1988 Oil pressure gauge / sender mystery... Now with Pics!

[scouthead]

A few weeks ago my new (well almost 12K miles now) engine (350 - .060 over) in my '88 TBI Blazer started having low oil pressure... turned out to be the gauge or sender - I think...

I unscrewed the sender behind the intake (next to the dizzy) and plugged in my mechanical gauge: 40-60 psi cold. lowest I have seen it go when hot is 19-20 psi at idle after a good long high rpm freeway run in the middle of the day.

Now get this:
The sender plug is hanging behind my motor not attached to anything... My oil pressure gauge in my dash still is acting like it did before I unplugged it (or thought I unplugged it).
Needle goes to 0 at idle, but comes up like the needle on a tach if I rev the engine, and then back to 0.

There is some other oil sender-ish thing on the driver's side of the block. It is just behind and above the exhaust manifold. I was not sure what it was for when I replaced the motor, so I put it in the same hole it had been in - in the origional block. There is one wire (think green) going to it. Attaches with one spade connector. There is a "cage" around the sender, and it is a royal pain in my ass to get to it with the engine in the truck. I am assuming the pressure gauge in the dash must be running off this thing. When I feel like twisting my arm up in there I will unplug it to make sure, but...

What the heck is the sender on top of the block for then? Only the computer? And I have not thrown any check engine lights with it unplugged... should it? WTF?

 

[4X4HIGH]

I replied to your PM.

 

[scouthead]

I was going to try and hide from the office tomorrow to bend some tube for my saturn buggy anyways...
I will pull out the old block and an intake to snap a few pics. Have the sender I pulled out on a shelf...
will try to get pics of the sender on the side of the block, but exhaust and clutch slave make it REALLY tight right there!

 

[scouthead]

O.K. This is the sender that I removed to plug in the mechanical gauge. This was the only oil pressure sender in All my other TBI trucks...
The hole circled on the block is where it came from.

 

[scouthead]

Ok here is the "other sender" under my truck.
Located just above the oil filter. I think it is actually an orange wire (only one wire).

 

[scouthead]

I got my arm up in there somehow and unplugged the wire, and my gauge in my dash pegged at like 70 PSI...
so my dash gauge definately is connected to this, and I will assume the gauge is still good.
Pic is of the motor running with the wire unplugged. If I plugged it back in (too hot to do it right now), it would be somewhere betweeen 20 and 0 psi depending on RPM.
O.K. I will replace that sender If I can find one.

But why do I have 2 oil senders, and what the hell is the normal one being used for? Still no SES lights or anything...

 

[4X4HIGH]

Ok, the first pic is a 3 wire oil pressure sender (you might not have 3 wires going to it though) that controls the oil pressure gauge and it is also the backup fuel pump sender (if there is at least 5 lbs. of oil pressure then it will keep the fuel pump running in the event the fuel pump relay goes bad). Some vehicles use a separate oil pressure sender for the gauge and this is why the 3 wire sender might not have 3 wires going to it since you have another oil pressure sender that you know works the gauge.

 

[scouthead]

ah ha...
so if I don't reconnect my sender on top, and my fuel pump relay were to crap out... game over for fuel delivery...
didn't know the oil sender had anything to do with the fuel system.

very interesting!

 

[4X4HIGH]

ah ha...
so if I don't reconnect my sender on top, and my fuel pump relay were to crap out... game over for fuel delivery...
didn't know the oil sender had anything to do with the fuel system.

very interesting!

That is correct. The oil pressure sender is just a backup for the fuel pump relay. I keep an extra FP relay in my blazer at all times just in case.

I'm curious though, how many wires are in the actual sender plug for the sender that is next to the distributor?

 

[scouthead]

I'm curious though, how many wires are in the actual sender plug for the sender that is next to the distributor?

I don't know...
I will put the sender back in the top of the block tomorrow and post what I find here.

 

[Dogman12]

I have an 85 and swaped in and 89 tbi 350 out of another blazer, I have the exact same issue and also the 3 wire sender. If you figure it out please post up, I suspect the sender but its such a pain to get at I keep putting off. Good luck

Jesse

 

[British Slave]

I wish someone would make this a sticky.......

General Motors 0-90 Ohm Gauge Diagnosis Procedure​

Especially as cars age, it is often somewhat difficult to pinpoint the root cause of instrument cluster gauge failure, thanks to questionable wiring, normal wear and tear, and sometimes even incorrect diagnosis procedures. The following diagnosis procedure is the method that GM publishes in it's shop manuals, will work on oil pressure, temperature, and fuel gauges, and should apply to all but the most recent GM-made vehicles with computerized gauges.
Before You Begin
You will need either Kent-Moore tool J-24538-A, if available, or tool JG-GM-1, a homemade GM gauge testing jumper. To build tool JG-GM-1, you will need the following materials:

• 1 Jumper wire with alligator clips (about $2 US from Wal-Mart)
• 1 220 ohm, 1/2W resistor (Radio Shack #271-1109, pack of 5, $0.99)
• 1 150 ohm, 1/2W resistor (Radio Shack #271-1111, pack of 5, $0.99)
• about 1" of 1/4" heat-shrink tubing, or vinyl electrical tape

Construct the jumper as follows:

1. Cut the jumper wire in half, and strip about 1/8" of the insulation off of the cut end of each wire. Tin the bare wires using a soldering iron.
2. Take both resistors and twist their leads together on each end, so that the resistors are joined in parallel. Twist the leads tightly, cut off the excess, and tin the twisted leads. If using heat-shrink tubing, make sure that the resistor assembly is about half the length of the tubing, or less.
3. If using heat-shrink tubing, slip the tubing over one of the jumper wire halves.
4. Solder each half of the jumper wire onto the opposite ends of the resistor assembly.
5. If using heat-shrink tubing, slide the tubing over the resistors and solder joints and shrink using a low-temperature flame or hair dryer. If using electrical tape, tightly wrap the tape around the resistors and solder joints, ensuring that the solder joints are completely covered.

Diagnosis Procedure

1. Disconnect wire from sending unit, and connect tool JG-GM-1 from the wire connector to ground.
2. Turn ignition ON.
3. If the gauge being tested reads at the high end of the scale (i.e. full for gas, 60psi for oil, etc.), the sending unit is at fault and must be replaced. Retest after sending unit replacement.
   Else, go to step 4.
4. If the gauge responds, but not accurately, go to step 5.
   If the gauge does not respond at all, go to step 7.
5. Disconnect engine harness connector at firewall bulkhead. Connect tool JG-GM-1 from ground to bulkhead connector lead that goes to gauge. If the gauge now responds accurately, check the wiring between the sending unit connector and the engine harness connector, repair any faults, and retest.
   Else, reconnect the engine harness connector, and go to step 6.
6. Remove the gauge from the instrument cluster and check the speed nuts holding the ceramic shunt across two terminals on the back. If they are loose, tighten them and retest gauge; if tight, the gauge is at fault and must be repaired or replaced.
   NOTE:You might be able to repair the gauge with a kit from Fix-A-Gauge, and/or you might be able to manually reposition the needle where applicable. Retest after repair.
7. Remove the instrument cluster and check for bad connections at the gauge terminal clips, instrument cluster connector, and both the instrument cluster and sending unit ground. If the connections are good, inspect the wiring from instrument panel to firewall harness connector, and repair any faults if found; if bad, repair the connections, reinstall gauge, and retest. If the connections and wiring are both good, the gauge is at fault and must be replaced.
   NOTE:A gauge diagnosed as bad through this step most likely can not be repaired.

 

[scouthead]

well, the damn sender went out again on the side of the block... this time the gauge still works, but the sender is pissing oil all over my block and exhaust and everything under my truck- everything. so much for a clean easy to work on blazer- hello oily grimy nastiness...

I think its just too damn hot for the sender next to the exhaust manifold... i suspect the replacements i am getting at the parts store are not the highest quality. This last one lasted about 10 months. I am not replacing this thing every 10 months. might as well just replace the clutch slave too at the same time since those only seem to last about 12-14 months... damnit.

anyways the sender on the side of my block is stuck, and my wrenches have rounded off the surface to turn it from, and i need to see if i can cut the damn thing off with a sawzall, and then unscrew the brass elbow from the block. I think ill plug the hole, and then try to wire my gauge to the sender behind the intake (on top of the motor).

not the kind of project i wanted today- hooray!!!!!!

 

[scouthead]

ok, cant get a pic of the various sending units right now, because theyre all still mounted on motors... but heres the plugs that go to the sender on top of the motor (behind intake)...

1989 suburban:

 

[scouthead]

and 1988 blazer that has another sender on the side of the block for the gauge:

 

[British Slave]

If you are trying to fix your oil pressure gauge, you don't need to even look at the sending unit on the back of the intake. Period! The oil pressure sending unit is located right above the oil filter and is single wire connector.

 

[scouthead]

no... im actually trying to fix the oil spurt... im going to switch to the later style sender, and splice my gauge into the middle pin on the connector.

im tired of parts failing because they are right next to the exhaust manifold.

so, i think i can end up with NO sending unit on the side of the block... thus making me just a little bit happier.

 

[British Slave]

Is that sensor a 0- 90 ohm sensor? Remember your gauge is a 0-90 ohm instrument.

 

[scouthead]

im almost sure the oil pressure gauge actually came out of the 1989 suburban i had, so... I will assume they are both 0-90 ohm senders and gauges.
will prob be the weekend before i can play around with anythng... oh well.

 

[British Slave]

A GM gage is a GM gage. Both 90ohm. The sending unit for the gage is a 0-90 ohm reostat. The sending unit on the intake is a switch with open and closed contacts. A gage will not work with the sending unit on the intake.