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Fuel Gauge/Sender fix notes  3 Saabers Like This Post!
Posted by TimN [Email]  (#2379) [Profile/Gallery]
(more from TimN) on Sun, 5 Jul 2020 12:17:01
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Greetings,
I was having the fuel gauge problems similar to a number of people. Here is what I found.
The top left image is from Bentley’s (371-48) showing the electrical schematic. #46 is the sender unit. The fuel gauge is 47A and empty-fuel light is 47B.
My initial reaction was to follow the reports of the 7mm bolt coming loose on the instrument panel where the wires are clamped to the fuel gauge and going through the speaker for access. I had a hard time with it until I cut off the box end off a crappy 21-pt wrench and JB Welded it to a piece of aluminum. The right picture is the bolt and clamp piece along with lame tool #1. The pieces are from the junkyard. For sure you don’t want to remove the bolt. If you don’t have a helper to guide you, I used my iPhone (or better a USB camera) linked to my Airbook and QuickTime (New Movie Recording mode). It’s a tight fit for an end wrench when you are blind. The bolt was indeed rather loose. The gauge still didn’t work.
I uncovered a post by Matt Grimes where he had a broke sender unit. My thinking is that statistically if people are seeing a failure then it is probably an end of life (or mean time to failure) part issue with all Saabs. Like Matt, shorting the Black lead to the Brown lead resulted in the Empty light coming on. Shorting the Black lead to the Grey showed a full tank. Measuring the resistance of the sender leads was inconclusive since I wasn’t sure what the values should be.
Then the ah-ha moment for confirmation. When I turned on the car the gauge seemed functional. If I shift into forward or reverse with a quick acceleration then a broken gauge would roll around the tank causing the spasms. And that was exactly it. Had I been more clever I would have done this from the get go and not wasted all the time with the 7mm bolt.
Next I had to get the sender unit off. More machining as I couldn’t turn it. I tapped a pair of 1/4-20 holes in another piece of aluminum, cut a square hole for the ratchet (with a drill then Dremel) and threaded two 1/4-20 screws to grab at the sides of the sender cap. Naturally my dimensions were off so I added nuts to decrease the diameter. That worked sweetly.
The next picture is virtually identical to Matt’s. But several parts are lost in the gas tank.
Fortunately, Saabs were quite common in New Mexico. The crashed ones are ending their days at local U-Pull junkyard. I thus came home with not one but two sender units. And they allowed me to verify what I expected.
The sender is a sliding potentiometer along with an on/off switch. The sender pins are marked S L Gnd connecting to the gray, brown and black cables respectively. When the slider float is at the empty position L to Gnd is low ohms (on), any other position it is high ohms (off). In the empty state S to Gnd reads 66 Ohms for both units. As it slides up, the ohms decrease. I measured 6 Ohms at the highest position.
The third picture is the broken unit, replacement and lame tool #2. With the new unit, the fuel gauge no longer behaves like in the standard horror movie where the ghost screws around with the person before revealing itself.
This probably answers questions posed over the years about calibration, off readings, etc. If for any reason - bad connections, sticking slider, etc, the Ohms will read higher than minimum, then the fuel gauge (i.e. ammeter) will not show a full tank.
Now I still have parts in the gas tank but when I run it empty I will try to fish them out and see how it is put together. Appreciations to everyone on Saabnet with discussions and suggestions for the fuel gauge fix.
TimN
1994 900 CE Convertible with a functioning fuel gauge
2007 9-5 Aero
2008 9-3 Convertible
_______________________________________ TimN
, Sun, 5 Jul 2020 12:17:01 <-- Viewing This Message
3, Tue, 7 Jul 2020 09:58:05
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