As part of the work this week on Superblue, I decided to finally have the two new oil pressure senders (for idiot light and pressure gauge) that I bought some time back installed on her. The problem was the usual one with the OEM style (and probably REALLY OEM) gauge pressure sender – showing all kinds of wild (and inaccurate) pressure readings on the gauge. Also, the idiot light sensor went wacky on me about the time I r/red the oil filter a year or so ago, and the warning light is always on since then. I’m thinking I probably bumped it in the process and knocked that rubber insulation sheath off of it, or some such, causing it to ground out to the block (?).
First up for replacement is the pressure gauge sender/transducer, being the “upgraded” version this time. The tech had a tussel getting the old one out (probably the original), but finally did and installed the new one. However, when we went to fire the car up, instead of reading mid-range on the gauge, it reads all the way to the left (i.e. no pressure). I’m thinking the possibilties for this are: (1) somehow the new sender is grounding out to the side of the block (or, if so, would that make the gauge read all the way to the right instead? ) (2) I bought a blinky sender (although it was still in the Jag packaging, being NOS) or (3) possibly the false “on” status of the idiot light sender is somehow causing a signal to be sent to the pressure gauge sender (or to some electrical interface common to both senders) to indicate that there is 0 pressure.
Of course, if the problem is #3, then when the idiot light sensor is r/red later tonight the gauge pressure sender should start reading as it should.
Thoughts, anyone? Another possibility is that there is NO signal coming from the new sender, and hence a false 0 pressure reading. ?
Is it possible that the lead to the sending unit has an open/broken connection or place in the wire?
Do you have a rheostat you can connect to the lead and connect the other end of the rheostat to ground, then watch the gauge as the rheostat is moved to different settings? Without knowing the resistance swing of the sending unit, at some point of moving the rheostat through its range from maximum resistance toward 0 resistance, the gauge should begin changing its displayed reading.
Well, one thing I decided to look for tonight is whether, when the ignition is turned off, the needle on the gauge falls even further to the left or not. It in fact does, but only a minute amount, since there is apparently only that much “room” left on the gauge. This tells me then that the gauge is indeed getting some input from the sender (vs. just “dead”), but just not enough to bring the needle up to the mid-way reading. This in turn makes me wonder whether there is a ground, short or break in the new sender and, perhaps as you mention, in the wire part of same (assuming that is part of the new sender, as I know the old wire was in good shape).
The needle going up a tad when ignition is turned on is likely going up to ‘0’.
If you’ve ever used an old analog ohm meter, you ‘adjust for 0’ when touching the leads together, then measure the resistance of whatever you are measuring.
The needle dropping below 0 with ignition off is common. No voltage lets the needle drop to it ‘off’ analog gauge position. Ignition on puts 12 volts on the meter with sensor in the circuit, with the sensor having a ‘0 pressure’ reading, which raises the needle to ‘0’.
Increased pressure increases the meter reading.
I was actually referring to the wire you handle and connect to the sensor. Wires are flexible, to a point, then the begin to break, then do break, with not much bending separating the two conditions.
Nothing “passes the time of time” as time “is still testing it” and it just hasn’t failed … yet …
The window lift relay in my car recently “failed the test of time”… after 40 years it quit working.
I finally got it out today, tested the relay, and … nope, not working when I put 12 volts across the relay coil.
I will be buying a new relay tomorrow, and saving my 40 year old Lucas relay as evidence that not everything Lucas, Prince of Darkness, touches is bad.
I’m beginning to think your “bad wire” theory is in fact correct, Jerry. I noticed tonight when I returned from the store that, after I shut the ignition off, the needle moved slightly to the usual “far left” position it was doing as before. However … out of curiosity, I then restarted the car and was surprised to see that the needle did not move to the right at all. This time instead it remained at the same position it was in when the car was off.
by way of update, tech of mine decided to spray down the ground connection to the engine block from the instrument panel with some degreaser (it was pretty fouled up ) … I noticed today when I started Superblue up, in fact the oil pressure gauge climbed to about “3” (almost midway) and was there for a few minutes, but then gradually sunk back into the red again. My thought on this is the degreaser spray, being conductive, temporarily increased current flow to ground … as soon as the engine started warming up, though, the degreaser dried up , causing the system to go back like before he sprayed it. Next step is to actually remove the ground connection and clean everything bright with a metal scouring pad, or some such, before putting back together again. We’ll see if that does anything …
I’m thinking I probably bumped it in the process and knocked that rubber insulation sheath off of it, or some such, causing it to ground out to the block (?).
) (2) I bought a blinky sender (although it was still in the Jag packaging, being NOS) or (3) possibly the false “on” status of the idiot light sender is somehow causing a signal to be sent to the pressure gauge sender (or to some electrical interface common to both senders) to indicate that there is 0 pressure. 
) … I noticed today when I started Superblue up, in fact the oil pressure gauge climbed to about “3” (almost midway) and was there for a few minutes, but then gradually sunk back into the red again. 
My thought on this is the degreaser spray, being conductive, temporarily increased current flow to ground … as soon as the engine started warming up, though, the degreaser dried up , causing the system to go back like before he sprayed it. Next step is to actually remove the ground connection and clean everything bright with a metal scouring pad, or some such, before putting back together again. We’ll see if that does anything … 