I have an aftermarket oil pressure guage and on a normal vehical it works fine but when i put it on my bus i get a reading that is only about half the amount regestering at the mech guage in the back. I know it has something to do with the length of wire I had to run in order to wire it. Has anyone else run into this and how did you fix it?
1960 4104 number 4900
I expect that you're on the right track, David. You know that these things read by resistance, right? There are three things that often happen:
1) The sensor (which acts as a ground) isn't making good contact with the engine block. Sometimes, people put in sealant on the threads (not needed), or sometimes the thread is rusty, or sometimes the sensor is loose. Whatever, run an ohmmeter from the base of the sensor to the engine block - you should be seeing basically no (or at least as little as possible) resistance. And, of course, you could have a bad sensor - what happens occasionally (besides the usual age-related internal electrical failures) is that you get oil leaking into the sensor and that will screw up the reading.
2) As you've guessed, wiring. Sometimes, people have wires that are too long and the resistance leads to a bad reading but more often the problem is in a loose or bad connector. This is another place for the ohmmeter; you should see no/ALAP resistance from the connector on the back of the gauge to the connector on the sensor. These wires don't have to carry much power (amperage) so wire size usually isn't an issue but the ohmmeter will tell you.
3) The sensor and gauge have to be matched. There are different resistance ranges between manufacturers (I think at that two you often see are the "Ford type" and the "S-W type", and there are other ranges specified from other manufacturers, too). If you don't have the right match, the gauge on the instrument panel will read wrong. And, guess what, a Ford type sensor looks just like a S-W type sensor (used on GM and I think Chrysler) so you can't tell by looking at them. Are you sure that the gauge and sensor are matched?
Those are the three big things to look for, but - as usual - it's the "unusual" stuff that makes you spend hours with your head stuffed into the engine compartment in either fiery hot or freezing cold weather. So, check your resistances and trace everything.
Good luck, B H NC USA