fuel sender
11-26-2001, 03:26 PM
#1
Track Day
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fuel sender
I have a wacky fuel level reading. No matter what the fuel level is, it registers at the 1/4 mark. In addition, when I ran out of gas a few weeks ago, I wasn't graced with the reserve light warning. OK, seems simple enough, either the float is stuck or fuel is not entering/exiting the sender tube...
I pull off the sending unit connector and I jumper the gold pin and the one right next to it and the fuel gage reads full. Then I jumper the gold pin and the third remaining pin and the reserve light illuminates. Great, so I continued. I removed the sender and I cleaned everything CAREFULLY. My DVM read 4 ohms when I move the float to the top and 64 ohms when I move the float to the bottom, just like it should. The float slides up and down when I tilt the whole assembly so I know it's not hanging-up on anything. The ports that allow fuel in and out of the tube are clear too.
Then I checked the little contacts at the bottom which are intended to ground out the reserve light circuit. It worked as it should but I could not get continuity between the reserve light contact and ground. The copper wire that goes from the reserve light contact to the pin at the top had infinitely high resistance. Guh? How can a (non-insulated) copper wire not conduct? I am baffled. It's right there for all to see, two contacts connected by one wire with soldered joints and no conductivity. Whatever. So I say screw it and I put the thing back in. Wow, it still reads a 1/4 tank when it's half full.
This is one of the simplest operations (among many electrical) that I have performed on this car and I have no clue. Any ideas?
I pull off the sending unit connector and I jumper the gold pin and the one right next to it and the fuel gage reads full. Then I jumper the gold pin and the third remaining pin and the reserve light illuminates. Great, so I continued. I removed the sender and I cleaned everything CAREFULLY. My DVM read 4 ohms when I move the float to the top and 64 ohms when I move the float to the bottom, just like it should. The float slides up and down when I tilt the whole assembly so I know it's not hanging-up on anything. The ports that allow fuel in and out of the tube are clear too.
Then I checked the little contacts at the bottom which are intended to ground out the reserve light circuit. It worked as it should but I could not get continuity between the reserve light contact and ground. The copper wire that goes from the reserve light contact to the pin at the top had infinitely high resistance. Guh? How can a (non-insulated) copper wire not conduct? I am baffled. It's right there for all to see, two contacts connected by one wire with soldered joints and no conductivity. Whatever. So I say screw it and I put the thing back in. Wow, it still reads a 1/4 tank when it's half full.
This is one of the simplest operations (among many electrical) that I have performed on this car and I have no clue. Any ideas?
11-27-2001, 05:09 AM
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Burning Brakes
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Hi Vic
There must be a current limiter (probably a zener diode module) in the power to the float somewhere.
The inside of the fuel tank is classified area (explosion!!) and it is simply not that smart to release an current to the float that could cause a spark in there, so there must be a limiter.
Since the float is OK and the indicator seems to be, the next possibility is this limiter. I would expect it to be in the dash behind the fuel gauge but am not sure.
There must be a current limiter (probably a zener diode module) in the power to the float somewhere.
The inside of the fuel tank is classified area (explosion!!) and it is simply not that smart to release an current to the float that could cause a spark in there, so there must be a limiter.
Since the float is OK and the indicator seems to be, the next possibility is this limiter. I would expect it to be in the dash behind the fuel gauge but am not sure.
11-27-2001, 11:15 AM
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Track Day
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Thanks Hans...but wouldn't a malfunctioning curent limiter just reslut in an erroneous gage indication (or zero indication) rather than a continuous 1/4 tank indication? Plus, when jumped, the gage reads 4/4 and the resistance in the sender is spot-on Porsche (VDO) spec...
11-27-2001, 12:46 PM
#4
Burning Brakes
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Vic
Dont know how this one limiter actually works, just suspect it would be there somewere.
It is some electronic circuit that I expect to contain a zener diode.
If it fails completely, the read out should be zero, but what if it goes only half way I do not know.
What if the tank actually gets below the 1/4 full? Does the indicator start showing life then?
Dont know how this one limiter actually works, just suspect it would be there somewere.
It is some electronic circuit that I expect to contain a zener diode.
If it fails completely, the read out should be zero, but what if it goes only half way I do not know.
What if the tank actually gets below the 1/4 full? Does the indicator start showing life then?
