Modified temperature gauge - no resistor??

[g8dhe]

http://www.g8dhe.net/bwm/bwmanualp.htm?1995/Page_54
Resistor is placed across the two lower legs. Lower left leg is to temp sensor (one wire) and to ground. If there is really nothing else between the coil, the sensor and ground, is the coil part of a voltage divider? If so, why does the voltage across the sensor "decrease" with increasing temperature? Don't temp sensors normally reduce in resistance with rising temperature? I am confused, sorry. I know I'm thinking about this too hard, just to save myself a trip to Maplin, but I'd still like to understand the circuit I'm modding.

The temperature Gauge is actually a bridge circuit;

So with the Mod applied, and worse case situation, then the top half becomes 38Ω and if the sensor becomes dis. then the lower half is 44Ω, which gives about 6.4Volts across the lower half so about 0.41Watts total dissipation and with a pair of 1/2 watt 220Ω resistors that's no problem.

Edit: Ah we posted at nearly the same time, but tracked down the circuit of the gauge at last so no problem.

[Driver+Passengers]

Nice one, Geoff - thanks for digging out the circuit. I'd (perhaps naively) measured the resistance from the sensor pin of the dash connector to ground - at 10degC it was reading 14K, suggesting a 10K sensor?? Not sure about my method though. Ordinarily, I'd continue on this to further my understanding, but it's the wee man's 4th birthday today, and he'd rather be out driving in the van than watching the back of my head.

Now I've got an idea of what the gauge looks like with a healthy coolant system, I'm ready for the coolant change on Monday.

[splitpin]

When I modified my gauge acouple of years ago the only resistor I had to hand was 0.25 watt , this has worked finefor me so far.

[Driver+Passengers]

I've just put a 100R resistor across the terminals of my 5V supply (50mA, which is a 0.25W). It gets pretty hot to the touch. Next time you've got your cluster out ( ), give the resistor a poke with the ignition on to see how hot it gets. It may well dip below 0.25W once the engine is up to temperature, but I suspect a cold start will result in the resistor drawing beyond it's rated capacity. If it's not hot, I've got something wrong.

[splitpin]

Next time you've got your cluster out ( ), give the resistor a
poke with the ignition on to see how hot it gets.

If I ever need to get it out again I'll try that but as I said it's been working fine for over two years so happy to leave it as it is for now.

[Steptoe]

I got a pack of 5 x 2W resistors delivered for £1.29 and did this mod including bridging the diode.

Now The temp reaches around the mid-point even when flat-out up a long hill, but coasting down the other side the temp drops back to 'C' or even further. 'Normal' driving has the temp sitting around a third of the way to mid-point and the needle moves freely depending on the situation. This is great improvement on no movement from 11 O'Clock position, and if it strays beyond half-way I'd be getting concerned. May play around a bit with values, but I'm happier with it than I was before. I would like it to read slightly higher in normal driving, so that I at least know I have water in the system! (I do have a low coolant alarm as well)

[mikexgough]

Not sure what Dave Mason has in his unit... but with it fitted the temp gauge on my Bongo... it sits dead halfway unless under heavy loading and it rises slowly.... it will creep across to the H... when in traffic (like Rouen) and 5mm from the H line "kick" the fans in and back to midpoint again... with Air con running the "normal" running is I guess 11:30 point...from the fans running with the air con

[mikeWalsall]

I used a £1.50 ceramic ballast resister from Maplins .. but I did not short out the Zenner diode .. so my guagemodification is a waste of time .. it moves to up by about a third on the dial as soon as the ignition is turned on .. then it never moves from cold to hotter back to cold ..

[Steptoe]

I used a £1.50 ceramic ballast resister from Maplins .. but I did not short out the Zenner diode .. so my guagemodification is a waste of time .. it moves to up by about a third on the dial as soon as the ignition is turned on .. then it never moves from cold to hotter back to cold ..

I used a 100 Ohm resistor but I DID short out the diode.
Works a treat:
0 -> quarter - normal light driving - will drop right down to zero if cruising down a hill on little throttle.
quarter -> three-quarters - long steep hill (esp. towing).
three-quarters+ - WARNING!

[mikeWalsall]

I am thinking of selling my V6 LPG JAL Bongo as I need a vehicle more suitable for my disabled son .. so I think I will remove the clocks again and do it properly ..