cooling problems

[Hawksfield]

Late this summer driving short journeys lasting half an hour or so was causing temperature to reach 120c. I suspected the transmitter was at fault and fitted a spare TT6811-1 without success. I was thinking blockage in rad, water pump, seized stat. As I was puzzling the problem I noticed the fuel gauge was slightly rising and falling with engine revs. Finally the light came on voltage stabiliser!, tested same and it was rising from 10 to 12 volts.
Bought a replacement from ebay for ?10.45 which stabilised the voltage at 10V.
I decided after this I would carry out a test to establish the accuracy of my gauge.

Carried out a test of the TT6811 in a pan of raising temp water results below/

Water temperature sender Smiths Type
Temp Centigrade 99 90 85 80 70 50

Resistance ? 53 75 82 98 130 265 Smiths TT6811-1 old spare

Resistance ? 54 71 83 98 130 257 Wipac TT7 New

Test method: rising water temperature on hob

My car gauge test against a temp measurement probe at the Thermostat housing

http://s102.photobucket.com/user/John-H ... uge%20test
Test carried out running engine to record rising temperature

My gauge is not accurate but I now know when it is over heating

[The Veg]

Interesting.

I don't know if this will be relevant, but it sounds similar to something I saw with a different Lotus:

I used to own a Turbo Esprit, and another TE owner in my local area and I both observed our cars doing something similar, in which the fuel and temp gauges would VERY slowly rise and fall together...a very subtle oscillation, if you will, of an amplitude of maybe 1/3 of the range of the gauges. The other guy installed two or three new voltage stabilisers in short order, and when none of them cured the oscillation, he gave up.

Observing all this, I accepted the mystery and learned to live with it.

Then some time later, after the other guy had sold his car, I was doing an oil and filter change on mine, and was at the point of cleaning up. On an Esprit this is very important as the engine leans 45 degrees to the left, and the oil filter is mounted horizontally on the upward-facing side, which means that there is no way to remove the filter without oil spilling from it and it gets all over the engine. So I was underneath the car, wiping-up the oil that had run down the block and sump, and when I wiped around the engine-mount to which the earth strap was secured, I noticed that the bolt that held the strap was loose. Thinking nothing of it, I tightened the bolt. And then I was surprised while driving that from then on, the oscillation had ceased!

To this day I have no idea why the engine's main earth should have anything to do with the operation of gauges in the dashboard, but it did.

[bitsobrits]

"To this day I have no idea why the engine's main earth should have anything to do with the operation of gauges in the dashboard, but it did."

I would think that if the engine was not properly grounded, then any electric sending units mounted on the engine would not be properly grounded either. Unless they had a separate ground wire, of course.

[Chrispy]

I had similar issues with mine. I worked out the additional resistance needed to give the correct readout at 90degC, then added the closest resister to this value. Works well now!