So my rebuilt L98 fired up and all seems fine for now except my oil pressure gauge reads max (pegged upwards). I put the pressure sensor back in the same location as the 1989 LB9, down by the filter. Is there a difference with these blocks? There were some loose, short brass pipe sections from the 1988 L98 (given to me after the engine was machined) that I couldn't figure out where they went. so I am assuming the 1988 L98 block would have the same oil pressure take off port as the 1989 LB9. I could be wrong though. Just want to tap the experience here before I go diving in.

I will post a proper thread on the apparent success of my project once it gets broken in and shows me I haven't messed anything up. I would've taken it for a spin but not have an oil pressure reading is not optimal. I'm not hearing and lifter noise so I figured pressure is just fine, but still.

 

My 86 305 TPI has it mounted by the dizzy. There is a long brass hexagon (i think) shaped tube that lets me mount the pressure switch and sending unit under the mounted dizzy. This unit is bell shaped, the one by the oil filter looks similar to a coolant temp unit. This was factory on mine, so when I did the swap I am currently finishing, i put this back on my new block the same way so that i could use factory harness pigtails at same location. If you are using a harness that has it at the other location, (because they did it both ways dependent on the years), then your fine too. Just make sure you are using the correct unit/pigtail for each location (they are different), and none of the wiring is bare, broken, etc. of course.

 

Thanks, that is it. I plugged that port up by the distributor and located the sensor down by the oil filter. I did have two sensors in my hands, one from 88 and one from the 89 and I don't know which one I used. If the wiring is different it could explain the bad reading. I will get a new one anyway.

 

Well, the pressure sensor circuit is part of the car, rather than the engine. So you'd want to use the same sensor from your old engine, in the same port as it was installed on your old engine. HTH.

 

Drew, let me make sure I have this right. The pressure switch, goes through the ECU and connects to the fuel pump. The switch opens up when there is oil pressure ( 3 or 5 lbs before opening the circuit) for the fuel pump to turn on (as a fail safe). The oil sender is just for a gauge / straight to the gauge. So, if the pressure switch is not connected at all, the car would never start because the switch would never send anything to turn on the pump. Or is the ECU actually reading this oil pressure and sending the signal to the pump? I'm asking this because although I have installed my sender and switch (under the dizzy on mine) I'm in the process of figuring out my wires for them now. I cant seem to find the original pigtails from the drivers side harness for the gauge, and haven't looked at the original engine one yet for the switch, or maybe both on this harness?, that I removed for the new Holley HP ECM and (Holley TPI) harness that I've started to put in. I have been pouring over threads about the wiring, but there is so many variations in the years and the diagrams are not that clear to me. I don't know if I have to have the switch wired with the Holley ECM, and then wired to a fuel pump wire? It's confusing because the Holley harness has a pigtail marked fuel pressure, and one marked oil pressure, but are definitely not the style of sender/switch I have. I figured the oil pressure one was the later 3rd gen style by the filter, but I don't have a fuel pressure gauge, so I am beginning to think they sent me the wrong harness. If you could straighten me on the wiring, then maybe someone could chime in on the difference with a aftermarket ECM.

Thanks guys

 

In 86, you should have a sending unit for only the dash gauge, the typical giant silver bell style. Then you'd also have a smaller oil pressure switch for the fuel pump circuit. Later thirdgens have the sender and switch integrated into a 3 pin unit that does both functions.


The ECM doesn't really read the pressure, it just sees a dumb signal more like an oil pressure yes/no. As far as the exact interaction with the fuel pump circuit, I couldn't really tell you, beyond when you turn the key to run initially the fuel pump relay is energized to prime the pump. There's a redundant feature that the ECM detects when the engine starts making oil pressure, and energizes the pump bypassing the relay. I'm fuzzy on the exact how and why of it, and I don't have any 86 service manuals around. Your best bet would be to get a copy of the GM service manual for your car and study the diagrams until it starts to make sense, or wait and see if someone more familiar with the 86 system stops through. Add in an aftermarket ECM and I really don't know.

 

This was my situation. The 88 block I swapped in had a separate gauge sender up by the distributor and only the oil pressure fuel pump switch down below. My harness was from an 89 LB9 which uses the integrated unit down by the oil filter. When I looked at the switch I installed from the 88, it was only a switch so my gauge wire had no connection. I installed a new, correct one and I've got oil pressure!

The oil pressure switch is a parallel power source to the fuel pump. Either the oil pressure switch OR the Fuel Pump Relay can power the fuel pump. Apparently the oil pump switch is a backup for a failed fuel pump relay.

 

So does anyone know if I don't hook up the switch part to it's harness, but hook up the main fuel relay, will it start/work without issues?

 

Yes, it should.