M.G. Z Magnette Temperature Gauge |
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| Looking through the bulletin board messages, I see a number of
questions about the operation of the Magnette temperature gauge. I would like to take this opportunity to
clarify some points about its operation. The temperature-sending unit in the late MGBs and all modern cars is a negative temperature coefficient device, i.e. as the temperature goes up, the resistance decreases allowing a greater current flow. This means that as the temperature of the engine increases, current flow through the gauge increases and the needle deflects toward H. The temperature-sending unit and gauge in the Magnettes do not operate like this. First of all, notice that when the ignition of the Magnette is turned off, the temperature gauge reads maximum H. When the ignition is turned on, the gauge swings down to C and then works its way toward H as the engine warms up. To operate this way, the sending unit consists of a bi-metallic switch, with the switchblade wrapped with resistance wire. The switch contacts are closed when everything is cold. When the ignition is first turned on, current flows through the resistance wire, the switch contacts and the temperature gauge, causing the gauge to move to the C position. The current through the resistance wire in the sending unit heats the bi-metallic switchblade and causing it to bend away from the stationary contact and opens the switch. When the switch is open the bi-metallic blade cools and straightens, closing the switch contacts. This cycle repeats as long as the ignition switch is on. This is exactly the way a flasher unit in turn signals works. Everything is factory calibrated--that is what the adjusting screw on the switch is for--so that the switch opens and closes at the required rate for the gauge to indicate C.  This bimetallic switch action also acts like a voltage stabilizer so
changes in battery voltage will not cause the gauge to fluctuate. When the engine heats up, the heat is transferred to
the bi-metallic blade in the sensor unit causing it to stay open for a longer period of time,
interrupting the current flow to the gauge for longer periods of time. Since the gauge also
consists of a bi-metallic blade that deflects with current generated heat,moving the needle
to C, the longer periods of no current flow allows the needle to move back toward H. In
effect, they have made the temperature sensor act as a positive temperature
coefficient unit (the higher the temperature, the higher the effective resistance causing less current
flow). This is all a rather complicated method to accomplish a relative simple procedure
and one that has a lot of areas for failure. I suspect that those of you who are seeing the gauge work backward have a car in which the sensor failed and someone has replaced it with a sensor from a late MGB (rather than pay the astronomical price for a Magnette sensor). The sensor in my Magnette failed shortly after I bought it (it failed on the freeway between Portland and Seattle, scaring me out of my wits when I saw the temperature gage climb up to and beyond the H). After checking the prices and finding out how the system worked, I purchased a late MGB sensor and built an add-on circuit to reverse its operation. After calibrating the sensor-converter-gauge combination, the temperature gauge in my Magnette now works the way it was intended to and if the sensor goes bad, it is relatively inexpensive to replace. If anyone is interested in doing the same thing on their Magnette, download the diagram and instructions on how to build the circuit and calibrate it with the temperature gauge. This is a fairly involved project and requires a good working knowledge of electronics and construction, soldering and calibration practices. I would not recommend anyone without this skill and knowledge undertaking this project. ~David DuBois,USA Instructions Tempconverter&Diagram Another article from Safety Fast with the same topic The theory in praxis:Like many other owners with "B" series engines I have had problems with apparent overheating and have taken a number of actions to try to prevent it. Recently I decided to check the accuracy of my temperature gauge to see just how hot my engine actually was. I drained the radiator and removed the temperature sender (great care! the original type is irreplaceable). Then I set up a test rig as described in the
above instructions . I connected the wire back to the sender, and earthed the casing to one of the thermostat cover bolts with copper wire. I removed the connections from the coil and the fuel pump so that I could leave the ignition switched on for the duration of the experiment without over-heating the coil and to reduce the drain on the battery. Then it was just a matter of heating the water to a set temperature (measured with a thermometer) and checking the reading on the temperature gauge. I found it was necessary to switch the heater on and off a few times to keep the temperature steady long enough for the reading on the gauge to stabilise. I discovered that with the temperature sender in boiling water the needle went exactly to the calibration point (the small dot under the right leg of the "H"). In case it is of interest to other "Z" owners I made a composite photo of the temperature gauge readings at different water temperatures. I now know that when my engine is overheating the temperature rises to 90°C. I wish I could stop it doing this, but at least I know it is in no danger of actually boiling. Obviously other users may find different results - particularly if the sender unit has been replaced with a later type. Another solution:I had a problem with my temp sensor (who doesn't?). The problem is now
solved for good----knock wood! You may have noticed the lengthy thread on
Dave DuBois's device. It became longer because I started talking about my
quest. In the past we were trying to work around the original sensor. I
solved the problem with my car by replacing the "works" in a
Magnette gauge with the works (needle and all) from a Jaeger temp gauge
#BT 2203/02, a Intermotor sensor # GTR101. Spacers 3/8" thick were
needed to raise the new works high enough for the needle to clear the
dial. (A couple extra holes were drilled in the original case for
electrical leads and terminal protrusion.) The electrical connections were
moved inside the case since the terminals were too short to go through the
case. The length of the gauge terminals also required attachment of the
works to the case be achieved with epoxy cement. (the spacers were epoxyed
in place also). The only other modification needed was the shortening of
the gauge needle so it would clear the case. I painted the needle with
casein paint so it's color would match other needles on the dash. The only
way that you can tell my modification---needle is slightly wider than gas
gauge needle (BIG DEAL) and when eng is not running the needle is to the
left. All in all very please to report my simple idea worked!!! Arch
Boston, USA A Cheap & Easy Solution :For those with a bum sending unit, note that replacements are very
expensive and hard to find. A carefully chosen series resistor ( about 44
ohms, but experiment to get it right ) will produce a reading about
between the normal and high bars at normal temperature, when used with a
late MGB sending unit (plentiful and only $8 + change apiece). Pay
attention to the wattage rating of the resistor, as it will be dissipating
about 1/4 W under normal conditions, much more under over-heating
conditions, and will be operating in an elevated ambient. Now the BIG
question, why settle for this?! Because it's extremely EASY and gets you
essentially all the info as the properly functioning gauge. NOTE, that in
this mode, the indication is reversed, so it starts out indicating full
hot (as when stopped BTW), and will tend toward cold as it heats up. A
slightly lower resistance will have you in the N bar under normal
conditions. I rejected this because I only had a 44 ohm resistor of suffi! Steve Giannoni, New York State, U.S.A. |
