gauges oil pressure & fuel

[SirRobyn0]

Ok, for those that didn't see my intro, the truck is new to me as of a few weeks ago. Temp, volt, and speedo work just fine.

Oil pressure gauge acts oddly. most of the time when I go to start the truck, turn the key on and it points to L, fire it up and it's a little over half, with a hot idle at the 1/4 mark. I believe when it is behaving like this it is reading accurately. Other times when I go to start it, I'll turn the key on and it'll read 1/2, once firing it up it'll go all the way up to the H, and hot idle at 3/4 mark. Sometimes it'll go from reading one way to the other while driving. I'm leaning toward replacing the sending unit and seeing what happens. Mostly because it's the easy first thing to try, but thought I'd see what you guys have experienced.

Fuel gauge. Never reads more than 7/8 when the tank is full and will stay at 1/8 tank until it runs out. I replaced the sending unit and it made no change. I did not change the ground wire on the sending unit because my bed dumps, it was easy to get to the sending unit, but the ground wire connects somewhere under the cab. Not wanting to remove the tank, I think the next step is to replace the ground wire, but I need to know if it connects to the frame or the cab. And what do you all think, ground issue or gauge?

Thanks,
Rob

 

[75gmck25]

The stock gas sending unit has a ground tab on it. The ground wire goes from that tab just about straight down, then through a grommet in the frame (the power wire comes through the same grommet) and is then grounded to a bolt on the inside of the frame. Its quite easy to reach the wire once you crawl under the truck. Just unscrew the bolt, clean up the frame with a wire brush, and reinstall the bolt.

Bruce

 

[Camar068]

If what 75gmck25 said doesn't fix it, make you a jumper wire out of wire with gator clips on each end. Make it about 10-15' long. The clip on the tab and hook the other end to the negative post on the battery. If there's a change you need to check all your grounds or add more.

I made two of the jumper wires way back when and they are handy when troubleshooting stuff like this.

 

[SirRobyn0]

After I made my post I thought about the ground wire thing. I think what I'll do is fill her up, and connect a jumper wire for the ground. If the gauge reads full I'll know it's the ground. If not then it's probably the gauge. Thanks

 

[chengny]

Ok, for those that didn't see my intro, the truck is new to me as of a few weeks ago. Temp, volt, and speedo work just fine.

Oil pressure gauge acts oddly. most of the time when I go to start the truck, turn the key on and it points to L, fire it up and it's a little over half, with a hot idle at the 1/4 mark. I believe when it is behaving like this it is reading accurately. Other times when I go to start it, I'll turn the key on and it'll read 1/2, once firing it up it'll go all the way up to the H, and hot idle at 3/4 mark. Sometimes it'll go from reading one way to the other while driving. I'm leaning toward replacing the sending unit and seeing what happens. Mostly because it's the easy first thing to try, but thought I'd see what you guys have experienced.

Fuel gauge. Never reads more than 7/8 when the tank is full and will stay at 1/8 tank until it runs out. I replaced the sending unit and it made no change. I did not change the ground wire on the sending unit because my bed dumps, it was easy to get to the sending unit, but the ground wire connects somewhere under the cab. Not wanting to remove the tank, I think the next step is to replace the ground wire, but I need to know if it connects to the frame or the cab. And what do you all think, ground issue or gauge?

Thanks,
Rob

The oil pressure gauge issue will most likely be due to an intermittent bad connection - it could be at the sender end, bulkhead connector or at the instrument cluster. While you're checking the connection at the sender, that's good time to be sure there is 12 volts available on the TAN wire that feeds the wire round resistor (with the ignition switch in the RUN position). The resistor is located under the bell shaped cover. The TAN lead is easy to identify by it's offset terminal boot.

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Engine oil pressure acts on a diaphragm which is connected to the resistor's wiper arm by linkage.

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If you can confirm 12 a volt supply into the variable resistor, you would be wise to check the operation of the diaphragm/linkage. Use the matrix below and interpolate an approximate expected resistance for whatever your truck's oil pressure runs at normally. A better method would be to pull the oil pressure switch and temporarily install a mechanical gauge.

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That would be helpful for two reasons:

1. You would be able to observe engine oil pressure in real time, and

2. It would let you rest easy knowing that the system was operating at design conditions.


Approximate sender resistance at three representative oil pressures:

(Low - i.e engine off - 0 PSI) sender resistance should = ~0 Ω

(Normal operating condition 30, 40, or 50 PSI) sender resistance should be = 30 Ω

(High, 60, 80, or 100 PSI) sender resistance should be = 60 Ω



If, after doing the sender diagnostic steps, then you can inspect the associated wiring. While the following may be true, I would rather trouble shoot with a VOM than start tearing into the wiring harnesses any day.



Gauge failures are often caused by defective wiring or grounds. The first step in locating trouble should be a thorough inspection of all wiring, terminals and printed circuits. If wiring is secured by clamps, check to see whether the insulation has been severed, thereby grounding the wire. In the case of a fuel gauge installation, rust may cause failure by corrosion at the ground connection of the tank unit.

 

[chengny]

Ok, for those that didn't see my intro, the truck is new to me as of a few weeks ago. Temp, volt, and speedo work just fine.


Fuel gauge. Never reads more than 7/8 when the tank is full and will stay at 1/8 tank until it runs out. I replaced the sending unit and it made no change. I did not change the ground wire on the sending unit because my bed dumps, it was easy to get to the sending unit, but the ground wire connects somewhere under the cab. Not wanting to remove the tank, I think the next step is to replace the ground wire, but I need to know if it connects to the frame or the cab. And what do you all think, ground issue or gauge?

Thanks,
Rob

I don't know if you were aware of this TSB. I know it only references 1986 models but that was a longtime ago and things get fuzzy - not to mention changed out. You might want to look into it. The first thing that crossed my mind when I read your OP was a mechanical interference issue. I'm a bit confused by this statement:

Never reads more than 7/8 when the tank is full and will stay at 1/8 tank until it runs out.

Does that mean that if you top the tank off, the gauge will initially show only 7/8. After you fill up, it reads normally, and falls off - until the gauge reads 1/8. And once at the 1/8 mark, it just hangs there - until you finally go dry? If that is the case, I don't think you have an electrical problem.





CHEVROLET NUMBER: 86-T-216


SECTION: 6C - Engine Fuel


DATE: January, 1987


SUBJECT: FUEL GAGE WON'T READ BELOW 1/4 TANK

MODELS: 1986 C, K PICKUP TRUCKS

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Some late built 1986 C/K trucks may have been built with 1987 fuel tanks. The 1987 fuel tank contains a fuel baffle which can cause interference with the fuel sender float arm and not allow the fuel gauge to read below 1/4 tank of fuel. If the truck is equipped with dual fuel tanks, this condition can occur in either tank. If this condition is encountered, the fuel tank must be replaced. The 1987 fuel tanks can be identified by removing the fuel tank and fuel tank sender and checking for the reservoir (plastic) which is attached to the bottom of the tank by two studs which are approximately 4" from the center of the fuel sender unit, See Figure No. 1. Refer to section 6C-Fuel System for fuel tank removal and reinstallation procedures.
PARTS INFORMATION

Part
Number Description Application Quantity


14071994 Fuel Tank 1986 C,K 1,2,3 1
right hand, left hand all engines 16 gallon tank


14071984 Fuel Tank 1986 C,K 1,2,3 1
right hand, left hand all engines 20 gallon tank

 

[SirRobyn0]

Never reads more than 7/8 when the tank is full and will stay at 1/8 tank until it runs out.

Does that mean that if you top the tank off, the gauge will initially show only 7/8. After you fill up, it reads normally, and falls off - until the gauge reads 1/8. And once at the 1/8 mark, it just hangs there - until you finally go dry? If that is the case, I don't think you have an electrical problem.

Thanks for all the information to answer your question, I think you understand the problem correctly, but I'll rephrase. Fill the tank it reads 7/8. It will slowly go down as fuel is used, once it gets to 1/8 it will stop going down any further, once the tank is completely empty it will still read 1/8. Does that make better sense?

 

[chengny]

Thanks for all the information to answer your question, I think you understand the problem correctly, but I'll rephrase. Fill the tank it reads 7/8. It will slowly go down as fuel is used, once it gets to 1/8 it will stop going down any further, once the tank is completely empty it will still read 1/8. Does that make better sense?

Yeah I get what you mean - and it seems like we're both saying the same thing. Unfortunately (and especially since you have already installed a new sender), the symptoms you describe don't normally suggest a problem with the gauge circuitry.

It seems like everything is working as designed (other than no level indication below 1/8).

Rather than pulling/installing another sender, check the circuit on both sides of it first. On the hot side, first confirm that there is 12 volts available on the pink lead. It is wicked easy. Just do as Bruce suggests and access the two leads by reaching up from under the truck. You will see the hot lead and ground leg. Your sender wiring will look like this:

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Pop the pink lead off the brass pin and check that there is 12 volts to the sender's variable resistor (as with the oil pressure sender, have the key in RUN). If there is voltage, snap the connector back on and move on to the ground leg. You can use your temporary jumper or just clean/tighten the connection to the frame rail. The sender end is soldered and it should be apparent if is loose. If you are concerned about the copper conductors inside not being intact, just check that there is continuity from the sender to the frame across the blue/black lead


After those checks are done - and there is no improvement - the cause starts to become a head scratcher.

 

[SirRobyn0]

I kind of doubt that both the original sender and the new one would have the exact same issue. As mentioned previously my truck has a dump bed so I was able to access the sending unit with the bed lifted. My ground wire is definately not attached to the frame in the same spot as the diagram shows. Mine runs way forward under the cab. I'll likely just connect a new wire to the frame in the location the is on the diagram and go with that. I'll let y'all know if I see improvement. Thanks