Coolant temp gauge

[924549]

Looking for some electrical advice. I am using a Westach temp gauge on my 582 coolant system. Had the system wired as per 1992 Kitfox instructions with separated mag on off switches. I have since installed a ignition switch with left right mag and start. That all works fine. What I have noticed is that with the engine running and master on it indicates 160deg. If I turn the master off it indicates 170deg. Any thoughts and suggestions would be appreciated.

[Mnflyer]

That's what mine did, it is different because of the voltage difference between having battery / regulator voltage and just operating off the alternator.

[jtpitkin06]

Help me understand a few things.
First What type temp gauge do you have? Is it all electric or is it a mechanical type with a Bourdon tube?
Assuming it is all electric, then comes the next question…
Why is the gauge powered with the master switch off? Do you have the alternator on with the battery off and the engine running?
John Pitkin
Greenville TX

[924549]

Yes it is an electric gauge. It gets its power from a terminal on the ignition switch not the master. That is how it was wired from the original plans. It has worked fine for a number of years.

[cap01]

probably need to get the meter or test light out and see what the terminal on the start switch is doing . or rewire the indicator to the airplane side of the master relay and then see what it does .

[jtpitkin06]

Please read this posting as a friendly explanation of electrics. It is not an admonishment.
Yes, the temp readings are incorrect with the battery out of the system. That’s because the alternator is driving the system over voltage.
Think of the battery as a very big damper or capacitor. It smoothes out the voltage ripples from alternator output.
The normal alternator output oscillates a few millivolts when there is a battery in the system. Without the damping effect of the battery, the voltage varies wildly. Without a battery, the regulator senses the alternator output as the system voltage and it tries to charge itself. The voltage runs up to the limit of the regulator. As electrical load vary, (say when keying a microphone,) voltage fluctuates wildly as the regulator tries to keep up with the un-damped alternator.
You will not see it on a simple voltmeter, but put an oscilloscope on the DC bus with the scope selector set to AC and you will see a very different story. The DC power from an alternator without a battery in the system is at best a very dirty DC. Voltage will oscillate much as 2 volts at 1200 Hz.
This dirty power supply can cause DC instruments such as a temp gauge to read higher than normal. It’s like plugging the DC gauge into AC power. It can also cause audio problems in your intercom and radios. It’s about a 1200Hz tone or a high and annoying whine that varies with rpm. You may not hear it in your side tone when you transmit, but others on the frequency do.
The other thing to worry about is damage to your radios. The spiking voltage can run way above the design maximum of the electrical equipment.
As a solution, wire your system so the alternator field switches off at the same time the battery switches off. Wire the alternator so you can turn off the field in an emergency. Being able to disconnect the field when an alternator fails will make the battery last longer.


John Pitkin
Greenville, TX

JP

[Av8r_Sed]

Pieter,
I suspect with the 582 you're running off the 170 Watt lighting coil with the original Rotax 866080 regulator. This regulator requires a minimum load of 1 Amp in order to regulate the voltage. With the battery in the circuit, it provides the necessary load. When the master is switched off the voltage will rise with a significant A/C component. This can't be good for the Westach gauge.

From the Rotax Installation Manual:
To avoid excessive voltage in conjunction with the rectifier regulator 866080, a constant minimum ballast load of 1 Amp is required (example lamp 12V, 15W).

The easiest fix might be to power the Westach from your Master Bus after the switch.

Alternatively, Aircraft Spruce and others sell a Key West regulator/rectifier for around $60 US that doesn't require the minimum external load in order to function. Rotax may have a different model as well, but it's likely much more expensive.

Regarding John's advice, if you don't have an alternator, you can't disconnect the field winding.

-- Paul S

[GDN]

Exactly, with such an old regulator i burnt my radio cause it wasn't consuming enough Ampère so the voltage increase and broke my radio.
Same engine as you and now new radio and new regulator