Got puzzled

[chris_cussen]

For some reason the temperature gauge on the Marlin has started to show a slightly higher reading over the last couple of months. Whereas it used to be rock steady on 80C it seems to have crept up to 90C. Just enough to make me a bit anxious.

So I decided to see if there was anything obviously wrong. On the modified Zetec engine there is a water rail which contains the thermostat and next to it is a tapping for the water temperature gauge, and another one for the ECU temperature sensor. The ECU is from Omex and is supposed to put the fan on, via a relay, at 90c and turn it off at 85.

I fitted a LED to the relay output and tested the relay worked using a wire to activate it. The LED and fan came on, so I positioned the LED to it just stick out under the bonnet and could be seen when driving.

I planned to drive to Montacute house near Yeovil to marshal on todays Parkrun. As I joined the A303 the gauge showed a temperature of 50C but the LED came on. Bugger I thought that is not good and pulled into a layby. As I did the LED went out. I checked the fan wasn’t running, and the engine certainly was not hot. Back on the A303 and the LED came on again, until I slowed until less than 50, then it went out. Meanwhile the gauge was slowly going up, but hadn’t got to 70C yet.

As I approached Montacute the gauge was indicating 90c, the LED was on until I slowed down and then the LED went off.

Puzzled I did my marshalling stint, then on the way home, with the same kind of results I realised what was going on.

A motor and a generator are mechanically the same, except with a motor you put electricity in and get rotation out, but with a generator you put rotation in and get electricity out. So the fan was windmilling when I was going fast and producing enough electricity to light the LED.

I’m now going to change the temperature sensor.

[Tony Stott]

Re: Got puzzled

why not put a diode in the wiring between the fan motor and the relay to allow power in one direction, that would stop the led lighting?

[scott h]

Re: Got puzzled

What about trying an infra red temperature probe? Can be had for not too much money and will allow you to check the actual temp of whatever you point it at, and see if the gauge is at fault.

[milliemarlin]

Re: Got puzzled

An LED is in actual fact a Light Emitting Diode so another diode will not help. I think a series resistor so the LED only lights at 12 V might do the trick. I doubt the fan will generate more than a few volts. LEDs will often light at about 2V. Or better still connect it to the relay coil.

I too have a Zetec/Omex set up. There is a discrepancy between the Omex sender and the gauge although they are both in the same housing. They differ in absolute temperature and in time. The gauge is not as responsive as the Omex sender. You can set up your laptop to monitor the Omex and that will tell you the temperature reading in the unit and when the fan is activated.
Paul

[Tony Stott]

Re: Got puzzled

It would however prevent a back feed which could destroy the led, often used in r/c speed controllers!

[chris_cussen]

Re: Got puzzled

Thanks for the suggestions. I've got an infra-red temperature ray gun gadget which I tried out after the LED attempt, It showed the temperature of the water rail as somewhere between 70C and 90C depending on which bit you pointed it at. I don't think that having the exhaust manifold directly below the water rail helped getting a consistent reading.

I did consider using a 25 watt stop bulb in place of the LED as it would need 2 amps of current to illuminate it not a few milliamps, but I've just decided to change the sensor.

[Dane_Rescueman]

Re: Got puzzled

A few years ago I put a post on the forum describing the LED problem. I just put a high power diode in the main fan feed. An LED was then connected upstream of this power diode. When the fan operates, the LED lights up. When the fan is windmill in, any generated power can't pass back through the power diode thus not Illuminating the LED. It is all still working correctly after 25000 miles

[Tony Stott]

Re: Got puzzled

That was the basis of my suggestion probably not described properly!!

[chris_cussen]

Re: Got puzzled

Can't remeber exactly how I did it but I used a combination of diodes to control the reversing lamp so that it would only come on when the ignition and lights were on. Initial attempt meant that if you put the reversing light on it kept the engine running even after you switched the ignition off. Should have made a note of how I did it but since the bodge seemed to work I left it as it was.

As to the LED for the fan it was just a temporary modification to see if the fan was coming on. Using a 25watt bulb instead as the fan doesn't produce that much power.