Temperature Gauge Problem

[Ian Slade]

Geoff, If you connect the wire of the sender to earth at the sender end the gauge should read maximum, if it doesn't it's either wiring or the gauge, so back o testing the gauge.

[zuurb]

Hello Geoff,

Before you invite the exorcist i advise you to check how the outlet pipe extension to the radiator top hose ( nr 145 0n plate 3 in the in the spare parts catalogue ) is mounted. This part has a bulbuous shape on one side and that should point downwards. When that bulbuous shape is pointing upwards it could cause that your sensor is ( partly) hanging in hot air instaed of hot water.

Kind regards,
Simon zuurbier

[Ian Slade]

Simon, it is immaterial whether the sender is in hot air or water, it is screwed into hot metal, the temperature variation would be almost negligible, Geoff has a problem of about 100 deg F.

[Soundmike]

Have you got the CORRECT sender? SP250 & V8250 have different gauges and senders. You need a multimeter to check resistance of sender. I'm away from home at present so can't give you the figures, perhaps someone can assist here.

[Fossil]

Apologies to all who responded for my failure to conclude this story.

Ian, thank you for your reminder about earthing the sensor lead, I'd forgotten to do that. Both gauges show full scale deflection. Another suggestion I should have thought of, offered at the DLOC AGM (thanks Matthew!) was to disconnect the rad fan, and upon doing so the gauge needles do in fact continue their upward motion. As to why the gauges don't show a more accurate temp reading, perhaps their calibration has been disturbed or something like that.

Anyway, armed with these insights I will now more happily set off southward on June 4th with my trusty digital thermometer in the glove box, reassured by the fact that the gauge does respond, and of course provided that I can complete all the other things that still need doing!

Ta everyone!

Cheers

Geoff

[Ian Slade]

Geoff, the original units are sold as a matched pair, today this would be too expensive, better manufacturing and different materials for modern cars and no actual temperature overcomes the problem. The solution is simple but time consuming, on the rear of the gauge there are 2 nuts, these nuts when undone allow the magnets in the gauge to move which in turn changes the needle deflection, using your digital thermometer, a 12v battery, a suitable container ( old kettle) and a bracket to hold the sender and a couple of lengt6hs of wire. Connect the gauge and sender, loosen the nuts, heat the sender to 90deg C or its Fahrenheit equivalent and adjust the magnets to set the needle at that temp, check the accuracy at 10 deg either side of 90 deg C, anything else doesn't really matter, ( your original set up was no more accurate) we only notice when the reading doesn't agree with what we expect at normal running running, or reads low when the car is overheating.

[Crossley Stephenson]

Just been through the recalibration of my temp gauge. The sender is a thermistor - as temp increases resistance decreases - at 90F the resistant should be 83 ohm, at 185F the resistance should be 43 ohm and at 230F the resistance should be 3 ohm.
Buy three resistors of the three values mentioned, disconnect the wire from the sender, connect to one of the resistors and the other end to earth (a couple of wires with clips helps) and check the gauge. Adjust as per previous post for each value.

BTW after changing sender, recalibrating temp. gauge, new thermostat, flushing rad. and engine block the problem was the rad. - 60% blocked. The gauge was correct, the engine was running warm, not overheating but too warm with no margin for a hill on a hot day. A Laser thermometer was a bit misleading as it reads external temp. and an allowance of about 10-20F is common.

Good luck,
Michael

[Fossil]

Ian and Michael

Thank you for your further advice on recalibration, yes I know the two small nuts on the back of the gauge, but sorry to say this will have to wait for another day! I fastened the dash centre panel back into the board this afternoon and must press on with the remaining work if there is to be any chance of taking the dear machine to the annual gathering in June. However it sounds like fun for one of those future "what to do when there's nothing to do" rainy days. Takes me back to the days of early blood gas analysers, the late 70s, when the machine required a high and a low calibration to be done prior to every analysis, and because the device was in the ward and not the lab, the dear doctor had to do it by himself, and the results were far from certain. Those were the days, perhaps?

Regards

Geoff

[Sydsmith]

Hmmm!!! sorry to be pedantic but a the resistor values quoted by Mike are not easily obtainable, you could use a small 100 ohm variable preset and set it for each temperature value. Syd

[Crossley Stephenson]

OMG, and we thought you guys were still years ahead of us. I went to Jaycar (local electronics retailer) and bought he resistors over the counter; about £1.50.
Cheers
Michael