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Fuel pump relay


Bob Kuykendall

Many Bricksters have experienced no-start problems that were later traced to the fuel pump relay unit. On closer inspection, it was often discovered that the source of the problem was bad solder joints inside the unit where the the actual electromagnetic relay is soldered to the circuit board. The bad joints would work for many years, but would eventually get very hot because of their increased resistance. This heat would often show up as burn marks on the circuit board or on the relay, and in extreme cases would discolor or melt the plastic casing of the unit.

In many cases the bad solder joints can simply be resoldered. Even if you do decide to replace the unit rather than repairing it, resoldering the joints of the old one will often yield a perfectly usable spare.

fuel pump relayDescription

The fuel pump relay unit on 1978-on (or so) K-jetronic-equipped 240s is actually more than just a simple relay. It also contains a circuit that listens to the ignition system to determine whether the engine is running. When the unit detects an ignition signal, it switches the fuel pump on. When the ignition signal stops, it switches the pump off.

Locating the Fuel Pump Relay Unit

The fuel pump relay unit is located under the dashboard in the vicinity of the steering column. I have found them clipped to the crossmember where the steering column mounts; and I have also found them mounted higher up between the instrument cluster and the firewall. The unit is usually either black or mustard yellow and has a six-blade connector on one end.

Opening the Unit

After locating and removing the relay unit, pop open the plastic casing by prying the case apart so that the end with the connector slides past the retaining wedges. This is a little bit difficult, but can be done without damaging the unit.Burn marks on tape

Inspecting the unit

Once you've got the unit open, the first thing to do is check for a faint burnt odor. This would indicate whether or not it's gotten too hot in there.

Next, check for burn marks that would indicate excessive resistance. You'll usually find them on the circuit board near the offending connections or components.

Next, put your thumb on the big relay on the circuit board and try to wiggle it. If the big part of the relay (not the small hinged part) wiggles at all, its solder joints are bad and must be re-soldered.

Bad solder jointsCheck the contact points between the hinged and stationary part of the relay. It's hard to say how much pitting or discoloration is normal, but all of them that I've looked at (including the perfectly good ones) have had a little pitting. If they're really bad, you may be able to extend their life by sanding them, but's impossible to say how much

On the solder-side of the cicuit board, check that all of the solder joints are smooth and shiny. A Dull or crinkly appearance indicates poor soldering.

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