Lotus Elan

10 volt Circuit

PostPost by: KevJ+2 » Sun Jun 21, 2015 5:43 pm

Hello all,
I have now got my dash back in and wired up, and before I move onto the big start up, I am testing all my wiring.
All seems good apart from the 10 volt circuit re: the fuel and temp gauges. Before I installed the tank, I tested (briefly) the sender with 12 volts and the fuel sender, gauge and low warning light worked fine.
In the car, (with new stabilizer) I am getting - 10.4 volts one side of the fuel gauge and 8.3 the other side and also at the sender. I suppose with this drop, I will not get a reading or low level light?
The temp gauge reads 10.4 one side and 9.4 on the other and at the sender. I have heated the sender with a hot air gun until hot, but there is no movement on the gauge at all. All very odd!
I am getting good earth's at the senders and gauges, but don't know what next to check.
Any ideas?
Regards,
Kev.
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PostPost by: Hawksfield » Sun Jun 21, 2015 6:22 pm

Hi

Measure the resistance of the temp sender whilst heating with your heat source you should see a change in resistance ie decreasing also earth the wire at the temp connection point it should read full scale at the guage

The sender is a bit different does it have two connections and earth at the sender if so earthing either of the non earth connection should give a full deflection at the gauge or the low level light.

I am making assumptions here as I don't Know your wiring diagrams but these test's help to identify where your problems is

Good luck
Regards

John

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PostPost by: bob_rich » Sun Jun 21, 2015 7:38 pm

Hi Kev

when I rebuilt my +2S I tested the fuel gauge and temp gauges from a 10V supply with a variable resistor in series with the gauges instead of the sender to confirm the gauges worked OK. I then tested the sender in beaker of hot water and wired it to my test meter to confirm the resistance change with temp. when testing in a beaker of water do not let the terminal immerse in the water as this may effect the reading. The temp gauge info attached is not from the +2S but I suspect will be very similar for a Smiths water temp gauge. The fuel sender worked OK but it precise reading will depend on how the float move relative to the fuel level in the tank. provided there is a resistance change and the gauge works as indicated in the picture all should be OK.

The low fuel level is a switch and does not use need to run from the 10V as it just lights a bulb that is fed from the ignition switched battery feed.

I have attached pictures from my rebuild diary if they may be of help

Best of luck Bob
Attachments
Smiths Temp gauge.png
Smiths Temp gauge.png (16.04 KiB) Viewed 889 times
+2S fuel gauge.png
+2S fuel gauge.png (17.63 KiB) Viewed 889 times
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PostPost by: KevJ+2 » Wed Jun 24, 2015 6:46 pm

Thank you both,
Today I took out the fuel sender, tested it, and all seems good. Fuel warning light lead changed so that works too.
The temp gauge reads 92c but the sender in hot water (and checked with IR) was in fact 78c.
I can't see how to calibrate the gauge, so can anyone suggest an answer, possibly a variable resistor?
Kev.
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PostPost by: mbell » Wed Jun 24, 2015 7:41 pm

KevJ+2 wrote:The temp gauge reads 92c but the sender in hot water (and checked with IR) was in fact 78c.
I can't see how to calibrate the gauge, so can anyone suggest an answer, possibly a variable resistor?


I don't believe you can adjust the gauge with out modifying it. You should be able to get the gauge to be closer aligned by adding a resistor inline with the sender. This will basically offset the gauge so it might not be 100% accurate for other temps but it probably isn't now.

A few ways you could do this:
- Stick in a variable resistor and adjust it with the engine up to temp.
- Use a multi-meter to measure the resistance at operating temperature and calculate the difference between that and the correct value (see Bob's post). Then insert that much resistance.

The system is just a basic potential divider circuit with the sender acting as variable resistor and the gauge measuring the voltage across a fixed resistor. So you can calculate want is going on with V= I x R.
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: KevJ+2 » Thu Jun 25, 2015 11:59 am

Would this be a good variable resistor to put in line with the sender to enable calibration?
If not, could some kind person put a link on here to the correct one.
Thanks,
Kev.
http://www.maplin.co.uk/p/250-andohm-ca ... eter-n37br
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PostPost by: billwill » Thu Jun 25, 2015 2:57 pm

I think you probably need one with a higher power rating to stop it burning out.

If the figures above are correct max current is 123 mA .123 amps, voltage is 10 volts.

To be safe allow the variable resistor to have at least the power rating of the fuel sensor.

0.123 x 10 = 1.23 watts

That variable resistor was only a tenth of that.

Look for a wire wound one.


Possibly this one:
http://www.ebay.co.uk/itm/5W-Wirewound- ... 1642417519



~~~~~~~~
Oh you meant the Plus2 electrical temperature sensor. Still the calculation is the same because that table above shows the temp sensor to be 15 ohms at 130 deg C same as 15 ohms for a full fuel tank.
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PostPost by: billwill » Thu Jun 25, 2015 3:19 pm

Here's another topic where this matter was discussed:

lotus-electrical-f38/temp-gauge-accuracy-t25080.html

Stu came up with about 9 ohms in series so perhaps the 20 ohm wirewound resistor is the appropriate one:

http://www.ebay.co.uk/itm/5W-Wirewound- ... 1642417519

Image
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PostPost by: billwill » Thu Jun 25, 2015 3:34 pm

Here's a message from a Mini-Minor website. Which sugests opening the front of the guage, with the sensor in boiling water, pull off the needle and then put it back on the 100 deg C/212 F mark on the dial.


http://www.minimania.com/Temperature_Gauge_Calibration
Introduction.
Summer is with us once more and the Mini overheating is on everybody's mind, while the temperature gauge won't keep your engine cool at least it will let you know what is happening.

To check that it is working correctly

With a capillary type gauge (mechanical)

Remove the sender unit from the head .Fill a small pot with water and bring to a rapid boil using a propane torch be careful to keep the flame away from any fuel or body parts. Insert the sending unit bulb into the boiling water. Your gauge should read 212? F or be on the start of the "H" area of the later models. If it reads otherwise, and the gauge can be pulled apart (eg Smiths) remove the gauge from the dash, and remove the chrome bezel , Twist the rim until the slots line up, remove the glass lens (it will usually needed cleaning anyway), gently pull the needle off its shaft, and replace it at 212?. Reassemble the gauge, replace the sender unit, top up coolant.

With an electric type gauge

Remove the sender unit from the head as with the capillary gauge. Using as before a small pot with boiling water, Insert the sender unit with another wire from an earth on the car . Once again your gauge should read 212? F or be on the start of the "H" area of the later models. If the gauge can be pulled apart (eg Smiths) it too can be recalibrated in the same way as the mechanical gauge.

Reassemble the gauge, replace the sender unit, top up coolant, and you are on your way, now with confidence in what your instruments are telling you.

If you don't have a propane torch or any other way of heating the water you can remove the gauge and do all this on the kitchen stove, with an electric gauge you will need a powersource and some wiring as well.
Bill Williams

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PostPost by: billwill » Thu Jun 25, 2015 3:46 pm

From an MG website, evidently some temperature guages can be adjusted.

http://www.mgexp.com/phorum/read.php?40,1090952
I tried a small pot' and it didn't work well at all. Just masks how hot the engine gets. A variation on "turn up the radio to drown out the engine noise"
Take it out and adjust the gauge. At the back of the gauge there are two holes which may have cork plugs in them.
The hole at the back cold end is for setting the running 'N' temperature. The hole at the hot end is for setting how far you want the needle to move from N, the max range, the H.

So run the engine to normal running temperature and set the "cold" adjuster end to "N".
Run the engine to just short of boiling, then set the "H" hot end.
I did the latter by un-plugging the fan and watching the outlet of the radiator cap closely, adjusting as it crept up. Then dropping every thing at the first sign of bubbles and switching in the fan. If you have a mechanical fan you might try putting a piece of cardboard in front of the radiator.

How do the adjusters work?
They are not electrical in nature, no variable pots like you might think. They are mechanical.
The gauge operates like this, the top of the indicating needle is held between a springy piece of metal and a bi metallic strip that has an electrical winding around it. as between your thumb and fore finger. The temperature sensor in the motor is a temperature sensitive resistor, the hotter it gets the less resistance it has. This allows more or less (depending on temperature) current to flow to ground , through the winding in the bi metallic strip in the temperature gauge. Generating heat, depending on the engine temperature and the current flowing, the winding heats the strip which either pushes harder or less hard against the springy piece of metal, thereby moving the needle.

The bases of this bi metallic strip and springy piece of metal are riveted to the baseplate at he back of the gauge. Off centre from the rivet is an oval hole. This allows you to lever and rotate the spring or bi metallic strip around its rivet thereby controling how hard each presses up against the other, which controls where the needle points as the top of the needle is balances between the two. As between your thumb and forefinger.

In practice this means a small rounded screwdriver may be 'twiddled' in oval hole to adjust the needle. warning , a little twiddle does allot.

Bill Williams

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PostPost by: bob_rich » Thu Jun 25, 2015 4:21 pm

Hi again Kev

I C U said that gauge said 92 and sensor only 78 with an IR temp sensor. You need 2 B very careful with those gadgets. Some of them have a small LED dot used as a pointer and they convey the sense that the temperature is read at that point. In practice many of them have very wide viewing angles and so unless very close will tend towards the average temperature of the viewing field and thus maybe a bit low.

Viewing boiling water I believe has an emissivity of 1 so thermal imaging kit can be calibrated by viewing that but be careful not to steam up the sensor. For your test and if U R in the UK at a reasonable altitude then if the sensor is reading if it is in boiling water will be close to 100 so I suspect that it may be OK.

Most of these gauges are approximate but consistent especially when fed from a regulated supply of 10V so provided there is not much change when U R running I dont think I would worry. I worry about gauges that change from "usual and familiar" while I am driving!

Hope this helps best of luck

Bob
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PostPost by: KevJ+2 » Thu Jun 25, 2015 6:20 pm

Hi Bill and Bob,
Thanks for the info, much appreciated.
Bill, much as I'd love to twiddle, I think the resistor you kindly attached will be a good option. Also, I think I'll opt for a solid state stabiliser as, although my old style one is new, it does give hit and miss figures.
Bob, I think the IR gauge is accurate as I use it for work. I tested the sender in a cup of just boiled water, but I will calibrate to boiling water to be certain.
All I need now is to sort out the coil / tacho issue and the big start up will follow!
Thanks again,
Kev.
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