Sunday, March 6, 2011

Temperature Sensor

Hi All,

After my recent battle with cooling I've been off doing what I should, DRIVING :-)
Lots of fun driving, but I'm still not convinced on the cooling.
The new temperature sensor shows her running warm, almost hot, not good in the Desert.

A bit of a search on the MG Experience and it looks like there are two sorts of sensors.
BUT not clear on what which sensor with which engine etc. so out with the kitchen physics kit and time to plot some graphs.

These are the two that I've got.

Part GTR101
Dia : 8.39mm
Sensor length : 13.3mm
Connection : Perpendicular

And then new : 

New Sensor
Part GTR104
Dia : 8.59mm
Sensor length : 12.4mm
Connection : Axial
The way the temperature gauge works is all about resistance. The sensor is a thermistor, the hotter it is the lower the resistance.
So out with the sauce pan and multimeter.


Plotting a graph for each sensor it is clear that the original sensor the gives a higher resistance.

Deg C  Old   New             Deg C   Old    New
   26     909   554                   70     181   146
   28     869   542                   72     168   134
   30     849   524                   74     160   119
   32     792   482                   76     142   112
   34     725   454                   78     132   109
   36     667   406                   80     124    98
   38     599   378                   82     116    91
   40     563   352                   84     108    86
   42     529   335                   86     101    75
   44     496   325                   88     95      69
   46     442   304                   90     90      65
   48     421   281                   92     85      59
   50     400   272                   94     80      54
   52     362   268                   96     69      49
   54     343   235                   98     61      45
   56     311   206                   99     56      47
   58     288   202
   60     270   184
   62     255   179
   64     244   170
   66     213   186
   68     197   180

Problem is that old sensor shows the engine running Cold (to cold in my mind) and the new one running Hot !
Time to get the laser thermometer out !


Richard B.


  1. Find out what resistance is delivered by 90C, then what voltage that allows through to the temperature gauge. If 90C doesn't put the needle in the middle, adjust the voltage with a suitable in-line resistor.

    Or mark where 90C is on the temperature gauge and drive the car. Of course, this is only sensible if 90C doesn't correspond with the needle up against the stop...

  2. Trying very had not to be dodgy.
    Easy to fit a small variable resistor and "fix" the problem.

    Going to get some real temperature readings (laser thermometer, or maybe borrow a termal camera from work) and see how hot the problem actually is.

  3. Ha Ha Ha.

    How many trips to the garage has your Patrol had to sort out the overheating ?

    Think I've got a few more attempts before I reach that lever ;-)

  4. I am currently in contact with an SMP Europe product engineer about the two MGB senders that have been giving false high readings for many years. Repair specialists like myself have to resort to good used senders to get out customers cars to read in the dead center like they were designed to.

    I have determined the resistive loads necessary to make the gauges read right, and they are nowhere near what is being sold, as you surmised.

    To add an elecrtical component such as a resistor skews the reading across the board, so is not a good solution. Until we can convince the manufacturer that there is a problem (and hopefully that will happen someday) and until all the defective units are out of the "pipeline", the only solution is to locate good used units from a salvage operation. There were two different senders. One for all MGB from 1968 to 1976, and one from 1977 and on when the engines had to run hotter for emissions standards, and the normal range of the gauge was shifted back to dead center by the different sender. All the gauges used the same input signals for low, Medium and High.

    Hope this helps.

  5. Thansk Glen. I also cheat and have anothed modern digital unit fitted.

  6. Since the thermostat does not start to open till 80 Deg and will not be fully open till 105 Deg the temperature detected is not represenative of the whole engine temperature as the full flow of water will not be circulating and hot spots will be present.
    With a pressurised system, the normal running temp will be above 105 Deg at which point your graphs have converged enough to be insignificant given the accuracy of a bi-metallic gauge