Thank you Ray.
If I understand correctly the bi-metal strip is opening and closing several times per second but the rate at which it does is determined by the distance between the bi-metal strip contacts based on how far the diaphragm is pushed by oil pressure.
The changing distance between the bi-metal contacts is what determines the duty cycle of the PWM?
Safe to assume the bi-metal strip or its contacts are the #1 failing cause of the OP switch?
And, if I were to pull the wire going to the OP switch I would get minimum oil pressure and if I were to ground it get maximum oil pressure?
The bi metallic switch’s secondary role is to act as voltage compensation to average what the gauge sees for varying voltage. The timing rate is affected by not only the pressure from the diaphragm for cycling, but by the applied voltage to heat the bi metallic wire. Higher voltage will compensate the cycle output accordingly, so that the average feedback remains more or less constant for the pressure at varying voltage…
I’ve done it and it didn’t work. Can’t remember what it did but no good. The 100 PSI sender matches an 80 PSI gauge, BTW (XJ starting around 1970, whenever the chrome bezels changed to black).
I have a unit repaired by a jag nut engineer many years ago.
His conclusion was the same as mine, for whatever reason, the Jag senders just dont work very well, the repros especially so.
Its not unknown for other vehicles to have defective senders, but they usually last a lot longer than the Jag units
The common experience I have had, is they (sometimes) work ok for a year or so, then gradually begin to read low, when the oil is warm. Eventually they read too low on idle to be satisfied
As I will not operate an engine without a reliable OP reading, that means either a mechanical setup, which I do not want, or digital, which is what I have done…until Ray invents a working sender ?!
Ray’s project is more ambitious, replacing both gauge and sender.
But a resistive (strain gauge) sender, as used by everyone else in the day, should work fine with the Jaguar gauge if the correct resistance versus pressure relation were found. I don’t know how hard folks have tried. You have the same problem trying to match temperature senders to gauges. I considered this a few times in my 45 years of ownership, but I have collected used Smiths senders instead, and selected one that worked.
The temp and fuel bimetallic gauges are both driven by resistive senders. If the OP gauge had a resistive sender (of the correct range) it would of course respond to changes in battery voltage (not a big deal IMO). Or, you could supply the gauge from the IVR (or a more modern IC voltage regulator, which is also used to replace the IVR).
I wonder how hard it would be to transplant the gauge internals from something reliable into the Smith case, and use the matching sender, presumably swept needle calibration would be the issue
If you wanted to go super high tech…an accurate sender, hidden computer, signal generator, bit of software…original gauge should work fine
I have done both, and trust me, the Smith’s gauges are simply squirrelly. My car has worked for most of the last 20 years with your second option, using a VDO sender. Well, SEVERAL VDO senders, as they also have a life of about 5-7 years. But the gauges are still not terribly accurate or repeatable.
What I’m working on right now is your first option - I replace the Smith’s gauge innards with a modern stepper motor movement, the same one used in all GM products for the last decade or more. There is also a small PCB with a microprocessor to control it. It also uses modern, solid-state senders for oil pressure and coolant temperature. As a bonus, they can also include RGB LED lighting, which lets you set the color and brightness of the lighting for the “Dim”, “Flicker”, and even “Off” settings of the instrument light switch. And they will flash bright RED if the sender reading goes dangerously high (or low in the case of the fuel gauge). Only the ammeter or voltmeter gauges are left stock (those use different movements from the others, and actually work ok).
Many years ago, when I first got my S1, the oil pressure gauge did not work. Not knowing it was a “special” sending unit, and being cheap, I went down to my FAPS and got what ever fit for a buck or so. I added a resistor to get a reasonable reading. I am sure it varies with alternator voltage, but I could care less. I do not quite follow the concern about an accurate gauge. I have no clue what the oil pressure is in any of my modern cars that I depend on everyday. Just a light. This is probably one reason OEM’s dropped them. They got tired of car guys complaining their new car’s oil pressure was not correct. As far as I am concerned, my gauge shows I have oil pressure. Although I would like to see good oil pressure, and if I just had my engine rebuilt I would expect to see good oil pressure, for me, at this point, 30, 60, no difference, as long as there is something, I will run it- and it does run quite enjoyable.
Tom
