Taking The Alfatross' Temperature (Post # 154)

Three senders for both oil and coolant temperature.

Veglia oil and water temperature gauges read degrees C.

I got a haircut today for the first time in two months. The barber insisted on taking my temperature with a forehead thermometer before I could come in. Temperature is an important indicator of whether things are running properly or about to crash and burn. The Alfatross has been keeping her temperatures to herself, but that's about to change. 

Suspecting that The Alfatross' olio and acua temperature gauges were not displaying accurate information, I set about trying to figure out if the problems were due to the gauges or the senders. The only way to do that conclusively is to remove the beautiful Veglia gauges from the dash and their senders from the oil sump and the radiator hose line and bench test them. In addition to the original senders that were on the engine when I got it, I acquired two others somewhere along the way, so I included all of them in the bench tests.

The laboratory thermometer in the bath  reads 87 C but the gauge reads 100 C!

Although the senders for the olio and acua temperature gauges are calibrated the same, and could be interchangeable, they look very different. The olio sender has a long "nose" compared to the acua sender. I don't know if there is a functional reason for this. Probably not. The testing station allowed me to check the compatibility and accuracy of various gauge and sender combinations by immersing the senders in a water bath set to different temperatures. I quickly learned that here at 7,000 ft elevation water boils at 92.7 degrees C (198.9 F) so I could not test temperatures at the top of the range--which is the most critical part! Later, I learned from Joey at MoMa that if I had used commercial coolant (antifreeze) instead of just water, I could have created much higher temperatures in the test bath. I tested needle movement on the gauges and changes in resistance readings with changes in bath temperature. It was immediately apparent that at least one of the senders was inoperative and the acua gauge always seemed to read too high. All that it really told me was that I needed to take the gauges and senders to MoMa Manufacturing in Albuquerque for a "second opinion".  It took Joey at Mo-Ma only a few minutes to diagnose my problems: the acua gauge needed adjustment and the two long-nose senders were defective.  The good news is that the two short-nose senders were OK and could be put back into service.    

Convincing the Benzina Gauge and Sender to Work Together (Post # 153)

Preparing The Alfatross for her appearance at the Arizona Concours and the Santa Fe Concorso, we put only a few gallons of gas in the tank and didn’t worry much about how accurate the gauge was.  Now it was time to correct that. After filling the gas tank up completely for the first time I was disappointed to find that the gauge still read “0”. Because the Veglia BENZINA gauge was restored recently by Mo-Ma Engineering I figured the problem was more likely a faulty sender. With the sensor out of the tank, moving the cork float arm up and down produced erratic readings on my ohmmeter, so something was wrong inside. 

Top of the benzine gauge sender. Gauge
connects to W terminal, T connects to
warning light, third terminal is unused. 

Bottom of sender with float rod.

A quick surf on the internet revealed that modern reproductions of the Veglia sender with more reliable solid-state electronics inside are available for purchase. But I had some reservations. That might be the easy way to solve the problem, but on the other hand maybe reproductions would not be calibrated to work with the original gauge--and those reproductions are expensive! 

Reproduction senders look very similar to the original.

Never having dissected a mechanical fuel level sender before, I saved the reproduction option as a last resort if the original was not salvageable. Opening the casing revealed that the components inside seemed to be in good condition in spite of their age. At the heart of the sender is a very Old School, very delicate, coiled wire rheostat attached at either end to terminal screws and curved copper blade contacts positioned above and parallel to the sides of the rheostat coil.  A fine wire from one end of the coil ran to a third terminal screw on one side of the case. Removing the terminal screws and rheostat coil revealed the grounding “brushes” mechanically attached at the bottom of the case to the float arm.

The rheostat coil, blade contacts, and terminals. Tips of
the brushes are out of sight.

The grounding brushes below the rheostat coil in the
"full tank" position

The sender performs two functions: monitoring the fuel level in the tank and illuminating a warning light inside the gauge when the level gets too low. As the float arm sinks, the brushes move along the sides of the rheostat, reducing resistance. The gauge translates resistance into movement of the needle from full to empty.  When one of the brushes makes contact with the copper blade near the end of its travel, it completes the light circuit at the third terminal screw. Turns out, this was the problem area.

The rheostat, a little discolored but otherwise completely functional.  The
insulated fine wire making the connection to the warning light was
shorting out on the casing, causing erratic readings.

After cleaning the contacts I bench-tested the sender with the gauge to make sure the old team agreed to work together again, and they did. 

Testing the cleaned up and reassembled sender  for compatibility with the gauge.  The problem was easy to fix-- 
but hard to pinpoint.  

So the BENZINA gauge is reliable now, but what about the ACUA and OLIO temperature and OLIO pressure gauges?  They're coming up next, and I have a feeling we're going to have problems with them, too. I have some spare senders for both oil and water, but what if there are problems with the gauges themselves?  We'll soon find out.