Modified temperature gauge - no resistor??

[Driver+Passengers]

http://www.igmaynard.co.uk/bongo/member ... pgauge.pdf

If I were to begin the job by removing the cluster and bypassing the zener diode, but then fail to find a suitable resistor in my many boxes of bits before I next need to drive the van, what would happen?

[gasace]

I think the gauge would read high, when I did mine it was reading high and I had to add a higher resistor to bring it down, so with no resistor it would probably be on H all the time

[Driver+Passengers]

...with no resistor it would probably be on H all the time

Cheers, ace. This is what I thought - I'm concerned that if it was "trying to read far, far beyond H" that the coil would draw to much, or something...?

I've found a bag of resistors, but pretty sure they're all 0.5W. From the factsheet: "For mechanical strength, the thermal rating of resistors should be 2 Watts."

I know I can run a few higher valued resistors in parallel and get somewhere around the 100 ohm mark with the appropriate power rating. I wonder if anyone can provide the necessary figures to back up to 2W requirement?

[g8dhe]

The power isn't the critical factor, its the strength of the leads to hold the resistors in position with the vibration, provided you use a couple of 1/2 watt resistors and twist the leads together close to the body then you will be OK, try a couple of 220Ω to give 110Ω in total, but as above twist the leads starting from the body on each side of the resistor for there full length before terminating and removing excess.

[Driver+Passengers]

The power isn't the critical factor, its the strength of the leads to hold the resistors in position with the vibration, provided you use a couple of 1/2 watt resistors and twist the leads together close to the body then you will be OK, try a couple of 220Ω to give 110Ω in total, but as above twist the leads starting from the body on each side of the resistor for there full length before terminating and removing excess.

Thanks, Geoff!

If I rotate the two spade connectors such that I can solder the resistor to them with minimal exposed wire, would a single 0.5W do? Threading the lead through the hole in the spade and folding back towards the body of the resistor, twisting round once and flooding with solder... I would like to think that would be sufficiently mechanically stable. Provided 0.5W is sufficient... (I have not found a pair of 220R yet...) Also, I'd probably prepare the solder joints before mounting, save excessive heat soaking down through the posts.

Your thoughts?

[g8dhe]

I didn't check the power dissipation when I did mine, but the mounting method was similar to what your describing. Might be worth measuring the voltage across the final combination, then we can calculate the wattage being dissipated. Likewise I didn't physically remove the Zener, merely carefully twisted the component a couple of times so that the wires shorted out thus removing the Zener from the circuit.

[Driver+Passengers]

I didn't check the power dissipation when I did mine, but the mounting method was similar to what your describing. Might be worth measuring the voltage across the final combination, then we can calculate the wattage being dissipated. Likewise I didn't physically remove the Zener, merely carefully twisted the component a couple of times so that the wires shorted out thus removing the Zener from the circuit.

When up to temperature?

[g8dhe]

It would be best to measure the power from cold to overheat, but I doubt that you would want to do that But I really can't see the power changing very much from the normal mid-point!

[Driver+Passengers]

Vtyp = ??

Vmax = 12V
R = 100R
Imax=Vmax/R = 0.12A
Pmax=VmaxImax = 12*0.12 = 1.44W

Suggests a 2W resistor is advisable for "worst case".

Edit: it's in parallel with 80R, but that doesn't change the calculations above.

[Driver+Passengers]

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.

[Driver+Passengers]

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.

[Driver+Passengers]

5R resistor from T-U to the connector. At 10 degC, there appears to be a -4.1V potential between that pin and the E pin. Suggesting a 0.5W might not be enough if this voltage were to increase with a drop in temperature.

No further forward, other than I think I'm not going to risk a single 100R 0.5W resistor. Will try and source a 2W...

[g8dhe]

I have a link to the original mod's for this type of gauge, you will need to use IE as its quite old and used the Microsoft format HTTP pages www.tyrion.net/miata/tempgauge/90-93-MTGLP.mht, I also found some other stuff at the time which gave more info on the coils involved but I don't seem to have bookmarked the site(s). I am pretty certain you won't have a problem with a pair of 1/2 watt resistors, I mounted mine a little clear of the flexible PCB as well to ensure that any heat generated didn't degrade the "board".

[Northern Bongolow]

[quote is the coil part of a voltage divider? If so, why does the voltage across the sensor "decrease" with increasing temperature? [/quote]

as i understand it, it is. the difference in the 2 legs is measured and shown on the gauge, i just bolt the 100 or 110 ohm on to the back of the external circuit track, and leave the zenner alone, i dont bridge or by pass it at all, works for me

[Driver+Passengers]

Got 2x 220R resistors from a local TV repair shop - either 1W or 2W, but up shot is 108.4R >=2W.

Idled for a minute or so got temp sensor reading 15-16 degC. With keys turned but engine off, voltage across resistor(s) was 6.1V. Drove around, got my temp gauge reading 78-80 degC. Again with engine off, voltage across resistor(s) was 4.6V (dropped around 0.5V with headlights on).

My conclusion is that a pair of 0.5W 220R resistors to give 1W 110R is "probably" OK, but a single 0.5W is definitely NOT. To be safe, either accounting for Vmax in event of some component failure or to give a 2x margin in normal operation, 2W should indeed be the minimum rating.

Sorry for not being unconditionally trusting...


Don't have a picture of needle position, but starts moving at around 65 degC (on my digital thingy) and at 80 degC (normal running temp) is "the same angle as two/thirds between 1/2 and 3/4 mark on the fuel gauge".

Job done - thanks, Geoff. Happy I went with >=2W.