348 "Electrical wonderment" question on Oil Pressure gauge

(If you mean the RPM of the engine doesn't change enough to cause the drop then) One thing that can contribute to this effect is a poor ground from the gauge to the chassis if that gauge's ground connection is also used for its internal lamp ground -- i.e., when you turn on the headlamps, which puts +12V on the gauge lamps, the whole "ground" network of the gauge (or instrument cluster) can be "pulled up" a bit (to like +0.3V) relative to the true chassis (0V) ground. Usually it's a "half-a-needle-width" type change so can be ignored, but your reported change seems more significant.

I don't know the specific wiring architecture of your 348, but usually the connection from the chassis ground to the instrument cluster ground is a tenuous maze of bolted/flaky connections (that often have too much paint IMO). Is it only the OP gauge that changes its reading? If it's got an independent ground network connection it might be worth checking (although the first thing to do is measure the voltage at the battery terminals, engine running, with headlights on and off just to make sure it doesn't change too much -- i.e., the alternator system is quasi-healthy)...

 

Does this only happen when you turn on the headlights, or are there other electrical components that have the same effect ie wipers, indicators etc.)

The problem will more than likely be a poor ground connection, try looking around the relay / fuse board area, I recall that there is a "common" grounding point there.

If the problem becomes more in depth and you need wiring diagrams I will post them for you.

MW

 

Maybe the headlights burn oil for light, like in the old mining days. My 20+ PSI?? hehe

Check grounding on the gauge. It's not likely to be grounding at the sender, but it would be easy to test. You're best bet is to run an alternate ground from a known good ground source to the points in question.

 

According to the old Fchat archives the oil burning headlights were used on GT4's destined for the middle east. ;-)

DJ