So here we go were triple posting but only cause I've had an interesting discovery. So went on a 100 mile drive today in 85 degree heat with the ac on. Water temp worked beautifully and I never went over 200 though I got close with the ac blasting climbing a hill. Seemed to like to stay between 185-190 ish.
Oil pressure was great at first in the morning with the windows down and the radio off. You'll see why this is important in a minute. I had 13-17 ish idling hot I believe gauge isn't exactly the best to read at low pressures but it was somewhere in there. Climbing a hill at 2600 rpm it was around 40 ish. I was happy.
I turned the ac and radio on and looked 20 minutes later and had 15psi at 2600 rpm. What? Well no panic required because apparently having the feed wire for the gauge (it's an electric oil pressure sender) in the fuse box for the radio gives all sorts of false readings when things are bein used from the wire that feeds the fuses. I then shut off everything else and it was back to 38ish and the engine was way hotter. Those good readings go along with what a mechanical gauge showed us when we first installed the electrical gauge and verified it was accurate in the driveway to a couple of psi at most rpm hot or cold. The damn radio hitting, reverse lights, basically anything that has a fuse from the fuse box causes the gauge to drop some from the supply voltage dropping ever so slightly.
So now my plan is to get a relay (this seems stupid) wired directly to the damn drivers side battery, and have the trip wire for the relay be something that is 12v switched from the fuse box (basically where it's wired now) so that the supply wire doesn't lose or change its properties as much when stuff gets turned on. I'm too far in to just switch to a mechanical gauge now, and I know the readings were correct from plugging a mechanical into the T when I installed it so now I just gotta either try what I'm thinking or know to just judge the ratings accordingly. I went as far to test running a super long wire from the + battery to the gauge input and cranked the ac, radio, whatever I could think of and then the gauge only changed by 2 psi at a known good setting.
Figured I'd type this all up just in case anyone ever comes across this issue, but when I add my gauges to Fred (my good 300-6 truck), I'm gonna do a mechanical with a copper line for the oil gauge more then likely to avoid navigating stupid electrical issues. Which probably wouldn't happen if all of fords wires weren't super tiny and kinda weak. But hey, gotta do what you gotta do. Least my oil pressure didn't turn out to be crap, looking on the bright side.
