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Posted By Discussion Topic: Oil pressure sensor -- page: 1 2     mzahorik 06-28-2021 @ 7:21 AM reply profile send p.m. Member Posts: 209 Joined: Nov 2009 I'm rebuilding the engine in my '37 Ford. It's a bitsa engine, a combination of many different years. The rebuild will have a new 80# oil pump in it. My oil sensor is a 50# unit. I found a 80# sensor, but after testing it with compressed air, it doesn't read correctly with my dash gauge. After some research I found that these sensors have a vibrating contact in parallel with a shunt resistor. Yet my ohm meter doesn't read any resistance terminal to ground on either the 50# or the 80#. Does any one know or could measure what the resistance is on these oil sensors? Thanks Mike. 37RAGTOPMAN 06-28-2021 @ 7:43 AM reply profile send p.m. Senior Posts: 1992 Joined: Oct 2009 HI Did you change the pressure relief valve ? or using what it in the block ? I use a 80 lb pump and used the 50 lb sender, with no problems, in my 37, and use the original pressure relief valve. oil pressure gauge seems to read correctly., hope this helps 37Ragtopman mzahorik 06-28-2021 @ 7:55 AM reply profile send p.m. Member Posts: 209 Joined: Nov 2009 When I first dissassembled the engine, I checked the 50# sensor and gauge. I found that the gauge was reading about 60% low. I found that I could calibrate the gauge with a small lever in the gauge. It then was reading much better. I had not intended to change the releif valve, maybe I should? The old spring could be weak? The 80# sensor makes my dash gauge read very low, maybe 15-20# at 80#. I don't think that I can adjust that out of my gauge. The 80# must use a different dash gauge. Yet I see in the doc's that there is a resistor in both sensor units, but they don't tell me the resistance. Common sense says the resistance must be low, but my meter reads it as open. Thanks Mike TomO 06-28-2021 @ 8:04 AM reply profile send p.m. Senior Posts: 7385 Joined: Oct 2009 Your dash gauge should read full scale when the sending unit has 80# of pressure and mid-scale when there is 40# of pressure. If you have a King Seeley unit, the reading from the terminal to the brass base should be infinity or an open circuit. The contact points in the sending unit will be open until there is enough pressure to close them. The description of the sending unit that you posted is not totally correct. The sending unit does not have a shunt resister. It has one contact point mounted on a piece that moves with increase pressure and the other is on a bi-metal strip that is heated when the contacts close. When the strip is heated, it opens the contacts until the pressure closes them again. The amount of current in the sending unit controls the operation of the dash unit, which also has a similar setup to move the needle. The more pressure to the sending unit, the more time that the points are closed and the increase in current is proportional to the pressure. I have attached a photo of the fuel sending unit. The oil pressure unit looks very similar. The shunt resister in the fuel sending unit is not in the oil pressure sending unit. The resister in the fuel sending unit is used to dampen the variations in readings as the fuel sloshes in the tank. If you post your gauge readings at 20#, 40# 60# and 80#, I may be able to help you adjust your sending unit to a more accurate reading or let you know if it can be calibrated by one of the services that repair the sending units. Sometimes it just take a few miles of operation to clean the contacts so the sending unit is more accurate. Tom This message was edited by TomO on 6-28-21 @ 8:10 AM mzahorik 06-28-2021 @ 8:15 AM reply profile send p.m. Member Posts: 209 Joined: Nov 2009 Tom, thanks for the reply. My original 50# sensor seems to work ok. In fact I re cal'ed the gauge to read 50# ar 80# air pressurre. My dash gauge reads 0 to 50# Air Pressure 50# sensor&dashGauge 80# 50# 70# 45# 60# 40# 50# 35# 40# 28# 30# 21# 20# 15# I didn't try this with the 80#Sensor. Thanks Mike mzahorik 06-28-2021 @ 8:23 AM reply profile send p.m. Member Posts: 209 Joined: Nov 2009 Tom, I have an appointment and will not respond until after 6 tonight. Thanks Mike carcrazy 06-28-2021 @ 9:57 AM reply profile send p.m. Senior Posts: 1978 Joined: Oct 2009 What crankshaft rear seal method will be used on your '37 bitsa engine? If you are using the system used on the 1941 and later Flatheads you will probably be OK. If you are using the earlier "labyrinth" system, beware as the increased pressure and volume of the oil will overpower the capability of the sealing system and the engine will have a massive continuous oil leak past the rear main bearing. TomO 06-29-2021 @ 5:13 AM reply profile send p.m. Senior Posts: 7385 Joined: Oct 2009 Mike, because you have adjusted your dash unit to read lower with a 50# unit, your 80# unit will be very inaccurate. I like to keep the dash unit as sent from King Seeley and then use the 80# unit as is also. This gives the readings that I posted previously. 50# units are harder to find than 80# units. Remember that you really don't care what the real oil pressure is, just that it is in the normal range for your engine and does not drop when you accelerate. As Carcrazy pointed out, more oil pressure is not always a good thing. You just need enough pressure to keep the engine lubricated. Tom mzahorik 06-29-2021 @ 7:29 AM reply profile send p.m. Member Posts: 209 Joined: Nov 2009 The engine is a 1946 59 series engine. It's a bitsa engine. Hardly anything is 1937. Mike mzahorik 06-29-2021 @ 7:39 AM reply profile send p.m. Member Posts: 209 Joined: Nov 2009 I understand that it is inaccurate, but I do know what it is. If the gauge reads 40#, I actually have 60#. It would be nice to have a more accurate gauge reading, but I'm not sure how. My dash gauge has marking of 0, 10, 20, 30, 40 and 50. The 30 thru 50 marks are close together, so accuracy is low to begin with. I thought of setting the dash gauge so that when the dash gauge reads 20#, there is 40# on the engine. The rest falls where it may. Mike

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