Volts Are Up, Oil Pressure is Down

[BrentBTK]

Hello all. I installed new Dakota Digital gauges in my 71 GMC I just picked up. I'm still working through some speedo and fuel issues, but the pressing thing is what I noticed on my first real drive of the truck yesterday after putting them in. When running down the highway between 2-3k, my voltage swings from about 15.5 to almost 19, spending most of its time between 17 and 18 volts. That seems awful strange to me. The other issue is the oil pressure. When running down the highway, its a strong 60 or so psi. Sitting in traffic with it in gear it drops as low as 9. If I put it in neutral to bring the revs up, it'll hang around 20. That makes me a whole lot more comfortable than 9... Does anyone have any thoughts? Should I start looking at alternators and hit the panic button on the engine? The oil pressure sender is mounted on top of the block behind the intake manifold, which I thought was odd. Thoughts? Thanks in advance!
BTK

[pjmoreland]

When I first bought my truck, it had an internally regulated alternator installed, and it still had the original external regulator. In addition to that, the wires to the alternator were backward. The voltage was going high like yours. Post some photos of the back of your alternator and of the wires down at the external voltage regulator.

[Lee H]

Welcome to the forum. I would not be concerned about 9 at idle with a 350. Take a volt meter and measure the alternator output and compare to the gauge reading.

[dagnabbitt]

That is a normal location for the oil pressure sender: at least, it is where mine is, also. An oil pressure reading of 9 would concern me, too, maybe the engine is a little tired.

I am guessing that you went from a mechanical sending setup to electronic when you put in the new gauges... you might have some grounding issues that were not apparent before. Before you hit the panic button I would add some grounds, get some straps going from the engine to the body, and from the engine to the frame then reevaluate.

[BrentBTK]

Thanks a bunch. Attached are a few pics of the back of the alternator. It's a mess. The brown wire coming out of the plug on the back of the alternator goes to some janky plug. The ground wire goes to the chassis, and the red wire goes to that same janky plug on a 3 way splice from the battery, to the alternator, and I think into the cab. The spliced in plug goes nowhere...

Edit* After looking through some videos, it looks like the "janky" plug used to go to an external regulator. Do I take this to mean it's internally regulated, or is that the problem?

[dagnabbitt]

Looks like the guy who rewired my '70 (using ALL red wires) might have been in Georgia and done your truck as well. If it is in the budget you might want to get a new wiring kit (eg. Painless) and redo it all. Trust me, it would be worth it.

[BrentBTK]

Originally Posted by dagnabbitt

Looks like the guy who rewired my '70 (using ALL red wires) might have been in Georgia and done your truck as well. If it is in the budget you might want to get a new wiring kit (eg. Painless) and redo it all. Trust me, it would be worth it.

It's definitely in the plan, I just don't want to burn anything down until then. So, I've figured the plug used to go to the regulator. The blue and white wires go to an empty plug that doesn't match the current alternator, so I'm guessing this is in internally regulated one. Down the rabbit hole I go.

[pjmoreland]

It appears that the wires are going to the correct places. Two things stand out to me:

1) The short red wire going from the big terminal on the back of the alternator to the #2 pin on the alternator connector isn't usually how I've seen people wire that connection. Usually they run a wire from the original red wire on the four-pin external voltage regulator connector to the alternator #2 pin. Either way, you've got 12V connected to #2, so it's probably ok.

2) What do you suppose is inside the shrink wrap on the wire that goes to the alternator #1 pin? I believe you've got a 12SI alternator, in which case the brown wire from the external voltage regulator connector should connect straight to the alternator #1 pin. There doesn't need to be a resistor or anything in that wire. The stock under-dash harness already has a high-resistance wire that's feeding the brown wire.

[BrentBTK]

Thanks! My thought was it's a resistor, but I didn't wanna mess with it unless I was sure. I started the truck and checked voltage at the alternator. I was reading anywhere from 14.8 to 19 volts. For a while it ran a steady 14.5 to 15. Then it sounded like the engine bogged a bit and the voltage spiked to 19v. I read voltage to ground at all 3 points on the alternator and it seemed to be the same.

[pjmoreland]

The issue is likely either a bad alternator, or the wiring is causing the problem somehow. You might want to get the alternator tested at an auto parts store to eliminate that as the problem.

With your ignition in the run position, but with the engine off, what is the voltage at pin #1 on your alternator? I just tested mine, and I get 3.25V. I believe this is a result of the high-resistance wire that feeds this location from the ignition switch. When I start my engine, the voltage is a steady 14.66V on all three spots on my alternator.

[MARKDTN]

Originally Posted by pjmoreland

It appears that the wires are going to the correct places. Two things stand out to me:

1) The short red wire going from the big terminal on the back of the alternator to the #2 pin on the alternator connector isn't usually how I've seen people wire that connection. Usually they run a wire from the original red wire on the four-pin external voltage regulator connector to the alternator #2 pin. Either way, you've got 12V connected to #2, so it's probably ok........

Oldsmobile used to do it this way. Short wire from #2 straight to the back of the alternator. Back in the day when doing alternator conversions we used to get Olds pigtails from the junkyard because they made it easy and clean. The disadvantage is that you are not truly sensing system voltage like you are when you get #2 from the ignition switch/fuse box. It acts a bit more like a 1-wire setup, but it will work.

[notsolo]

Good advice from these guys, I would also check all ground wires. Clean under and reattach...body, frame, motor and there is a clip on the dash somewhere.

[BrentBTK]

Voltage issue solved! Thanks everyone. pjmoreland, you were correct. It was an unnecessary resistor on the brown wire. With it on the line, I was reading anywhere from 3.5 to 9 volts with the key on and not running. I took the resistor out, and read a steady 3.5v. Fired her up and got a steady 14.8v at every point on the system. I love an easy fix!

[pjmoreland]

Nice! I've spent a lot of time fixing electrical "fixes" on my truck made by previous owners. My favorite was the distributor power wire that was attached to the fuse panel with an alligator clip.

[Rich72C10]

Nice fix. The electronic geek in me wants something like the Dakota Digital gauges in my truck but part of the charm of the truck is the old analog gauges. I wouldn't be surprised at some point I'll go with cool gauges like you have!

I end up reading all these post that have anything to do with the alternator... I am still stock alternator / external voltage regulator - which is working perfectly at the moment. Though I'd image it is a ticking bomb waiting to die on me.

I already gotten from MAD Electrical a pretty big kit and even spoken to the guy (he can really carry on a conversation and I think he might be a zillion years old). I am totally in a holding pattern because currently it isn't broken and I don't feel like fixing something that isn't.

I am also so anal on doing things right & pretty I am finding myself being overwhelmed on breaking apart the wiring! That wiring you have in your engine bay that the previous owner did would keep me up at nights!