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A4 (B6 Platform) Discussion Discussion forum for the B6 Audi A4 produced from 2002-2005

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Old 06-02-2008, 01:48 PM   #1
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Default faulty gas cap sensor... how to check wiring?

So I paid $120 to my local shop to find out that my flashing gas cap is just a bad gas cap sensor. I replaced the cap, but still bad. How does that circuit work at the gascap? It drives me nuts!
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2002A4 1.8TQMSPX w/o sunroof and a bunch of (functional) mods...
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Old 06-02-2008, 03:24 PM   #2
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Default No Bentley?

Its $65 entrance fee pays for its self!


Audi > A4, S4 > 2002-2007
Fuel Supply System
20 - Fuel tank, vehicles with all wheel drive up to 06.03

Fuel Tank Lock Switch F314 , checking The Fuel Tank Lock Switch F314 consists of a reed contact on fuel filler tube and a fuel tank cap with an embedded magnet. If fuel tank cap is not fully screwed on, the reed contact switches on an indicator in the instrument cluster.

Special tools, testers and auxiliary items required

Portable multimeter V.A.G 1526 C

Connector test set V.A.G 1594 C

If indicator is lit, check the following points in order:

Fuel tank cap is completely screwed on.
Correct fuel tank cap (Identification: Threads are gray in color).
Switch - arrow - at fuel filler tube is fully inserted.
Electrical malfunction in switch.
Wiring connection to instrument cluster is faulty (check according to schematics).
Fuel Tank Lock Switch F314 , checking electrical


Remove rear seat bench Body Interior - Repair Group 72 .

Unbolt cover for right locking flange - arrows - .
Disconnect electrical connector - 2 - .
Note:
Ignore - 1 - .
Connect hand multimeter V.A.G 1526 B between terminal - 1 - and - 2 - for resistance measurement.
Fuel tank cap Specified value
Screwed on tight ? ?
Not or incorrectly screwed on less than 1 ?

If specifications are not obtained:
Remove right rear wheel housing liner Body Exterior - Repair Group 66 .
Replace Fuel Tank Lock Switch F314 - arrow - , thereby unclipping and clipping in the switch and electrical wiring at fuel filler tube.
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2004 A4 1.8t USP Quattro 6MT
K04-15/APR/TT 225 Injectors/Custom Stainless Exhaust/710N/RaceTec FMIC
034 HFC/Jet-Hot Cermi-coated & ported exhaust manifold
Spec Stage 3+ with OE DMFW!
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Bilstein Sport with H&R/ H-Sport Anti-Sway bars/E-codes
19x8.5 OEM Audi Skinny Fives with 235-35 Bridgestone 760 Sport Skins
B7 A4 brakes with StopTech Ceramic pads and Stop Tech rotors

Sirius Satellite Radio

Ross-Tech HEX-USB+CAN Equipped

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Old 06-02-2008, 10:58 PM   #3
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Default I think the "gas cap" warning light is actually a condition when the fuel system leak detection....

fails the self check. There is a leak detection pump that runs and either creates a vacuum or a low pressure on the fuel tank (I forget now, I have to review the lit.) and tank ventilation fuel vapor recovery plumbing. If the system doesn't hold the specified differential pressure/vacuum, then the "gas cap" warning light is turned on. I don't think there is an electrical cap sensor.
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Old 06-03-2008, 01:18 AM   #4
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Default I guess you didnt read my post... :O(

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2004 A4 1.8t USP Quattro 6MT
K04-15/APR/TT 225 Injectors/Custom Stainless Exhaust/710N/RaceTec FMIC
034 HFC/Jet-Hot Cermi-coated & ported exhaust manifold
Spec Stage 3+ with OE DMFW!
Neuspeed TIP/Neuspeed Snub/Optima 75-35/AFE Pro-Dry/FSI Coils
AMS Short-shift @ %40 redux(Sweet!)
Bilstein Sport with H&R/ H-Sport Anti-Sway bars/E-codes
19x8.5 OEM Audi Skinny Fives with 235-35 Bridgestone 760 Sport Skins
B7 A4 brakes with StopTech Ceramic pads and Stop Tech rotors

Sirius Satellite Radio

Ross-Tech HEX-USB+CAN Equipped

Escort 9500ix
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Old 06-03-2008, 03:13 AM   #5
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Default Hmmm. they steered me wrong then... It leak tested fine.

The first thing I had them do was smoke test it for leaks.


Rats...
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Old 06-03-2008, 03:16 AM   #6
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Default yeah, but I lost the disk! Dang it. Don't want to pay for it twice. :(

Hey, can you possible post the picture of it?

Thanks a ton for the description.... I'll just hot wire it shut once I see how to get to it.
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Old 06-03-2008, 03:17 AM   #7
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Default It leak tested fine. Thanks for the info MikTip..

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Old 06-03-2008, 03:21 AM   #8
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Default i had the same problem....(actually it JUST started again)

but i noticed that if i looked inside the gas cap area....there's a small wire.....i figited around for a few seconds with a screw driver and the light went out (for 4 months)
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Old 06-03-2008, 11:36 AM   #9
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Default OK, missed the reed switch with cap magnet in the Bentley excerpt you posted.

So, I was incorrect regarding the tank cap status switch. Thanks for the correction/info!
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Old 06-03-2008, 01:05 PM   #10
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Default I can't find the reed switch for the tank cap status anywhere in the wiring diagrams....

The description for the function is also incomplete. For example, the magnet in the gas cap, when placed in close proximity with the reed switch, can either close the reed switch contacts together and establish circuit continuity,(a N.O. or normally open switch,) or pull the contacts open and break circuit continuity at the reed switch. (A N.C. or normally closed switch). Depending on the design of the monitoring device, either switch or circuit condition as "open" or "closed" circuit can be used to infer the status as "cap installed" or "cap not installed". Therefore, testing the circuit continuity at the IC end, may be inconclusive if continuity does not exist with the gas cap installed.
This doesn't help much, I know, but is a basic description of the functional aspects involved, or not. :>/ Lacking clear documentation of the circuit/reed switch setup, as a practical matter, it can be assumed that the reed switch is N.O.. As such, if the circuit between the reed switch is broken or "open" anywhere from the IC to the switch, or the reed switch itself is faultily, will exhibit the flashing gas cap warning light with the cap installed.
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Old 06-03-2008, 01:05 PM
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2005, a4, audi, cap, faulty, flashing, fuel, gas, light, low, magnet, open, pressure, s4, sensors, warning


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