of similar resistance in line.

Sounds to me like the voltage stabiliser/regulator to the instruments might have failed - alternatively, check the earth from the gauge is good.

The stabiliser is usually tucked up behind the dash on the driver's side screwed to the ash frame under the scuttle. It looks like this:



and you can buy them from SVC for around a tenner:

https://www.s-v-c.co.uk/product/smiths-voltage-stabilister-negative-earth/

Note that the stabiliser needs to be earthed as well.

I'd agree it's likely to be the voltage stabiliser as my went last week on my 1972 4/4 with symptoms exactly the same as yours

I've replaced and put in an extra earth loop from the fixing screw to an earth point

Arwyn

Or go for compact and precise solid state equivalent.

I have a spare set of gauges of that period, if all else fails.
smiths, correct black surrounding. Mine were all replaced with chrome bezels

Where were you using the surface thermometer?
Next to the sensor/sender?
On a metal surface?
The differences measured seem very high.
Are the electrical contacts clean? Dirty ones will have a higher resistance.
Is there an adjustment screw on the gauge to recalibrate it?

Many thanks for your most helpful suggestions to my problem with a WTG reading high. When I bought the car 8 years ago I knew the gauge was wrong and ignored it -- relying on the fan to keep it cool -- all OK until recently I had a heater hose leak, lost cooling water and blew the head gasket.
The head has now been skimmed and refitted, new thermostat housing, thermostat, temp. sender and hoses/clips fitted.
This time I replaced the Instrument Voltage Regulator and rechecked Actual vs Gauge Temps. The Gauge was still reading high so I did some tests on the circuit from the sender and fitted a 22ohmn resistor into the circuit --- this has now cured the Gauge problem and it now reads the same as the actual water temp. --- taken from the casing of the header tank. So all is now well but I should have done this simple modification years ago and been in a position to see engine overheating when it occurred.
Many thanks for all your kind and correct advice.

David Thorp --- aka Greenmorgan