LouC wrote:
While that (wrong sending unit) is a possibility, those VDO panels were known to give bad readings. The only way to know for sure, is if someone on this forum has access to a manual that covers boats that used that instrument panel. Then you could look up the ohms specification at different temperatures. Keep in mind that this is not a Volvo or OMC engine per se. It is a standard, plain old Chevrolet 4.3 liter V-6 engine. As such, there is no European vs American sending units, the gauges can be different because they can be calibrated in metric units but the sending units I bet are the same. Its not even a metric thread in the intake manifold, I bet its an US spec SAE thread.
I would do the test I am suggesting with a normal gauge, see if it runs at the correct temp. Or even easier, get an infared temp measuring gun and measure the temperature on the intake manifold right under the themostat housing. If your engine is running at the right temp, with a 160* thermostat, you should see 150-155* F there. If so, then you can take out the sending unit and test it with an ohmmeter. As I listed the specs above. If that checks out and the engine temp is really correct as per the IR temp gun, it has to be the gauge, or wiring.....
look here at the resistance values for temperature gauges, you can see the ohms are the same for the corresponding C* and F* temperature ranges....
http://faria-instruments.com/site_manuals/IS0085E.pdfThanks a lot for your response.
Today I tested the old temp sender which was installed from the previous owner. I replaced it because of the issue. This sender has a resistance of nearly 200 ohms at 100°C/212°F. According to the specs of faria in your link, it cannot work with a "normal" instrument.
To test the instrument I'll install potentiometer with a defined resistance range at this weekend. Then I can see if it works at the right resistance. To find out the engine temperature I have to finde another solution, because I haven't an IR temp gun. I'll see and let you know.