[stuarted]

I've been having problems with the temp gauge on my '95 916 and after doing some basic checks from tips from another forum, suspecting the gauge rather than the sensor, I purchased a used gauge sourced from the USA (half the price of a new unit). Now I've fitted it and am finding it's giving the same incorrect readings as the original. Either both gauges have the same fault or it lies elsewhere. I don't want to return the unit until I'm sure that it's the culprit. I've been trying to isolate the problem myself but haven't succeeded and am wondering if anyone can shed some light.

When connected and with the coolant at room temp (about 15 deg C) on switching on the ignition, the gauge goes almost to full right scale (actually about 110 deg C). Disconnecting the wires to the sensor makes no cange to this reading. I've checked the sensor on the bike by measuring the resistance across the terminals and it falls in the range given for this room temp in the handbook ie about 5 kohms. When measuring the voltage across the wires to the sensor (sensor disconnected, ignition on) it's about 8 volts which seems a bit low but I don't know what it should be. Across the terminals at the gauge its the full 12 volts. Is the wiring at fault? What further checks can I do? Is there somewhere where I can get the two gauges checked? The bike runs fine by the way and the fan doesn't come on under normal winter running conditions.
Does anyone have any suggestions?

 

[tye1138]

It could be a faulty gauge, seen it before. When they are damaged internally, the voltage peaks the meter, even when the sensor is unplugged. I'd start wiggling cables to see if anything makes a difference, maybe even tap on the gauge a bit from behind with a screw driver handle, see if makes a difference. Funny enough, I have the same problem with a VDO gauge on my BMW right now, if you tap it, the gauge works. Otherwise, it just peaks. In my case, its not a cabling issue, its actually the gauge internally.

 

[Shazaam!]

I suspect the wiring — and particularly the three in-line electrical connectors — between the gauge and temperature sensor. Check for corrosion and water.

 

[stuarted]

Have tried wiggling wires and tapping gauge(s) but doesn't make any difference. I was concerned that plug to sensor was not making good contact but have bridged with wires and clips remotely and still no change. It's strange that disconnecting the sensor doesn't have any affect on the gauge.

 

[stuarted]

I suspect the wiring — and particularly the three in-line electrical connectors — between the gauge and temperature sensor.

Where do I find these?

 

[Shazaam!]

Where do I find these?

There's a wiring diagram in the back of your Owners Manual.

 

[stuarted]

There's a wiring diagram in the back of your Owners Manual.

Yes, but it's not at all clear what the temp circuit is as the labelling is almost non-existant!
I assume the sensor works as a shunt across the gauge so that when the coolant temp rises (and the sensor resistance decreases), more current goes through the gauge and winds the needle up the scale. If that's correct shouldn't I be seeing the full 12 volts across the sensor (ignition on of course)? In fact it's a lot lower suggesting additional resistance in that arm of the circuit. Maybe as you suggested from a poor contact somewhere.

 

[stuarted]

I think I've been barking up the wrong tree! I don't think the gauge has anything to do with the sensor but gets its signal from the coolant temperature transmitter and I think that's my most likely problem source. Trouble is they're $106 here in Australia so if I'm wrong it's more money down the gurgler.

 

[tye1138]

Ohh shit, I mis-read your first post… you replaced the GAUGE prior, I thought you replaced the SENSOR prior. My bad… YES its a bad sensor. Spend the money, problem will be solved.

 

[stuarted]

How I got confused is that I had read elsewhere that the gauge feeds off the sensor but actually the sensor feeds the ignition computer and it's the thermistor (sometimes called the coolant temperature transmitter) that sends the temperature to the gauge. At least that's what I'm assuming after decoding the wiring diagram! I've just bought a used thermister ($A35 including postage from Florida) from Gotham Cycles so we shall see!