1976 Commercial Chassis Fuel Sending Unit

Steve Ahlstedt

Steve Ahlstedt

For as long as I have known my 1976 Cadillac Superior Ambulance, the gas gauge has read full. To prevent any embarrassment, I watch the trip odometer. My summer project is to repair the gas gauge. I assured the sending unit has a solid chassis ground. I disassembled the rear license plate mount to access the brown gauge wire and when I touched it to the frame, the gauge dropped to empty - leading me to believe the gauge and wiring are intact.

My suspect is therefore the sending unit which I have been unable to source because the parts houses state “except commercial chassis”.

I purchased the 1976 Cadillac Shop Manual. Throughout the manual it will state “except commercial chassis” - however - I do not see where manual section 8 states there is any difference between a non-EFI standard auto sending unit and a commercial chassis sending unit.

Has anyone had success installing a “standard” unit in a 1974 - 1976 commercial chassis?
 
the problem is return hoses and vapor lines. then there is some debate as to what tank they used. it may not be a a Caddy tank. Dave up in Canada has some interesting takes on it. capacity would be your indication. i have been lead to believe the tank was smaller then the standard car. most likely your best bet is one of the universal ones. get a stainless steel one or it will get ate up.

as you have all ready preformed the test and determine the sending unit is bad. the next step is to do the visual and see if lines come out of the unit or some were else on the tank. and compare what you see with pictures. rockauto.com is one good place to get a look at pictures of parts. it might be just a case of plugging off lines you don't need. let us know what you find out
 
These guys also:https://www.bobsspeedometer.com/1/120/index.asp

Family owned company, I personally have dealt with them since 1969.

One thing also. Is the gauge reading "full" or over past full? If past full it might be an open circuit condition in the wiring to the sending unit. Worth checking or running a new wire between a known good point on the harness to the sending unit.
 
August Update

I removed the tank and the sender which read over 200 ohms regardless of the arm position. While I appreciated all the suggestions, I took the sender to the closest geographic facility: Tri Starr in Seneca Falls, NY. Paul Steinberg had stated others had used them successfully.

When the sender returned, it was re-installed and worked perfectly!

As to why a “standard” unit can not be used in a commercial chassis - I still do not know. However, I took the opportunity to take some measurements and photos in the hope they may help others.

All measurements are approximate:
Tank depth - 10 ½"
Cap diameter:- 3 ½"
Top legs extended 13 ½" from the cap center
Suction tube diameter - 3/8"
Return - 1/4" with the center tube slightly smaller.

The photos are:
The top of the tank we can not see when it is in place.
The sender as it was removed from the tank
The last two are the sender as it was returned from Tri Starr.
 

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Happy that my recommendation worked out for you. Many times, when I make a recommendation, it is based on second hand knowledge from others that I know and trust.
 
When you were reading 200 ohms regardless of float position, where was the probe not on the wire connection?

My experience with GM of that vintage has been pathetic ground connections to the tank itself.
 
Walter:
To minimize possible error, the sender was out of the tank and I had alligator clips on the multimeter probes. The clips were connected to the insulated rheostat post and the sender body.

Steve
 
Thanks for sharing. My series 62 conv. is going to the mechanic Monday to have a new sending unit from McVeys installed. I hope that we have the same success.
 
I'm sure a standard Gauge is what it is. but it is most likely not a Caddy gauge. which is why the books all say except CC. did you look for a number stamped into it? might lead you to what was used.
 
Steve Ahlstedt said:
Walter:
To minimize possible error, the sender was out of the tank and I had alligator clips on the multimeter probes. The clips were connected to the insulated rheostat post and the sender body.

Steve
Click to expand...

Thanks Steve.
As Charlie Chan said Vely intelesting.

S-10 Blazers around here were famous for corroding off their ground connection to the sender/fuel pump via the rivet that carried all the grounds into the sending unit/pump assembly. That was so fun on cold nights when the fuel pump stopped working. I know of 3 Blazers that got access doors cut through the floor so grounds could be reconnected to the combination unit with a 300 watt soldering iron. Beat hell out of buying replacements and fighting to seal them into the corroded flange.
 
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