Where to hook up mechanical oil pressure gage? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Jason Spears · 04-18-2003, 12:19 PM

I'm in the process of hooking up a mechanical oil pressure gage in my truck and I'm not sure where to hook it up. I have an '87 1/2 ton with a 305 TBI. There are two oil pressure taps on the engine. One is beside the distributor and the other is just above the oil filter. The one beside the distributor has two wires going to it and the one by the oil filter has one. I have all of the factory service manuals and wiring diagrams but they seem to contradict themselves as to the function of each of these switches. One switch is a oil pressure switch which gives a permissive to the fuel pump to come on once the oil pressure reaches 4 psi. The other is a sending unit for my idiot light. Now according to the wiring diagram the one with two wires (beside the distributor) is the switch for the fuel pump and the other is for the idiot light.

In the section on the instrument cluster in the service manual it says when you have a problem with the gages to check the sending unit beside the distributor. I've also read in other posts about a tee fitting beside the distributor in trucks with factory gages. Is the other switch present in trucks with gages?

Where has everyone else hooked up their pressure gages? I just want to make sure I'm getting the correct reading so I can diagnose my oil pressure problem correctly. Thanks for the help.

Jason

raceman6135 · 04-18-2003, 12:47 PM

I installed a 'T' in the port by the distributor so I could use both my factory idiot light and my aftermarket gauge.

My 1986 350 block (carbureted) isn't even tapped by the oil filtre. Sorry, can't help you with regard to which senders/sensors/switches do what functions, but if you 'T' either of them, you should be safe, because then you're not bypassing any factory systems.

DJS

ElGracho · 04-18-2003, 12:54 PM

Well, I have some experience with this, but not on an 87. The sending unit by the distributor is for the idiot light or the gage, the switch near the oil filter on earlier models is tapped into the choke wire (this is why your choke light comes on when your engine dies, no oil pressure and therefore no voltage goes to the choke coil) I can only assume that they used the same arrangement in 87 but substituted the switch for the fuel pump for the electric choke switch.

Jason Spears · 04-19-2003, 08:55 PM

After doing a little more research I determined that the port just above the oil filter actually intersects the bore going up to the port by the distributor so they both read the oil pressure after the oil filter. I installed the pressure line for the gage at the port by the oil filter because that is where the sending unit for the idiot light is. It was also much easier to access than the one by the distributor. This way I can see exactly what the pressure is if the light comes on since the gage and the light are on the same port.

I also found a pretty easy way to get the stock sending unit out. I just used a ordinary basin wrench. It reached the unit no problem and I didn't have to buy a sending unit socket or bust my knuckles reaching up between the block and the exhaust manifold with a pair of vise grips!

Now I can start trying to figure what made my oil light come on when I was driving home on Thursday. Lots of fun!

04-18-2003, 12:19 PM	#1
Jason Spears Registered User Join Date: Dec 2002 Location: Stoney Creek, ON Posts: 89	Where to hook up mechanical oil pressure gage? I'm in the process of hooking up a mechanical oil pressure gage in my truck and I'm not sure where to hook it up. I have an '87 1/2 ton with a 305 TBI. There are two oil pressure taps on the engine. One is beside the distributor and the other is just above the oil filter. The one beside the distributor has two wires going to it and the one by the oil filter has one. I have all of the factory service manuals and wiring diagrams but they seem to contradict themselves as to the function of each of these switches. One switch is a oil pressure switch which gives a permissive to the fuel pump to come on once the oil pressure reaches 4 psi. The other is a sending unit for my idiot light. Now according to the wiring diagram the one with two wires (beside the distributor) is the switch for the fuel pump and the other is for the idiot light. In the section on the instrument cluster in the service manual it says when you have a problem with the gages to check the sending unit beside the distributor. I've also read in other posts about a tee fitting beside the distributor in trucks with factory gages. Is the other switch present in trucks with gages? Where has everyone else hooked up their pressure gages? I just want to make sure I'm getting the correct reading so I can diagnose my oil pressure problem correctly. Thanks for the help. Jason

04-18-2003, 12:54 PM	#3
ElGracho Gentleman Jim Driver Join Date: Nov 2002 Location: Poulsbo, WA Posts: 1,553	Well, I have some experience with this, but not on an 87. The sending unit by the distributor is for the idiot light or the gage, the switch near the oil filter on earlier models is tapped into the choke wire (this is why your choke light comes on when your engine dies, no oil pressure and therefore no voltage goes to the choke coil) I can only assume that they used the same arrangement in 87 but substituted the switch for the fuel pump for the electric choke switch. __________________ Joe '75 GMC Gentleman Jim '84 Chev C10 Short Wide - Super duper plain (manual steering, manual brakes, no dome light, no cig lighter) '85 Chev C10 Short Wide - Super plain Vortec 4.8 4L60E trans also: '81 K30, '83 C30 Crew Dually, '84 M1028 CUCV, '85 M1009 CUCV, another '85 C10 SWB, '89 R3500 Flatbed

04-18-2003, 12:47 PM	#2
raceman6135 Dino burner Join Date: Nov 2002 Location: Calgary, Alberta, Canada Posts: 602	I installed a 'T' in the port by the distributor so I could use both my factory idiot light and my aftermarket gauge. My 1986 350 block (carbureted) isn't even tapped by the oil filtre. Sorry, can't help you with regard to which senders/sensors/switches do what functions, but if you 'T' either of them, you should be safe, because then you're not bypassing any factory systems. DJS

04-19-2003, 08:55 PM	#4
Jason Spears Registered User Join Date: Dec 2002 Location: Stoney Creek, ON Posts: 89	After doing a little more research I determined that the port just above the oil filter actually intersects the bore going up to the port by the distributor so they both read the oil pressure after the oil filter. I installed the pressure line for the gage at the port by the oil filter because that is where the sending unit for the idiot light is. It was also much easier to access than the one by the distributor. This way I can see exactly what the pressure is if the light comes on since the gage and the light are on the same port. I also found a pretty easy way to get the stock sending unit out. I just used a ordinary basin wrench. It reached the unit no problem and I didn't have to buy a sending unit socket or bust my knuckles reaching up between the block and the exhaust manifold with a pair of vise grips! Now I can start trying to figure what made my oil light come on when I was driving home on Thursday. Lots of fun!



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