The silver three prong switch for my 242 finally broke a couple of weeks ago. I bought a replacement on eBay, but it turned out to be the black 4 prong switch. Is it possible to use this switch in my car?

Like Randy, I've got a three-pronged one that, surprise, surprise, didn't fit my later 4-prong light switch connector. Yours if you'd like it - PM me.

Thanks,
Zach

I have one from a car I parted out. As is my practice I saved the connector as well. If you are interested please contact me: jrstarkieathotmaildotcom

Thanks for the switch and harness. I'm also going to use a relay as suggested by volvodad. I've upgraded the headlights from the sealed beams to e-codes, and the higher current draw will be an issue for the existing circuit. Will update as things progress.

The 4-prong headlamp switch is for later 240's which used a relay to handle the headlight power demands. It's a good thing it won't fit the earlier harness connector as it wouldn't last long.

Our 1983 melted its plastic harness connector, too. I found a junkyard unit which had an only-moderately-overheated harness connector. I have yet to see one in perfect shape. I installed a 30-amp relay near the headlamp beam switching relay, which gets power from a 12-ga wire straight from the battery's positive post. This means the headlamp switch only handles the current required to run the parking/tail/licence/instrument lights, much like the newer 4-pin switch setup in 240's (and 700's)

That sounds like a good solution. Too bad Volvo didn't figure this out until 1986. Where did you get the power for the relay switch that runs through the headlight switch? Or am I not asking this question correctly?

The relay gets power direct from the battery. I cut the Yellow lead from the headlamp switch - which normally supplies all the power to the beam-switching relay - near the BS relay. Connected the relay output terminal to the BS end of the Yellow wire, the headlamp-switch-end of the Yellow to the relay's "trigger" terminal, and the relay's other trigger terminal to ground. Now the headlamp switch Yellow wire just turns the relay on, the relay then connects battery power to the Yellow wire into the BS relay. Now the headlamp switch carries ~3 Amps instead of the ~18 in stock form (with high beams on in 4-headlamp system).

This also has the advantage of increasing available voltage at the (brighter) headlamps, although on low beam they still use the bulb failure sensor circuit wiring.

...$11.97 for "aftermarket" at FCPGroton HERE



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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Does your ebay find have terminal numbers on it?

Battery (red) is terminal #30.

Headlight (yellow) is terminal #56.

Parking lights (white) is terminal #58.
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Art Benstein near Baltimore

Have an awesome day and know that someone has thought about you today...I did.

I had the terminal numbers on my old silver switch. It had three prongs to match the harness. The new switch is black. It has four prongs and obviously does not match the harness. That is not really an issue for me as the harness was half melted and I didn't plan on using it. According to the Bentley manual, the "extra" prong is for a link to an accessory power switch, presumably so that headlights and parking lights can't be left on when the ignition is turned off.

Is my best option to source a silver three prong switch? I have a two terminal Radio Shack switch jury rigged in there to give me parking or headlights (not both at the same time), but it's not sufficient to handle the current. I get a burning plastic smell about ten minutes into usage, and the switch gets really hot.

Is my best option to source a silver three prong switch?

Yes. I don't know for certain, but I think the switch you might have bought is one for the later cars that use a relay, and way too light duty for the 79. Best option is just as Lucid posted - get the correct switch and repair the spade connector contacts in the harness end for good connection to it.
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Art Benstein near Baltimore

The professor discovered that her theory of earthquakes was on shaky ground.