Re: temp guage reading hotter than it is

Posted by HH56 On 2021/9/4 13:17:55

In the 48-50 22,23 series cars the gauges are different from all other years.

Any year other than 48-50, the sender is a straight resistance unit where a changing fuel, oil, or temperature parameter will change the resistance and vary the current to the gauge so heat generated by a heater element wrapped around a bimetal strip on a thermal type gauge or in other years, vary the strength of an electromagnet coil on magnetic type gauges. That change in current will make the needle move proportionally. While the resistance range of the senders did vary by year and gauge type or brand, modern senders can be found that will work on some years or on senders with completely obsolete ohm ranges, repros can be made by KM Lifestyle.mykmlifestyle.com/

In the completely different 48-50 gauges there are two bimetal strips -- one in the gauge and one in the sender -- operated by heaters wired in series with each operating their respective bimetal strips. The senders in those years do not provide a varying current but rather a straight on/off contact action of solid ground or an open contact. A closed contact will provide the 6v going to the gauge a solid ground and a lot of heat can be generated by the heater around the gauge bimetal. The frequency and duration of the on/off pulses is determined by how much physical pressure is being applied to the bimetal strip in the sender. That is determined by a diaphragm which is in direct contact with the respective fluid being measured. A weak pressure and the action is quick and frequent and gauge heater doesn't get much time to heat so needle stays near the off or resting end of the gauge, higher pressure and the on time delivering a straight ground to the gauge is longer and will make the heater move the bimetal strip and needle more so it displays the reading at the end opposite the resting end. The off or open contact period between the pulses allows the gauge heater to cool slightly so the needle stays fairly steady. If it did not have the cool off period the needle would continue to climb each time the ground was applied. The heaters in each component being connected in series along with the sender contact action affecting the sender heater at a different time makes them respond in complementary ways so a kind of feedback is generated and keeps things in sync.

It appears original NOS Packard senders for the 48-50 models are all but extinct and expensive if you find them. Ford and some other brands also used a similar type system and maybe something could be found from one of their vendors that will work and that may be what some of our vendors have done.

I do not know if KM Lifestyles can remanufacture that type sender but I kind of doubt it since there are many trying to find affordable working senders or make a straight resistance type work. As far as I know, while there have been attempts to find a modern resistance type sender that will work the the 48-50 gauges, no one has been completely successful.