What oil pressure sensor(s) does a '90 have?
'87--'89 uses a small switch by the distributor to bypass the fuel pump relay. There's a fist-size canister just above the oil filter that sends it's signal to the dash gauge. Those canisters are known to be screwy; I've had four of 'em on my '88; and I don't know if the one on the truck when I bought it at 170K was original. I'm getting about 50K miles out of them. They can read "too high", "too low" or randomly high/low. Sometimes when you unthread them from the vehicle, they slosh because they're full of oil inside--faulty seals.
They are, in short, poorly designed/poorly manufactured junk. Apparently, one company makes them (in Asia, of course) and fifteen other companies buy in bulk, put them in custom boxes sold under whatever the "House Brand" is.
The defining item is the green paint on the end, at the base of the hex. (
NAPA OP6729)
You must be registered for see images attach
Some parts stores catalogs show a different sending unit, but it's too big--won't clear the exhaust. (NAPA OP6649) Even tried it with different adapter fittings, but it just doesn't work.