Fuel Sending Unit

hatzie · Feb 16, 2018

The bypass valve is already eliminated.
The gauge goes to E with the selected sender plugged in and 3:00 unplugged.

If any of the wiring was shorted it wouldn't go to 3:00 with the sender unplugged. If the wiring were open it wouldn't go to E with the sender plugged in.

mavtricks71 · Feb 17, 2018

hatzie said:
The bypass valve is already eliminated.
The gauge goes to E with the selected sender plugged in and 3:00 unplugged.

If any of the wiring was shorted it wouldn't go to 3:00 with the sender unplugged. If the wiring were open it wouldn't go to E with the sender plugged in.

Yes I did unplug drivers side tank and it did go to 3 olock on the fuel gauge. Maybe the problem is my fancy Chinese made Spectra fuel sending unit.

hatzie · Feb 17, 2018

mavtricks71 said:
Yes I did unplug drivers side tank and it did go to 3 olock on the fuel gauge. Maybe the problem is my fancy Chinese made Spectra fuel sending unit.

So you tried it on both sides.

It's probably the senders.

You can test the senders out of the tanks. The top of the sender needs to be grounded to the frame (the blue wire in Jerrys' last picture) and the sender wire plugged in. With the ignition on move the float through its' travel and see what the gauge does.

If you only have 1 or 2 or 3 gallons in the tank the needle may not move off the E mark.

bucket · Feb 17, 2018

The factory senders aren't all that accurate anymore either. My '84 with the 16 gallon tank is always very empty by the time the gauge says it's empty. But my '79 with the dual 20 gallon tanks, both still have 4 or 5 gallons in them when the gauge gets down near empty. Which is different from in the past, because the left tank used to run out with 1/8 tank showing on the gauge.

hatzie · Feb 17, 2018

bucket said:
The factory senders aren't all that accurate anymore either. My '84 with the 16 gallon tank is always very empty by the time the gauge says it's empty. But my '79 with the dual 20 gallon tanks, both still have 4 or 5 gallons in them when the gauge gets down near empty. Which is different from in the past, because the left tank used to run out with 1/8 tank showing on the gauge.

There's supposed to be a "reserve" of something like 2-3 gallons after the gauge reads empty. Sounds like the old sender was worn out.

You can change your Empty point in the 16 gallon tank by VERY slightly bending the float arm.
I would put 4 gallons in the tank and set the arm so it has around 4Ω-5Ω resistance.

The other interesting thing I read in Chevrolet Service News from the mid to late 70's is GM changed the senders to be weighted so the gauge reads nearer full longer than they used to. If I can find the article I'll scan it and post it.

I bench tested several fuel, temp, and oil pressure gauges with some of my electronics test gear. A Benchtop Precision DC power supply and a General Radio 1432K (0.1Ω per step 1111Ω total) Calibrated Decade resistance box.

I recorded the readings and then I plotted the resistance to needle position curve in Libre Office Calc.
The results are enlightening.
You can see that F - 3/4 has a slightly steeper slope compared to 3/4 - 1/4 and just below 3/8 to E is very steep. You can see it's weighted to read in the mid range for longer than low and high. This is several gauges from several years that are very close.

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chengny · Feb 17, 2018

A TSB regarding improper applications of fuel tanks/senders - IDK if it pertains to this particular case or not. But knowledge is power:

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 gage 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

mavtricks71 · Feb 17, 2018

chengny said:
A TSB regarding improper applications of fuel tanks/senders - IDK if it pertains to this particular case or not. But knowledge is power:

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

You must be registered for see images attach

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 gage 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

Wow this is getting crazy lol

chengny said:
A TSB regarding improper applications of fuel tanks/senders - IDK if it pertains to this particular case or not. But knowledge is power:

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

You must be registered for see images attach

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 gage 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

All this information is great....I do think I looked in my old tank....I don't recall a baffle but I really need to check again, All info is great

hatzie · Feb 17, 2018

Yup.
The RV series TBI tanks definitely have baffles to keep the fuel pump supplied with petrol during braking, acceleration, and cornering. I never tried the CK senders in the 87 tanks I purposely put on my 76. It makes sense that the senders might have an issue if they weren't setup for the baffles.

You could probably use an 87 sender with a cut-off pickup tube and sock from an earlier unit in a baffled tank. Just couple the pickup tubes with SAE J30R10 hose and fuel injection clamps and tie the lower pickup half to the fuel pump support.

chengny · Feb 17, 2018

You'll get it. It's just hard to accept that you got 2 BOB senders.

Also, I apologize for confusing the issue by even bringing up the Pollak valve aspect - but I didn't have all the facts (as far as what, where & how you had tested the circuit).

But if - as Hatzie advises - you get

1. A max full indication with the common PNK grounded,

2. An MT indication with the PNK discoed

3. And no indication on the dash with the PNK 30 connected directly to the production tank...

it's pretty much sure to be a sender problem. The gauge and associated wiring (up to the transfer valve) are doing exactly what they are supposed to.

But what I haven't seen is the results for test #3 above (i.e the PNK 30 lead connected only and directly to the LH sender). Your original complaint was that the gauge would not budge off the E end - which could be the result of a short to ground either within the valve itself or between the splice point and the valve. And also, the extension lead is still an unknown - if I am reading this correctly.

You really owe it to yourself to disconnect the common (PNK 30) lead at the valve socket - that would only leave the two short runs of wire (PNK/WHT & PNK BLK) that go to the senders - and, unless you are extremely unlucky, only one should be grounded. The other should be capable of working the gauge.

Just as a general discussion on the gauge/sender/chassis relationship - and to make sure I have the theory of operation straight in my mind. This is not required material - skip it if you want:

The components mentioned above - when connected by the pink (hot) and blue (ground) wires - form the "sensing leg" of the gas gauge located in the instrument panel.

12 volts is supplied to the positive terminal of the sender (via the pink wire). It's source is the negative side of the gauge's measuring coil.

The less resistance in this total combined circuit, the greater the electron flow through the coil wires - and consequently the stronger the force is that it (the measuring coil) can exert on the gauge's magnet. The magnet is attached to the lower end of the indicating needle. Blah, blah blah…

In addition to the sensing leg, there is another - opposing - circuit called the "control leg". It is identical to the sensing leg in every way except that the negative side of it's coil is lead straight to ground (rather than through a variable resistance and then to ground like the sensing leg). Also, it's coil is located on the opposite side of the magnet from the measuring coil's.

So what you have is; two coils located on either side of an indicating needle - AKA the "ray". The ray is pivoted somewhere in the middle and has an integral magnet mounted at the bottom.

These coils create their own magnetic fields - the strength of which is a function of the electron flow (i.e. current) passing through them. When these circuits are energized - by turning the ignition switch to the RUN position - the opposing magnetic fields begin to "fight it out". Each coil tries to pull the magnet - mounted on the needle - towards itself.

Due to the fact that it's negative leg is run straight to ground - resulting in nearly zero resistance, the current flow through the control coil is essentially constant (as is it's magnetic force).

On the other hand, due to the variable resistance generated by the sender (as a result of changes in the fuel tank level), the current flow/magnetic strength of the measuring coil can be higher or lower than that of the control coil.

The location of the two coils - to the magnet - is such that:

1. The control leg is always trying to peg the indicating ray to the FULL end.

2. The sensing leg is always trying to peg the ray to the EMPTY end.

So as extreme examples:

1. If the pink wire to the sender is broken/disconnected the control leg exerts the greater magnetic force and pegs the needle Full.

2. If the pink wire is allowed to contact ground (before passing through the sender), the sensing leg has a greater force and the ray pegs low.

As GM explains it (better and way more succinctly than I can):

VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.

When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil.

When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.

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mavtricks71 · Feb 17, 2018

Boy that's lotta info , and I'm not good at electrical.....but I'll try research it...once I get out of hospital.

chengny · Feb 17, 2018

Jeez, I forgot you don't feel good right now. I'm sorry - seriously.

But here's an easy way to work from the other end of the problem. It should only take about 5 minutes and will verify whether the senders are good or not:

Take your meter (the bed is currently off correct?). Set it to a low resistance scale (like 0 -100 would be perfect). Remove any external wiring to the brass pin and attach the probes as shown below. The ground lead can stay attached. Obtain a ohmic value across the sender - might as well do both tanks.

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I am assuming the tanks are 16 gallon capacity, but that doesn't matter anyway. All you want to see is whether the senders's variable resistance rises as a function of the level of gas in the tank. The exact numbers are not important.

After you get the initial resistance. Dump a 5 gallon Jerry can of gas in the tank and recheck the ohms across the senders coil.

I believe the upper limit for a GM gas tank is 90 ohms. So even if the tank was empty for first check (and rang out at 0 ohms), the addition of 5 gallons will raise the float to about 1/3 of full capacity. That level should yield a reading of about 30 ohms. If it moves to anywhere near that point you will know that it is varying the resistance and might just be stuck or something.

I would do the second test with the ground side probe at the end of the lead where it bolts to the frame.

mavtricks71 · Feb 18, 2018

chengny said:
Jeez, I forgot you don't feel good right now. I'm sorry - seriously.

But here's an easy way to work from the other end of the problem. It should only take about 5 minutes and will verify whether the senders are good or not:

Take your meter (the bed is currently off correct?). Set it to a low resistance scale (like 0 -100 would be perfect). Remove any external wiring to the brass pin and attach the probes as shown below. The ground lead can stay attached. Obtain a ohmic value across the sender - might as well do both tanks.

You must be registered for see images attach

I am assuming the tanks are 16 gallon capacity, but that doesn't matter anyway. All you want to see is whether the senders's variable resistance rises as a function of the level of gas in the tank. The exact numbers are not important.

After you get the initial resistance. Dump a 5 gallon Jerry can of gas in the tank and recheck the ohms across the senders coil.

I believe the upper limit for a GM gas tank is 90 ohms. So even if the tank was empty for first check (and rang out at 0 ohms), the addition of 5 gallons will raise the float to about 1/3 of full capacity. That level should yield a reading of about 30 ohms. If it moves to anywhere near that point you will know that it is varying the resistance and might just be stuck or something.
So yes beds off truck, and I did put 5 gallons in drivers tank and gas gauge stayed on Empty....that's when I knew something was up, I did ohm it before I left and it was only 4 ohms....that's if I set it on correct setting , "as I'm not good w electrical" but my buddy is gona help me re read senders when I'm home...thanks for info appreciate it.

I would do the second test with the ground side probe at the end of the lead where it bolts to the frame.

mavtricks71 · Feb 18, 2018

Thanks you very much ....yes bed is off truck.....when I get home I'm gonna continue to figure it out and re read senders w ohms.

mavtricks71 · Mar 7, 2018

chengny said:
Jeez, I forgot you don't feel good right now. I'm sorry - seriously.

But here's an easy way to work from the other end of the problem. It should only take about 5 minutes and will verify whether the senders are good or not:

Take your meter (the bed is currently off correct?). Set it to a low resistance scale (like 0 -100 would be perfect). Remove any external wiring to the brass pin and attach the probes as shown below. The ground lead can stay attached. Obtain a ohmic value across the sender - might as well do both tanks.

You must be registered for see images attach

I am assuming the tanks are 16 gallon capacity, but that doesn't matter anyway. All you want to see is whether the senders's variable resistance rises as a function of the level of gas in the tank. The exact numbers are not important.

After you get the initial resistance. Dump a 5 gallon Jerry can of gas in the tank and recheck the ohms across the senders coil.

I believe the upper limit for a GM gas tank is 90 ohms. So even if the tank was empty for first check (and rang out at 0 ohms), the addition of 5 gallons will raise the float to about 1/3 of full capacity. That level should yield a reading of about 30 ohms. If it moves to anywhere near that point you will know that it is varying the resistance and might just be stuck or something.

I would do the second test with the ground side probe at the end of the lead where it bolts to the frame.

Update on my Sending units it turns out before I went into hospital I didn't have my dash switch hooked up/replaced with new one.....I now have a working fuel gauge but the needle likes to move around when I go around corners ect.....if I'm standing still its fine. Its good enough for me heck I'm just glad to have a gas gauge now.

bucket · Mar 7, 2018

Mine all move around during cornering, braking etc too. These fine folks here taught me that's due to a bad dampener on the back of the gauge. It's that ceramic looking thing that goes across the two studs.

Fuel Sending Unit

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