i want to install a temp gauge and oil pressure gauge on 73 sdv. i don't know where the sending units are. there is and sending unit on the left front head with nothing attached to it. any suggestions and help would be great.
Are your gauges electric, or mechanical?? The old sending unit that you say has nothing connected to it, is likely the place to install the new gauge sender, or capillary type probe , depending on which type you have.
Which engine is in there??
The oil pressure sensor, or switch if it has a light for oil pressure, is on the top of the block at the back end. If you have an oil light and want to keep that working, ( I would) you can put a brass pipe tee in the block, with room then, for both the light pressure switch, and a new pressure sensor ( or line if the gauge is mechanical)
I believe the thread in the block is 1/8 NPT pipe thread, and a brass pipe tee would be available at you local auto parts store.
Having the oil light come on at idle seems to be normal if the idle is set low, and the trans is in gear. These engines don't have a very high operating oil pressure, like 37 lbs max, and quite low at idle. The switch for the light may not be exact either, for the pressure to switch the light on. If the engine has a fairly high mileage on it, the pump may be a bit worn also.
My personal preference is a mechanical oil pressure gauge. Like I mentioned, you need to get to the back of the engine where the pressure switch is located, to get to an oil pressure port. A 1/8 pipe thread tee will give you two openings, one for the line to the gauge you want to install, and the other to use the switch for the oil light.
If you go with an electric gauge, you have to put the sender where the line would go for the mechanical gauge. Of course you would need a 12 volt power wire to operate the gauge, from the fuse block, and it would be switched power from the ignition switch.
Same thing with a temp gauge, electric or mechanical.
The mechanical temp gauge probes have an adapter which screws into the water jacket (in the block or head), for the gauge capillary probe to screw into.