Oil pressure meter and sender, November 29, 2005

There is quite a bit of confusion about the meaning of the figures on the oil pressure meter. Early Fulvia's have a scale 0 - 25 -50. Later S1 Fulvia's have a 0 - 30 - 60 scale. S2 Fulvia's have an instrument with 0 - 35 - 70 on the scale.

I have measured some gauges of various S1 and S2 Fulvia's. Mostly Veglia Borletti ones and some Jaeger types. The gauge itself, the meter part with the needle on the dashboard, is identical for all Veglia instruments. Only the figures on the scale are different. Even the Jaeger gauge is not too much different. The needle is on 0 when the resistance of the sender is about 300 Ohms. The needle is at full scale when the resistance of the sender is 0 Ohms.

The confusing part is the sender. So far I have come across types with ranges of 0 - 6 bar, 0 - 7 bar and 0 - 8 bar. These ranges are stamped into the bottom or into the side of the sender. I am talking about Veglia senders now. They are all about 300 Ohms, 10% at zero pressure and 0 Ohms at their full scale value. Because the pressure relief valve opens at appr. 6 bar (I have measured 6,2 bars on several cars) the actual oil pressure will never go over 6,2 bar. The 7 bar and 8 bar senders will never get to a value of 0 Ohms. The needle will never reach full scale.

Both Veglia Borletti and Jaeger instruments were used on the Fulvia's. In the Fulvia parts catalog there are two partnumber for the sender. One partnumber is for the Veglia Borletti one. The other partnumber is for the Jaeger. These numbers never changed between the first and the last Fulvia. Still, I find three different Veglia Borletti senders on cars, each sender with its different pressure range as explained above. I have not been able to find anything in the documentation clarifying what sender was used on which car.

It would be logical to assume that a 0 - 7 bar sender was mounted on S2/3 cars with a scale of 0 - 35 -70. The figures on the scale would then be decibars. The needle would stop at a value 62 as the pressure relief valve opens at 6,2 bar and oil pressure will not increase any further.

It is also logical to assume that a 0 - 6 bar sender was used on S1 cars with a scale of 0 - 30 - 60. Again, the figures on the scale would be decibars. The needle will go to full scale. Full scale coincides with the opening of the pressure relief valve.

What sender was then used on cars with a scale of 0 - 25 - 50? I have (so far) not found any 0 - 5 bar senders. At least no Veglia ones. Some Jaeger senders seem to be 0 - 5 bar, but I have not yet finished measuring them all. All my Fulvia's have Veglia gauges which explains my preference for testing Veglia gauges and senders but I will get around to measuring Jaeger ones. At present I also mount 0- 6 bar senders on S1 Fulvia's with a 0 - 25 - 50 scale.

On which car was the 0 - 8 bar sender used? I think it was on some Fiat cars, not any Fulvia's, but I may be wrong. If you have an 8 bar sender the needle will never go over 3/4 of full scale as 3/4 of full scale represents 6,2 bar, the value at which the pressure relief valve opens. On some cars the needle will hardly go over half scale because of the variation in sensitivity and variation in non linearity.

The most certain way of checking the type of sender you have is to take it off and read the numbers. Another less accurate way is to guestimate it from the maximum deflection of the needle on the gauge. You should have the maximum deflection with hot engine at 5.000 rpm or at 3.000 rpm with cold engine. This test is valid for Veglia senders with Veglia gauges.
If the needle goes all the way to the full scale mark, you have a 6 bar sender.
If the max. deflection is between 80% and 95% of full scale, you have a 7 bar sender.
If the max deflection is between 55% and 80% of full scale, you have an 8 bar sender.

I opened several Veglia senders with the different ranges. Inside they are very similar. The wirewound variable resistor is the same in all senders. So is the mechanisme driving the wiper. The only difference is in the copper membrane in the bottom part. Bad senders can often be repaired. I have a test set up with a a callibrated manometer and a smooth pressure regulator connected to the air compressor. When changing the pressure I measure the resistance with an digital Ohm meter. Usually they all function well up to and including the variable wire wound resistor. What goes bad is the contact between the external connector and the wire wound resistor. The external connector has a copper strip on the inside which presses against a pin on the substrate of the wire wound resistor. This contact is dirty and the pressure of the strip has become insufficient. Open the flange at the bottom with a small cisel and take off the top. Clean the contacts. Bend the copper strip a bit. Assemble the thing again.

If anybody has additonal information about the oil pressure meters and senders, please send me an email so I can include it.

Huib, huib@viva-lancia.com, November 29, 2005.