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MG MGB Technical - Temperature Sender Problem

Hello All,

After browsing the archives and finding some helpful info, I still needed to post a question.

I've been resurrecting a 75 that sat for several years. At this point, the car runs great (sometimes -still many gremlins), drives good and looks good.

The temp gauge always showed right at "N" no matter how long the car ran or drove. I was quite amazed, as my '80 with the electric fans is a little sporadic in the temp dept. While working under the hood a few weeks ago, I broke the little tab on the temp sender. I rigged it up temporarily and it still showed worked its way right up to "N" and stayed there.

Last week I ordered a bunch of stuff from Victoria British and purchased a new temp sender. The gauge now ready VERY HOT. Your opinions as to if the old unit was bad, the car is overheating, or the car is fine. I'm tempted to put the old sending unit back in.

Thanks,
Chris
CRH Hoebel

You can't be sure until you measure the actual temperature. I recommend using an infrared thermometer to measure the temp of the car at the sending unit when it's fully warmed up.

N should equal abor 195 F. If your's is much higher than that, the new gauge is correct. If not, it could be defective.
Dan



According to my Haynes manual,
there are three sorts of thermostat's
Hot climate(early models) 74deg C
Standard early models 74 deg C
Standard Later models 82 deg C
Cold climate early models 82 deg C
Cold climate later models 88 deg C. so that might have something to do with it!
Now it could be that you have a genuinely faulty sensor.
How can you tell for sure?
Heat a pot of water to boiling, put a cheap alcohol type thermometer in it. Take the water out to the car and stick the sensor in it. Compare as the pot cools.


I don't even own one of the cheap sort of thermometers but you can recalibrate the gauge to the new sensor easily enough without it.
It must be the season for temperature gauge/sensor problems, some one was just asking about this, here's a big cut and paste!

In the back of the gauge are two rounded slots, one either side. I am reliably informed by Paul Hunt that these may have cork stoppers covering them
One controls the bottom setting of the needle and the other controls the extent of the range of the needle movement past that point (the top setting). Bottom end one controls the bottom.
There are two small plates riveted in place inside the gauge. These plates are attached to and control the mechanism. The slots allow you to turn each plate around its rivet. The plates rotate around the rivet, not the slots, about 1 cm distant, so movement is off-center. They also don't move very far and small movements have a large effect.

I calibrated mine by first connecting the gauge and then warming up the motor to normal operating temperature, then setting the bottom end of the range to the middle of the 'N'. I then allowed the motor to get hotter while watching the radiator outlet closly. The moment the water in the over flow bottle started to rise and made a bubble or two (118 degrees C , pure water at 100KPa, or 15PSI) , I reconnected the fan and set the upper range on the gauge to the middle of 'H'.
I then repeated the entire performance to check.
The outlet of my radiater over flow is into an overflow bottle, so bubbles of steam are obvious and I have an electric fan. If you have a mechanical (not electrical) fan, you could possible place a piece of stiff cardboard infront of your radiater and pull it away the moment the engine shows signs of starting to boil. Don't use a cloth, it might get caught up in the fan and you with it, thereby qualifying you as a nominee for the Darwin awards.

The gauge works by having the base of the needle held between the ends of a springy strip of metal and a bimetalic strip with a winding around it. When the bimetalic strip is heated (by the electrical winding), it pushes harder against the springy strip, via the base of the needle. The Needle moves as a result. Like holding a pencil between two fingers, one finger pushing against another.
One of your slots controls the initial tension/pressure of the bimetalic strip and the other sets how hard the springy strip is pushing back. So you see, mechanical adjusters, not electrical.
If you open up the the gauge and have a look it is immediately obvious how they work. You take off the glass by twisting the chrome bevel until the spaces align and then undo the electrical terminals at the back, to get the mechanism out of the case. Just make sure you put the insulators and washers back in the same order, with the same orientation (brass tab' up or down etc). Take a moment to write the order down. This might be a good chance to paint the inside of the gauge white (one quick squirt from a spray can, dry in 10 minutes). This makes the light much much brighter at night.





It could also be that you in fact have a Range rover temperature sensor. The mid '80's RR's had a sender that looks exactly the same as an MGB one. My motor is from a '90's rangerover and that is why I had to recalibrate the gauge. There is a good chance that the '80's sender had the same resistance as the '90's one, although I have never checked this.
Temperature sensors often have a small number on them. Obviously it should be the same.
Obviously I didn't have any problem recalibrating the gauge to the rangerover sensor, which is meant to run at 87. Good luck.
Peter

There has been some discussion of late about senders and gauges, and incompatibilities if you get the wrong combination. A gauge will also read very hot of the wire is grounding anywhere, so the first step is simply to disconnect it from the sender and check the gauhe drops back past 'C'. If so, then put the old sender back in and try that. If that's OK then you know your new sender is either faulty or the wrong one. The biggest risk these days with replacement parts is that no matter what the part number, insulator colour, or numbers marked on the device it could still have the wrong characteristics unless you can get an OE or NOS item.

There are loads of threads on this in the archive and I can't find the most recent one with lots of info in it including temperature/resistance characteristics, but it is summarised here: http://www.mgb-stuff.org.uk/wn_coolingframe.htm and click on 'Temp Gauge'. If the reading is not a huge amount out it should also be possible to recalibrate the gauge to show 'N', *IF* you are sure that it is indeed running at a 'normal' temperature and it is just the gauge that is reading high.
Paul Hunt 2

Hi Paul, or anyone who wants to weigh in,
I'm currently having some trouble with my thermoswitch (otterswitch?) on the rangerover V8 thermostat housing. My turn to run a temperature!
This is the range rover aiconditioning switch and switches itself on at 94 degrees C. This worked perfectly for the last three years, suddenly it took to not switching itself off until the engine was cold. The hysteresis function out of whack. Not Every time, just every now and again, with increasing frequency.
OK I thought, switch failing. I put in my spare. This one did not switch on at all.
I shorted the switch connector to keep the fan on all the time while I went to buy a new switch.
Fitted the new one at the shop ($65 !!). This did not work. Changed the new switch for another one they had. Same problem.
Four in a row! The odds are about 99.99975 to one against. I have my moments of paranoia and angst, but even I am not that unlucky.
The switch operates a relay. Power from ignition to the earth on the relay. Relay works fine and ignition power supply OK, shorting demonstrates that.
I even stuck a couple of these switches in a pan of boiling water. The resistance was infinite at 100 degrees. They should switch at 94 degrees C.
I'm mystified, these switches are supposed to be trouble free.
I'm going to go fit a Davies craige switch (like the Ken Lowe one), but I hate leaving the problem unsolved and I rather liked the neat use of the Rangerover switch. The adjustable one is messy.
Any thoughts anyone?


I'll put this on the V8 site as well, it might be buried here and If I'm having trouble some one else also will, eventually.
Peter

Saw it there first so I'll copy my thoughts here:

SU Burlen do a replacement for the factory V8 Otter switch (so-called becasue it switches on when the water gets 'otter). It seems to have a slightly lower switch-on than my old Otter, no bad thing, but that could be because mine was on the way out. It also has a wider and more variable hysteresis, sometimes it doesn't switch off until the temp gauge is practically down on N which means the fans are running longer than they used to. At others the temp gauge hardly varies between on and off and the fans only run briefly but frequently.

The odds of getting a succession of faulty parts from the same supplier are actually quite high these days, especially if they come from the same batch. Replacement parts are rarely as good as the originals unless you can get OE or NOS, and AFAIK genuine Otters haven't been available for years.

The Otter in the factory V8 is a single terminal device i.e. when it operates it puts out a ground picked up from its physical mounting and that is used to operate a relay which is backed by a fused ignition 12v supply. Two-terminal thermostatic switches were used in the 4-cylinders cars and operated the fans directly i.e. no relay.

The air conditioning only comes on at 94C? Is that the economy version? :o)
Paul Hunt 2

Chris

The thread Paul referes to is in the 2007 archives with the title "Temperature Gauge / Sensor Problem"

Please post any temperature / resistance data you may get for your sensors. So far the data all seems to fall along the same curve with only minor variations.

Larry
72BGT
Larry Hallanger

Just in case some one wants to put in a adjustable therister and doesn't like the under the hose capillary, Ill copy here

Yes Paul, we are soooo.. tough in Australia that only a girly wimp would even consider turning on the air conditioning at less than 94 degrees Celsius!
Re reading my post I believe I was mostly looking for confirmation that the switches were bad (which seems to be the case) and partly hoping someone might have a miraculous solution.

I've installed the local version of the Kenlow switch. I was trying to ovoid that because I really hated the under the hose stainless capillary concept, Makes taking off the radiator hose potentially expensive. Also when I tried it was hard to get on and it leaked copiously, in spite of the rubber "seal" provided. No matter how hard I did it up. You'd need a very thick rubber hose for that to work. The installation instructions suggested that silicone be used to deal with persistent leaks (as opposed to the more usual intermittent leaks I guess!). I really didn't like that.
Obviously this is a common problem as Davies Craige have now just (March) produced an installation kit for $30 ex tax.

http://www.daviescraig.com.au/documents/Thermal_Switches/0409_instructions.pdf


http://www.daviescraig.com.au/main/thermal_switches.asp?prodid=4

Obviously I could buy a tap and install the compression fitting into the thermostat housing by either using a 22M plug/bush, or getting a blank housing. This I'll almost certainly do at some stage, in the mean time it works very well, it was quick and doesn't look too bad. The sensor points down and is more or less out of sight.
If someone had time and didn't want to buy the kit, they could easily source and buy the compression fitting body, compression nut and 6mm brass olive. or quarter inch depending on the sensor. The brass olive is the type that has concave ends. Longer in the center, receding to the outer edge. <ZZ> It squeezes the olive evenly around the sensor over about 5mm in length. It doesn't pinch the sensor. A search on the net pulled up a variety of suppliers. However I was in a hurry and I wouldn't have saved too much from $34 what with all the driving around.
Peter

Thanks. I'll probably work on it this weekend. Any thoughts as to where I can buy an infared thermometer? As for having the correct part, I did purchase it from victoria british, and there is one for older models, and one for later models and i have the correct one. I might order some stuff from moss next time, and since the part is less than $10, I might just order another one and see.
Chris
CRH Hoebel

Chris

I got my IR thermometer from Harbor Freight. They have a variety of models and mine is not quite the least expensive.

HTH

Larry
Larry Hallanger

An IR thermometer is going to be relatively expensive, a “one of” use, and not very accurate in so far as the interior of the motor goes. They are very useful for speed and ease of use. Not good for calibration.
Just go buy an ordinary cheap thermometer that goes to at least 88 Celsius, or 190.4 F equivalent. Any glass $5 one will be way more accurate and reliable than a gadget.
Unscrew the temperature sender.
Heat up some water to boiling, perhaps put it in a thermos. Take it out to the car.
Put the sender in the thermos (or other container) of hot water with the thermometer and wait until the water temperature to 88 degrees. Or 74 or 82 as appropriate.
The MG gauge should show N.
If it doesn’t, then adjust it so it does using the slot adjuster on the Cold end of the gauge. You now know that when your gauge reads N it is exactly 88 (or 74 or 82) degrees C.
If you want to calibrate the gauge to the Hot end of the scale then do as per previous.
Repeat the entire process for accuracy.


If you doubt the glass thermometer, then put it into boiling water, it should read 100 degrees C. If is doesn’t then apply a correction (ie minus 0.75 degrees) when you adjust the gauge. It doesn’t get more accurate than that!
Good luck
Peter
Peter Sherman

"into boiling water, it should read 100 degrees C"

To be pedantic water just *starts* to boil at 100C, and then only at sea level. Above sea level it will boil at a lower temperature. Boiling violently it will have a higher temperature than when just boiling.
Paul Hunt 2

True about the sea level thing, however it is less than 1 degree C for a 1000 meters. Also the local barometric pressure (i.e. big storm) will also have a small effect. For our purposes is doesn’t matter

However I must politely disagree on the latter point. If the water is reasonably pure it will stay at 100 degrees until all the water is gone. Then it (the pot) can get hotter again. The change in state uses up the heat energy. However if the bulb of the thermometer is resting on the metal of the pot it would be a little hotter so in that respect you are quite right and that may have been your practical experience. However, for calibration you must suspend the thermometer in the main body of the water.
Of course there is also the Phenomenon of superheating for super pure substance. A few unfortunates have encounterd this with microwave ovens.
A non pure substance (like petrol) will boil over quite a large range.

Actually when I calibrate a thermometer I usually just use the ice point, a slishy mix of ice and water. However the 100 degrees C is closer to the 88.

Peter Sherman

I think if the bulb of the thermometer were resting on the bottom of the pan it would read *a lot* hotter, and before the water boiled to boot. I was talking about being suspended.
Paul Hunt 2

Well, I do believe that the new sender is bad. Though I'm sure using boiling water in a thermos and a meat thermometer (which was what was available) is not 100% accurate, the dash continued to read well above N at and slightly below 88 C. The old sender stayed right around N. I'm still going to order another from moss next time and see what happens.

Thanks for all your help!
Chris
CRH Hoebel

Chris - you may get another one just the same. I have grave doubts as to the specifications of many replacement parts these days, fuel gauge senders is another one where replacements can be wildly different from the original, making recalibration of the gauge essential if you are not to run out (BT, DT!).
Paul Hunt 2

What did the car dash gauge read when the thermometer read 74 degrees?
If the car gauge read N when the thermometer read 74 then the sender is a later one.
You can reset your gauge to the new sender.
Peter Sherman

This thread was discussed between 23/09/2007 and 03/10/2007

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