Probably already know it’s bad, but how bad?

[bucket]

My '88 with factory 350 TBI and harness still:

Sender by the oil filter is for the gauge, it also has a shield to protect it:

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Secondary pressure sender, with wetherpack style plug:

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The pictures are kind of crappy. There's no trans currently and the engine is up against the firewall. There's also debris from rodents.

 

[8888]

Thanks for the photos. I have a photo of the original sender on mine down by the oil filter and it's included.

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The replacement sender I got today was exactly the same as the original and I installed it a little while ago.

The oil pressure is now pegged on the gauge down past the 60 psi mark and not moving instead of being at the 30 psi mark and not moving. I need to start it again and be sure it drops to 0 when the truck is off, the other sender did not allow the gauge to return to zero.

I'm going to change the oil once more with Rotella T 10w 30 and I'll check gauge zeroing when I start it afterwards.

The truck started up nicely after I installed the new sender with very little to no smoke. It still has a bit of a misfire but I have (4) plugs on the passenger side gapped at .045" and the (4) on the drivers side are gapped at .035" which may have something to do with it. I will get some oil smoke if I rev it but other than that, it seems much better. No oil drips at the exhaust manifold flange.

 

[Snoots]

Also there to shut the fuel pump off if you lose oil pressure.

 

[bucket]

Also there to shut the fuel pump off if you lose oil pressure.

Nope. The only thing it does supply a backup power to the fuel pump. The truck will run with zero oil pressure, my wife has proved that.

Like a said before, the whole thing about it shutting off the fuel pump has become an old wive's tale. It's the modern day version of "leaving a battery on concrete will kill it". A quick study of an electrical diagram will clear it up.

 

[8888]

The truck will run with zero oil pressure, my wife has proved that.

Nothing about that sounds good.

 

[bucket]

Nothing about that sounds good.

Amazingly it still lived a little while longer, after putting oil in of course. Has 200k on it now, but it's shot.

Here's a diagram showing the different places the fuel pump gets the power it needs. It also includes the hot fuel module which I think only came on some trucks, not all. But it shows that the oil pressure switch overrides the fuel pump relay.

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[8888]

Thanks for the schematic. I know I had a Chiltons manual for this era truck but I can't find it post move.

Along with assembling the roof rack and seeing if the oil pressure gauge returns to zero with the new sender before pegging when started, I may swap the old oil pressure sender in and see if it still goes right to 30 psi and never moves/returns to zero.

Do these gauges rely on the ECM to display a value or are they direct? Meaning, does the oil pressure sensor provide a voltage output and the ECM decides what the gauge shows based on the voltage from the sensor? I don't see a lead to the oil pressure gauge above so the output to the gauge must come from the ECM?

 

[bucket]

The gauges don't get their input from the ECM, just the individual senders. However, the back of the speedometer does have the VSS attached to it, and it sends a reading to the ECM.

 

[8888]

ok, thanks. Just trying to sort out the issues with gauge reaction to the two senders I've installed.

Since the signal bypasses the ECM, it's not at fault, which is a good thing. They probably aren't easy to find.

 

[Snoots]

Nope. The only thing it does supply a backup power to the fuel pump. The truck will run with zero oil pressure, my wife has proved that.

Like a said before, the whole thing about it shutting off the fuel pump has become an old wive's tale. It's the modern day version of "leaving a battery on concrete will kill it". A quick study of an electrical diagram will clear it up.

That's odd.
I know on some mid-80's GM vehicles the oil pressure SWITCH was overridden by the ECM during start but if the oil pressure dropped below ~5psi while running the fuel pump would shut off.
Maybe it's a totally ECM thing.

 

[bucket]

That's odd.
I know on some mid-80's GM vehicles the oil pressure SWITCH was overridden by the ECM during start but if the oil pressure dropped below ~5psi while running the fuel pump would shut off.
Maybe it's a totally ECM thing.

I dunno, every diagram I've had to look at involving the fuel pump on this era GM has been setup like this or nearly the same. Maybe the computer controlled carb setups were set up like you describe? I've never really messed with those at all, but I do know there were some cars that employed an in-tank pump along with the engine mounted mechanical pump.

 

[bucket]

And then there were also many of them that had an oil pressure switch enable power to the electric choke.

 

[PrairieDrifter]

And then there were also many of them that had an oil pressure switch enable power to the electric choke.

Yep my 84 was this way.

 

[Camar068]

And then there were also many of them that had an oil pressure switch enable power to the electric choke.

now that's a good fail safe that might be worthy on the LS swap I just did. Probably want to keep a spare switch handy in case it's the switch though.....and/or have a bypass switch just in case.

 

[8888]

Okay, today I fired it up and there was no visible smoke on startup (in the barn so I would have seen it) and little to none when revving it (outside the barn, so less easy to see but always seen in the past). I still have a bit of a misfire but I still have 4 plugs gapped correctly and 4 010" over. Also have nothing installed after the muffler, need an adapter to fit the new tailpipe (Walker aftermarkets appear to be a larger ID). Engine sounds smooth and strong for the 10 minutes I let it idle. Oil pressure gauge never drops when switched off, stays pegged beyond 60 psi when on, at limit of the gauge, I assume. I was going to install the old oil pressure sender but couldn't find it till I was ready to come in. Will install tomorrow hopefully and see if it goes back to 30 psi.

All things considered, it seems to be doing well.

A member sent me a PM asking for photos of the color combo. I can't figure out how to attach them to a PM so I'll post them here along with a shot of the option label inside the glovebox (assume color codes are on there)

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