THOSE "WOBBLY" FUEL GAUGES
Many of us, as 4CV owners, have experienced fuel gauges that had an "over-reactive" needle. "Overactive" to the extent of fluctuating wildly between full and empty, with possibly a hesitation in the movement.
Determining the actual level of the contents in the tank, with a gauge that behaves in such a manner, takes a fair amount of imagination, and it can lead to an unplanned roadside stop with no fuel.
The cause of this fuel gauge activity is most likely to be found in the sender unit mounted on the fuel tank itself.
Just briefly, the contents level of the fuel tank is monitored by a cork float attached to a pivoted arm that is part of the sender unit mounted on the top rear of the tank. Changes in the fuel level are transmitted via the float and arm to a pair of copper contact strips that move along the sides of an electrical restistance mat, One end of the resistance mat is secured to the body of the sender and is insulated from it, the other end is connected to the wire cable from the fuel gauge instrument on the dashboard. This connection is insulated from the metal exterior of the sender unit. Thus, the wiping contact strips moving along this resistance mat, from one end when the tank is full, to the other when it is empty, change the electrical value of the resistance. This electrical resistance value is "read" by the fuel gauge meter on the dashboard, when supplied with power via the ignition switch. So, a half-full tank would have the float at the half level, and the wiping contacts approximately half way along the resistance mat in the sender unit, earthing out the resistance to less than its full value. This should then show on the gauge needle as half full.
OK then, what can go wrong in the sender to cause problems at the gauge? Any of three main things:
- A saturated and soggy float that loses its buoyancy and will sink to the bottom of the tank and be read as empty or low on the gauge needle.
- Worn and slack wiping contacts on the resistance mat, that will only give an occasional true contact reading, thus throwing the gauge needle into confusion.
- Poor, or intermittent, earthing contact between the sender unit and tank body, or between tank and car frame. This can also send the gauge needle into a panic.
WHAT TO DO
Remove the sender unit from the tank. This can be done from underneath the car with the back wheels up on ramps, or through the removeable access panel gehind the back of the rear seat on the driver’s side. The unit is secured to the tank by three screws. Disconnect the wire from its stud or lug. (Note: Earlier type sender units have two wires to disconnect; take note which wire goes where, or the action of the gauge needle will be reversed).
Pull the unit upwards, then twist it to clear the float through the access hole in the tank. The float is made of cork, covered with a plastic epoxy type coating. This coating is frequently cracked and split, and this can lead to it becoming saturated and heavy. Test the float in a container of petrol. Any lack of buoyancy, and replace the float. Unfortunately, the float is not available as a spare part.
However, South Australian State Co-ordinator, colin Redmond, has had success in fitting a float from a V.D.O sender unit from a Renault 12. This float is made from a high-density foam material and would appear to be a worthy replacement for the cork. Colin suggests cutting the wire arm of the 4CV Jaeger sender at the inside edge of the old cork, leaving the bent corner on the arm, and the two bound together and soldered, using fine wire. Colin reports that this repair has served him well, and is still in use after many years. It is believed that there are some "general purpose" aftermarket floats available through some auto-electrical outlets, but a satisfactory means of securing them to the 4CV float arm would have to be determined.
Next, remove the steel cover, held onto the sender body by a small screw and a shakeproof washer. Check the sealing gasket. Under this cover is the resistance mat and the copper contact strips, the action of which will become obvious as the float arm is moved. Examine the windings on the resistance for breakage or damage. To ensure a firmer contact between the wiping strips and the resistance, position the strips mid-way along the mat and squeeze the top ends inwards towards each other, thus putting a slight bend on them. This should enhance their ability to remain in firm contact with the windings of the resistance, and encourage a steady reading on the gauge needle.
The resistance mat has been a replaceable unit in the past, having part No 8513394 for the early sender with two wire connections, and 8542606 for the later one with a single wire connection. However, availability of either would be very doubtful at this stage.
Finally, the "earthing" of this unit electrically is important. The earth return from the wiping contacts is via the body of the unit, but the unit is mounted on a stout sealing gasket on the tank, and this in itself offers no path for return current. The pressed steel cover on top of the sender unit sits and seals on either a cork of composition gasket — again, no return path. However the small securing screw, that holds the cover to the body, screws into the alloy and hopefully provides electrical contact from body to cover through a small shakeproof washer. Connection to thte pterol tank body is then made through the three mounting screws that secure the unit to the tank. The tank is further earthed out to the car frame via its mounting bolts. So check for, and ensure, good clean contact between the small securing screw and the cover, and the three holding screws at the cover: good shakeproof or "star" washers are necessary here. Remember, the 4CVs have only a 6 volt system, and good earthing is essential for any and all of the electric units on the cars.
George Hames from RENAULT 4CV REGISTER OF AUSTRALIA.
(based on the internet article by Jean-Pierre Delaunoy and with input from Colin Redmond