Fuel Sending Unit

[mavtricks71]

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.

Wow ok, So is that something I need to find NOS? NEW OLD STOCK....or is it common

 

[bucket]

Wow ok, So is that something I need to find NOS? NEW OLD STOCK....or is it common

I only recently found out about it. Honestly it doesn't personally bother me that much so I'll likely choose to leave it alone. If it doesn't bother you, it's not going to hurt anything.

 

[mavtricks71]

Yea I'll leave it for now

 

[hatzie]

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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Your lysdexia is showing... Easy to get this backwards.

1. Max Full needle is 90Ω resistance through the sender to the tank ground.

2. Empty needle happens when the circuit 30 wire is dead shorted to ground or 0Ω resistance through the sender to the tank ground.

3. Roughly 3:00 needle, well past max full, is a completely open circuit 30 wire (cut/damaged/unplugged). This is the default needle position with power applied to the gauge and no path at all to a ground on the Circuit 30 signal lead.

Your theory of operation text is dead on...

 

[Frankenchevy]

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

I intentionally ordered a 1987 tank for my sniper application to run an ep381 fuel pump. the tanks I received don't have any baffles below the sender and there are no studs like the diagram you posted. did I get the right tank or did I get shafted and need to return them?

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[hatzie]

I intentionally ordered a 1987 tank for my sniper application to run an ep381 fuel pump. the tanks I received don't have any baffles below the sender and there are no studs like the diagram you posted. did I get the right tank or did I get shafted and need to return them?

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That doesn't look like the innards of the 87-91 TBI tanks I put in my 1976 diesel project to get in-tank fuel pumps. Don't ask me to find the part number for the lift pumps I used. They are Walbro fuel-cell carburetor fuel pumps with 4-6PSI output. That's all I remember.
Did they sell you a diesel tank?