Wrong fuel gauge?

[one4fun]

My fuel gauge on my 86 K10 has been bouncing all over the place when the tanks are anything less than full. It does it with both tanks. I decided to try swapping out the gauge with one from my local picknpull. Looked exactly the same, installed it, no more erratic fuel gauge! But. now it only goes up to 1/4 tank when full! Both tanks, same thing. I don't know the model year I pulled it out of, but I am wondering if the sending units are different for different years and if so, this could be why the gauge does not read correctly.

If so, does anyone know what years/models I would look for a replacement gauge that would be compatible with my truck?

 

[MadOgre]

There are differences. Im not sure what differences that is, but I do know for instance that a diesel gauge only works with diesel sending units. Also there are 3 or 4 different styles of sending units with in our square years.

 

[marks86]

There are differences. Im not sure what differences that is, but I do know for instance that a diesel gauge only works with diesel sending units. Also there are 3 or 4 different styles of sending units with in our square years.

how come diesel units only work with diesel gauges?

 

[MadOgre]

I should correct that. Gasoline gauges do not work with diesel sending units.

I don't know probably different ohms or something im sure Jerry would know.

 

[marks86]

I should correct that. Gasoline gauges do not work with diesel sending units.

I don't know probably different ohms or something im sure Jerry would know.

well I have a diesel gauge with a gas sending unit and its not working

 

[MadOgre]

I figured as much. Im pretty sure they are set to read at different ohms

 

[MadOgre]

I swapped in a gasoline gauge when I pulled the 6.2 but it wouldn't work so I just blacked out the diesel on the face of my diesel gauge and put it back in. It works fine

It was the same thing it would only read like a 1/4 tank or something

 

[marks86]

I swapped in a gasoline gauge when I pulled the 6.2 but it wouldn't work so I just blacked out the diesel on the face of my diesel gauge and put it back in. It works fine

It was the same thing it would only read like a 1/4 tank or something

wow thanks man you are on to something!

rep for you

 

[MadOgre]

Thanks although Im not sure what I did lol

 

[one4fun]

I made another trip to pick n pull and got another gauge. Not sure if it has anything to do with this or if it was just coincidence, but the first one that i pulled out of a truck, which did not register the correct fuel level when installed in my truck, had an S or a 5 written on the round part behind the gauge face. I noticed the same thing on another one that I pulled yesterday and decided to look for another one. The one i ended up getting had an L or a 1 written on it. It came off a truck with dual tanks, used unleaded fuel and did not have a tach.

It works like a dream.

 

[bucket]

I figured as much. Im pretty sure they are set to read at different ohms

That's interesting. GM sending units are 0-90 ohms IIRC, Why in the heck would the diesel units work different? Do they have the same float design or is that different too?

 

[chengny]

In addition to the normal - float driven - variable resistor, the fuel level sender used for diesel applications also incorporates a sensor that detects a "water in fuel" condition.

It is most likely a 2 prong NO probe that utilizes the conductive properties of water. When the layer of water that forms at the bottom of the tank becomes thick enough to bridge the gap between the sensor's probes, it effectively closes the contacts and completes the WIF alarm circuit.


Because of this extra circuit in the sender, a third lead is required in addition to the pink & black/blue used for gasoline applications. Just going by the dwg below, it would appear to be yellow:

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It seems that this set up was abandoned at some point (the dwg above is from the 1981 Service Manual) and the WIF probe was moved to the fuel filter housing.

 

[highdesertrange]

just to clear up a few things. like bucket said all gm sensors are 0-90 ohms, been that way forever. the water in fuel sensor on the sending units was only used for a few years 82-83 I believe. after that the water in fuel sensor was moved to the filter. I have not been able to find those senders all aftermarket sensors are the same diesel or gas, except for the number of lines and none come with the wif sensor. highdesertranger

 

[MadOgre]

Ok so then why doesn't a gas gauge work on my diesel? only the diesel gauge works properly. I even changed the cluster to one out of a gas truck. tried it with several different gas gauges but no luck. Put the diesel gauge into the gas cluster and it works. Just to make sure, I tried 5 gauges in total 3 gas and 2 diesel. None of the gas gauges worked but the 2 diesel gauges did.

 

[hatzie]

Ok so then why doesn't a gas gauge work on my diesel? only the diesel gauge works properly. I even changed the cluster to one out of a gas truck. tried it with several different gas gauges but no luck. Put the diesel gauge into the gas cluster and it works. Just to make sure, I tried 5 gauges in total 3 gas and 2 diesel. None of the gas gauges worked but the 2 diesel gauges did.

All three gas gauges were not working and both diesel gauges were. End of story.
GM used magnetic gauges from 1965 till they slowly rolled out the stepper motor gauges driven by the ECM in the 1990s. The base of the gauge needle has a small magnet that "floats" in a magnetic field created by three coils. The coils are fed voltage through the IGN terminal on the back of the gauge. Inside, the voltage follows a split path. Part of it passes through all three coils to ground and part of it goes through only the first coil then on through the sending unit to ground. Depending on the resistance in the sending unit circuit, the strength of the magnetic field created by the first coil compared to the other two shifts the magnetic field one way or the other to deflect the needle on the fuel gauge.
http://www.chevyclassicsclub.com/the-gm-fuel-gauge-mystery-1965-newer/
GM didn't make special tanks with mystery linings to protect the zinc coated steel from diesel fuel or oogie boogie diesel only fuel gauges. I've heard all kinds of urban legends about both.
Diesel senders were different in that they had Water In Fuel sensors and tank drain straws on the early diesels.
All of the fuel gauge senders themselves were 0-90Ω rheostats from 1973-1991 regardless of the number of fuel, vapor, return, and drain lines or the presence of in-tank fuel pumps or Water In Fuel sensors or even an extra draw straw for draining water out of the tank.

I have bench tested over 20 GM squarebody fuel gauges. They were a merry mix of both gas and diesel Small and Large face and two Medium Duty Tach/Fuel-gauge combo units.
I use a 0-30VDC 0-20A Mastech HY3020E Linear Variable Regulated DC Lab Power Supply set at 14.5VDC and 5A (alternator charge voltage) and a, in calibration, General Radio 1432-M precision decade resistance box. Properly working gauges all sweep the same within roughly ± 3-4Ω.
Usually gauges that were malfunctioning on the bench were a squirrely calibration resistor.

Bouncy fuel gauges can be fixed by injecting a special silicone gel into the core to damp the shaft. The Gen 2 F-Body and the C2 & C3 Corvette guys get it done with their original fuel gauges that are made of unobtainium.

BTW you can test all of these air core gauges with a lot cruder equipment but I have it on my electronics bench so...