You may remember I posted last week about my fuel gauge which was stuck at the half way point no matter how much fuel was in the tank. Judging from the amount of mail I received this is a really common problem.
Today I removed the unit, disassembled it and gave it a clean, as well as cleaned the electrical connections and it now works perfectly. These were the steps I took:
Removing the sender unit.
1. Remove carpet from trunk. This is needed as the fuel can leak from the sender during removal. Place fire extinguisher within easy reach. Also, be sure to carry this procedure out in a well ventilated area.
2. Place towels inside trunk around sending unit to soak up any leaked fuel.
3. Unplug electrical system plug.
4. Remove filler cap to let pressure out of the system. (I don't know if this is required but I felt better about relieving the pressure at the filler cap rather than when undoing the bolts holding the sender in)
5. Remove the five 8mm nuts securing the sending unit.
6. Gently lift sending unit up from its position. I made sure to lift very slowly to allow the fuel to drain from the unit back into the tank rather than into the trunk.
7. Stuff a rag in the now empty spot where the unit was to stop the fuel vapors escaping and to insure against any dirt or pesky type rodents entering the fuel cell.
8. Check sending unit gasket to see if it's in good enough condition to use again. Mine was.
The sender is a long cylinder, about 300mm in length and 40mm in diameter with electrical connectors at the top and pin holes to allow the fuel to enter the unit at the bottom. It's secured by a 4mm nut and locking washer at the bottom.
9. With needle nose pliers, bend the small locking washer flat so you can access the nut to remove it.
10. Slide the float assembly from the housing.
Cleaning the Unit:
It has one thin wire which runs the length of the unit and then back up the other side, as well as a copper earth wire. In fact, the long thin wire looks like two different wires but it crosses from one side of the assembly to the other. This wire supports a cork like float with two electrical contacts that touch each side of the wire. The unit measures the resistance of the electrical circuit depending on the location of the float. There is also a contact at the bottom of the unit that the sender closes to activate the low fuel warning light.
11. Check the integrity of the active wire. The float should move freely up and down the wire. Also check the solders at each end where the wire connects for both the active wire and the earth. My earth was very dirty with what looked like some type of corrosion. If the float or the wires are damaged, you could try to fix them or maybe the easier path would be to order a new sender unit (p/n 901.741.801.00) as it's quite delicate in there.
12. Lightly sand the active wire and all the contacts with some emery cloth or fine glass paper, being careful not to rub too hard.
13. I then lightly sprayed the wires and contacts, as well as the outer electrical contacts where the lug connects with some WD-40 then left it to dry.
14. As per disassembly. The float assembly is keyed to the housing by a notch at the top so you can't assemble it incorrectly, though I can't see what difference it would make. I guess it's there for a stronger join. Remember the locking washer before fastening the nut.
15. Reinsert into fuel tank and tighten the five 8mm nuts.
16. Clean the connection points on the plug. I used a cotton bud (I think they're called q-tips in the US) dipped in WD40 and pushed it into the three connector holes. It came out filthy dirty so I think this was quite worthwhile.
17. Attach plug to connector on sender unit.
18. Reinstall trunk carpet
19. Turn on ignition and see the fuel gauge jump to the correct reading!
This process is not going to be the panacea for every fuel gauge problem, but it's probably something you should do to at least get a better idea of what might be causing your problem
1980 911SC Petrol Blue