Fuel gauge / sender help

"CAR_IS_MI" (car-is-mi)
03/19/2015 at 20:44 • Filed to: None

So I've been working on the electrical system of the 124 all day and I've gotten most things ironed out. The fuel gauge does not work and I've been trying to diagnose the system for a few now, and I think I am confusing myself more than anything so I figure I'll ask for your help as well oppo.

This is the diagram I am looking at (and yes, I have the same resolution here...)

So the sender gets its power from the gauge. I know this because without the gauge plugged in there is 0 volts. Power comes in on the yellow / black wire into the gauge, I have verified 12+ volts. I have also verified all grounds. When the sender has 12v in, the output wire is at 6.9v.

So heres the funny part. I bench tested the gauge. Using the specified ground from the diagram the only thing that works is the backlight. If I accidently use the ignition 12v input as a ground and send power to the backlight, it still works. using the in as an out and sending power to the low fuel light also allows the low fuel light to turn on. And finally, again, using the power as a ground, if I send power to the sender input, the gauge jumps to full.

Now its not likely that having my gauge wired exactly like the diagram says it should would be wrong...

So wheres my likely culprit, the sender or the gauge. And ideas on how to test for sure?

DISCUSSION (13)

. . > CAR_IS_MI
03/19/2015 at 20:48

If it's anything like my 126 it's the sensor arm shock absorber that's broken.

Bairclaw > CAR_IS_MI
03/19/2015 at 20:58

is that small circle on the bottom towards the left the sender?

CAR_IS_MI > Bairclaw
03/19/2015 at 21:29

yes

Ramblin Rover - The Vivisector of Solihull > CAR_IS_MI
03/19/2015 at 21:47

Senders go wrong more often than gauges. It's very possible that 6.9V is the same as zero as far as the gauge is concerned (no reading). It's often easiest to pull the sending unit and swing the arm through its range, measuring resistance - if the resistance doesn't change or at the least remains high, the sending unit is toast.

Joe6pack > CAR_IS_MI
03/19/2015 at 21:48

What year is your Spider? I have a '78 in the garage and pretty much everything works, so maybe I can help. One thing I do know is that Fiat pretty much changed everything every two years and the newer the car, the more wires. My '78 was originally a California car and had several unnecessary emissions valves/sensors/vacuum lines, etc. It is now running a 2.0 liter with a 1.8 liter head, a Weber 32/36 and headers.

CAR_IS_MI > Joe6pack
03/19/2015 at 21:53

mines a 79. My plan is to get it put back together, try to pass emissions and if not ill just register it as a classic rod so I dont have to smog it. granted, that limits me to 5,000 miles per year.

AMGtech - now with more recalls! > CAR_IS_MI
03/19/2015 at 22:51

At first I thought you meant Mercedes 124, not Fiat.

Does the voltage output at the sender change as you swing the arm through its range of motion? Do you see that same voltage input at the gauge? Ignore the backlight, it has nothing to do with the circuit you are testing.

Joe6pack > CAR_IS_MI
03/19/2015 at 22:59

Ok, I think they should be pretty similar in that the '79s got all of the '78 California nonsense, but compensated with increased displacement. If I read what you have said above, it seems that all of the lights and the gage work when when you use the ignition 12 volts as ground and input power to the other leg of the lights/gage. This could just be due to the differential in voltage between the ignition and your other power source. If there is enough of a potential difference, you will get current flow.

From my Haynes diagrams, it appears that the gage has a pigtail connector and all of the wire colors change in going from the harness to the pigtail. Assuming what you have matches my diagram, here is what I think is going on. The white/black wire in the harness is the ground for the gage illumination. It is labeled BN (Brown?) in the pigtail. White is power for the illumination and is labeled B (Blue?) in the pigtail. Yellow/black appears to be ground for the fuel gage and low fuel light and is labeled GN (Green?) in the pigtail, there are two wires coming from the sending unit as in your diagram. They are grey/red and pink. In the pigtail, the grey/red is labeled R (Red?) and the Pink is labeled S (no clue).

So, in essence, there are two separate circuits - one for illumination and one for signal. You might start by using a larger ground and applying power and see what happens. The white/black and yellow/black are daisy chained to the other gages in the dash, so it would make sense that they are ground. I have to go to bed, but I may be able to scan this diagram over the weekend. Hopefully I haven't confused you even more or sent you down the wrong path.

CAR_IS_MI > AMGtech - now with more recalls!
03/20/2015 at 00:28

Sorry, yes, Fiat. I have to dig the sender out still. I do see the same voltage at the gauge, I only mention the backlight because of the weird ground reversal thing.

CAR_IS_MI > Joe6pack
03/20/2015 at 00:31

close, but yellow/black is the 12v source. it jumps to multiple inputs on the gauge but its all piggybacked. Pink seems to send power through the gauge to the sender, and the grey/red seems to be the return signal.

AMGtech - now with more recalls! > CAR_IS_MI
03/20/2015 at 01:05

The weird thing you mention with the backlight is probably just voltage back-feeding through a different circuit to find its ground. Had to mention, I see seasoned pros get caught up on irrelevant crap all of the time. It's very important to stick to the main concern.

CAR_IS_MI > AMGtech - now with more recalls!
03/20/2015 at 02:00

yeah I just really need to stop being lazy and pull the sender and test it.

Joe6pack > CAR_IS_MI
03/20/2015 at 19:19

Looking closer, that makes sense. It looks like the yellow/black does supply power to the gages and ground comes from the sensor end of the circuit whether it is the fuel gage, the temperature gage or the oil pressure gage (if so equipped).