95 Transport Oil pressure sensor

Replaced the motor (3,8) in this Transport and in process broke the original oil sending unit. The replacement motor came from a Lumina with warning lights. The Transport has gauge. Picked up several sending units from a wrecker but none work properly. (None were out of the same year or model that is needed but some did have guages in dash). It appears the sender for warning lights has three pins. The broken one has 4 pins. Checking with a meter it appears there is a contact and variable resistor inside and this is confirmed when I cut it open. Two wires are for contact and the other two are for the resistor which changes when pressure is applied to the broken end where engine oil pressure would normally apply. I can get it as high as 100 ohms with finger pressure.

When one of the salvaged sending units is installed on the motor I get zero ohms engine off and very high ohms engine running. The dash gauge is pinned to top. (There is 40 lbs oil pressure engine hot idle with an external gauge installed) Maybe the dash gauge reading being so ridiculously high is due to miss match of sending units?

One question I have: why is there a need for a contact in the sending unit? The resistor part is simple enough.

In many GM models ( and this engine is one of them) the oil pressure switch does 2 functions: it has a variable resistance to operate the dash gauge, and the other side of it is a simple on/off switch that closes at about 4 to 8 psi to allow back up power path for the fuel pump ( primary through the PCM/ fuel pump relay). I haven't priced a new sesnor, but that may be the way to go for it
Good luck
Jeff

OK thanks. Priced here at $40. So I'm waiting for a unit to come through the local wrecker. They gets hundreds a months so it shouldn't take too long to find another. Would be nice to get a wiring diagram of the oil section/guage. There must be quite a variation between senders from other models that use this 3800 engine since the guage isn't even close now with the sender I'm using. For now I'm using an external guage plumbed in. Not so pretty but it works fine.