TR4A temperature sender resistance - a favour

[peejay4A]

Having failed to get my temperature gauge to make sense - it now over-reads - would some kind soul undertake a couple of resistance measurements. Stone cold (an estimate of ambient temp would be useful) and at normal engine operating temperature. Even the cold reading would give me a starting point.

Thanks.

[stuart]

1.52 ohms on 20K scale at 45 degrees F in the workshop. Chilly"!

Stuart.

[peejay4A]

Thanks Stuart, you're a gem. Mine measures 1333 ohms at 8 deg C so that explains the over-reading. (Presumably you measured 1.52k ohms). I have some others so I'll select on resistance and give it a go.

[Flyer]

Mine is 2.6K (2,600 Ohms) cold. Resistance will reduce with increasing temperature.

 

 

Chris

[peejay4A]

Crikey Chris, that's miles away from Stuart's and mine. Is yours a 4 or 4A?

[Flyer]

1967 4A and the temperature readings are OK.

 

I suspect the cold resistance isn't that critical, it just needs to be high enough that there is no significant current flow through the gauge.

 

I checked my other car (same type of gauge) which had run and was still slightly warm and that was 600 ohms.

[peejay4A]

At running temperature mine measures 78 ohms.

[AlanT]

I suspect you are going get a lot of arguments doing it this way.

 

Test meters apply a voltage, typically but not always 1.5V and then measure the resulting current.

Then they convert this to resistance.

 

This voltage will warm the sensor.

 

All I know is that the gauges have a resistance of 60 Ohms and it takes 160mA to get full scale.

 

Al.

[peejay4A]

Interesting Al. I wonder what current is required for half of full scale deflection.

[AlanT]

You can short the gauge to ground and check it goes to full scale.

Check the current with your meter.

 

Get another gauge, the fuel one would do.

Put em in series and you will have half the current.

 

Should be in the middle somewhere.

This is rough analog technology not digital precision. Don't expect too much.

 

Al.

[stuart]

As discussed elsewhere it depends on what the full scale temperature of the sender is. Some are 100degree full scale and some are 110 degree full scale and 120 full scale. There are usually three types around with black/green/red plastic collars on them. Mine is black collar and reads correct with a 4a gauge on a stabiliser.

Stuart.

[Flyer]

As I had nothing better to do I substituted fixed resistors for the sender, results were:

 

At the cold end of the scale (in the black square) 210ohms

Centre scale between the calibration dots 80ohms

Hot end (on the red square not full scale) 73ohms

 

So fairly non linear but the gauge sorts this out.

 

Mid scale is about 70 mA

 

I also warmed the engine and using an infra red thermometer and measuring the sensor got 1000 ohms at 30 deg C, 280 ohms at 50 deg and 150 at 70 deg.

 

Mine is a red sensor, as mentioned earlier there are many different colours. I checked another British Jaeger temp guage and this was also mid scale with 80ohms to chassis.

 

The other thing to eliminate is the voltage regulator which also feeds the fuel guage - is this reading correctly? If the contacts were not opening on this then both gauges would over read.

[peejay4A]

Thanks Flyer. I have an electronic regulator, rock steady 10v. Those are interesting readings. My red sensor reads about 50 ohms when the gauge is just under the red section. The temperature of the thermo housing was about 85 degrees.

[peejay4A]

Well there's a happy ending to this saga. More in desperation than anything I ordered a GTR104 from Rimmers - I already had one from Moss and that caused over-reading like the other two. Blow me down, it works perfectly.

[john.r.davies]

The same part for a six cylinder has a very similar part No. GTR108, and no doubt works in exactly the same way. I have had problems with them in the past and have measured their temperature response curve.

 

Uploaded with ImageShack.us

 

 

As you can see, two new ones had the same, expected response to changing temp. but and old one (KR) and a third new one were way out. I've done the same with nearly new sensors and found similar atypical response. Some modern repro sensors have a short life, not even lasting their shelf life it seems.

 

John

Edited March 5, 2013 by john.r.davies

[Westy]

My temp gauge also reads hot despite no signs of over- heating. Bought a point and shoot infrared thermometer gun to check. Gauge is now with Speedy Cables for testing along with original sender and two others that I bought trying to correct reading but only made things worse. Does anyone know whether a 1967 TR4A should have a gauge with 70 and 100 degree markings? Mine has no markings just a C and H at either end of the scale so wondering whether I have a later model gauge?

[peejay4A]

Mine's a C and H gauge, dated 1965 and I think original to the car.

[AlanT]

'66 4A has C-H on the gauge too. Also think this is original gauge.

 

Al.

[Westy]

Quick update. I chatted with Peter Galloway at Speedy Cables today (thoroughly nice chap) who explained the problem with my temperature gauge and sender. Mine is a '67 TR4A and I sent them the temp gaug, the sender that was in the car when I bought it and a couple of other senders that I had bought from Rimmers.

 

The gauge is a bi-metallic strip that bends at it heats (starting to remember my school physics lessons now) and the sender is a thyristor (solid state semi-conductor material) whose resistance decreases with an increase in temperature and so allows more current to flow, thereby heating the bi-metallic strip and moving the dial (apologies if most of you know this stuff already but it was good for me to learn). For my gauge the resistance should be 240 Ohms when cold (reading "C") and 20 Ohms when hot (reading "H"). At a stabilised 10V you can work out the current (V = IR). Problem was that the original thyristor had failed / broken down - basically died of old age. The other spare senders I sent were not a match for my gauge - so basically the wrong ones. So they will supply a correct sender and then adjust the gauge calibration/reset the movement to match.

 

According to Peter, when Triumphs were built in the factory the sender was installed in the engine shop and the gauge in the body shop and only brought together i.e. matched in the final stages of assembly. This wasn't too much of a problem because volume manufacturing meant that the senders were sorted into different resistances and the range (low resistance to high resistance) was tolerable. The senders today are made in low volume (Eastern Europe?) and the range is wider. So it's a matter of luck as to whether the new sender you buy matches your gauge and the only way to be sure is to have them matched.

 

Anyway, looking forward to getting my new gauge back complete with new dial markings and glass in time for a weekend trip to the North Norfolk coast in May..

[john.r.davies]

Thank you, Westy, for confirming by figures from a professional at Speedy Cables the numbers (resistances) that I found experimentally!

 

It's not a difficult rig to set up - just a multimeter and a pan of boiling water.

If you want to calibrate the sender and guage together, then put the sender on the end of extension wires in a water bath alongside the car, so you can power it and the gauge from the battery, and heat the water on a camping stove while you monitor the temperature rise of the water bath with your IR thermometer.

 

John

[Paul Harvey]

To add to the discussion, my temp standard 4A gauge (C-H markings) was reading just below the red with no sign of overheating (electric fan never cut in) after I fitted an oil cooler thermostat (previously the gauge had been reading very low)

 

Without changing anything else:

1. I bought Rimmer's sender GTR104 last week and fitting this immediately reduced the reading to about the 3/4 mark.

2. Today I fitted the proper bellows thermostat (a fortune from Moss - over £60!) and now the gauge reads just past the half way mark, ie. dead normal.

 

There's probably a bit of luck here so I've also bought a 'point and shoot' thermometer to check what the actual temperatures are!

Somehow I could never cope with the gauge reading nearly in the red even if I knew it was over-reading.

[ianc]

Rather than spending a fortune on a bellows thermostat, fit a Waxstat and restrict the flow through the bypass hose by inserting into the hose a lump of material with a small bleed hole throught the middle. Works a treat and the water temperature in the cylinder head will rise far more rapidly - even in really cold weather (such as we've had of late) and keeping to a maximum of about 2300 revs, my gauge is almost up to normal operating temperature within a mile and a half of home.

Ian Cornish

[peejay4A]

Mine with a sleeve stat comes up to temperature quickly.

 

(null)

[AlfredHitchcock]

Friends.

 

On starting my engine last week I saw that both the fuel and temperature gauge seemed to be working fine. As the temp rose the needle rose.

 

Then as the engine got hot they both suddenly stopped working.

 

Ive got a new modern voltage stabilizer and new ebay special temp sender. I checked the voltage in to the stabilizer was ok but voltage out was higher than 10v. On removing the vs I found it was cooked.

 

I measured the resistance of the temp sender at room temp as 643 ohms and the resistance across both gauge circuits as 200ohms. I'm stumped as what to do next because reading though this thread suggests that even shorting the senders doesnt overdrive the vs

 

Any ideas gratefully received.

 

Nick

[RogerH]

Hi Nick,

the modern VS is a simple solid state voltage regulator - 13V in = 10V out.

 

However you have to treat them right - if you used this device in a standard electrical circuit you would put capacitors either side.

 

On the input (12V side) try a 47 or 100uf capacitor and on the output (10V side) use a 0.1uf. One end of each cap goes to the relative in/out terminal. The other end goes to earth.

 

The regulators are not perfect devices and the 12V supply on the car is very very noisy. So you can easily get a ringing/resonance building up.

This can increase the current through the otherwise well protected device.

 

Also a heatsink wouldn't go amiss.

 

Roger