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Overdrive enLightenment
Neil McCabe

Introductory note: 122s and 1800s have a third contact built into their turn signal control stalks. On many cars, this was used as a high-beam dipper switch instead of the foot-operated dipper found on cars exported to the U.S. market. When coupled with a relay suitable to the needs at hand, a momentary pull on the stalk can be used to control a whole variety of electrical equipment. A wire from this switch is generally already in place; you can probably find the loose end taped up under the hood near the firewall. --Phil

Overdrive is a great upgrade for your 122. You get five nicely spaced gear ratios and quieter, fuel-conserving cruising. An M41 overdrive transmission will bolt right in, although there are a few related modifications that must be made (see note 1). This article focuses on a little detail which you shouldn't do without if you make this conversion.

You need to know when the overdrive switch is engaged. Otherwise, as you up-shift through the gears and hit 4th, you will unexpectedly find yourself in 5th instead, out of the power curve and a disgrace to Smooth Shifters!

Volvo's solution, when installing overdrive in the 123GT, the 122's racier sibling, was to install an indicator light between, and slightly above, the cigarette lighter and the fan switch. Very simple. I might have done it this way, if I could have found a 123GT light. Alternatively, I could have used a variety of other available lights, or a toggle switch with a plastic lever which lit up when toggled. But these solutions would have required drilling a hole in the dash.

Perhaps you wouldn't feel guilty drilling a hole in a pristine 122 dash. As you approach that precious metal, revving your drill, you don't expect a concours police officer to bark, "Freeze! Now just lay that drill down, nice and easy." But wait a minute there's another way. How about a completely "stealth" installation using an existing light on the dash, perhaps the green turn signal indicator, for a dual purpose? (See note 2.)

However, this required me to contemplate Electricity, one of those wonders of the world which I will never personally understand. Forty-five years ago it nearly knocked me off a friend's Whizzer motor bike and stood my hair on end when I attempted to re-attach the loose, insulated spark plug wire, yet it will refuse to flow through proper wiring from my pickup truck to my neighbor's car trailer the next time I hook up. Magically, I have solved many other electrical problems by simply removing a bulb or connection and polishing it on my Levis. But this project called for more. I needed Sage advice.

My friend Gerald ("Jerry") Sage is an electrical engineer. I posed my problem to him: can the turn signal light also serve as the OD indicator light? Jerry got a gleam in his eye. He thought something could be done. More importantly, he thought it would be fun!

Putting a dual-filament bulb in the original socket had already been ruled out, because the dual-filament bulb base is too big. Perhaps we could bundle three small LEDs (Light Emitting Diodes) and insert them behind the turn signal lens, one for each turn signal circuit and the third, of less intensity, for the OD light circuit. Or we could use a single filament bulb, but control the current flowing to it with diodes and resistors. This is what Jerry recommended.

Schematic of circuitHe prepared a small circuit board, using diodes and resistors. Three wires feed current to the board, one each for the left and right turn signals, and one for the OD light. The turn signal circuits utilize diodes. The OD light circuit utilizes a diode and resistors to reduce the intensity of the light when used in OD mode.

I remembered to disconnect the battery before crawling under the dash. I should have also checked my medical insurance policy: extreme contortions of the cervical spine are mandatory in working under the dash and could have necessitated physical therapy or chiropractic adjustment.

Once the circuit board was installed and the wires connected, it was time to re-connect the battery and perform the Smoke Test. Jerry advises that this test is performed the first time electrical current is sent to a new electrical device. If the device goes up in smoke, something is wrong.

In my case, the first circuit board did indeed go up in smoke, destroying the diodes. This was not caused by any flaw in the board, but instead by the author's inadvertence which caused contact between the circuit and the mounting bracket. In other words, I shorted it out. The second circuit board, or rather, my installation of it, passed the Smoke Test.

Then the Road Test. Voila! The turn signal light blinks brightly, as before. When the OD is engaged, the light glows at a lower intensity. When the OD is engaged, with the light glowing, and the turn signal is engaged, the light blinks brighter.

Everything on the dash looks completely original. The switch used is the headlight flasher switch in the turn signal, but using a 240 headlamp relay. The indicator light is the green turn signal lens. The circuit board for the light is installed out of sight, behind the dash (see note 3). I did have to cut the turn signal wires near the original turn signal light socket so that I could re-attach them to the circuit board. But I didn't drill any holes in the car. The installation is invisible, except in its function, and the concours police won't arrest me for willfully and wantonly desecrating a classic Amazon dash.

1. It is beyond the scope of this article to discuss fully the overdrive conversion. Suffice it to say that an M41 overdrive transmission will bolt right in but you will also need: 1) a drive shaft with a shorter forward section (use a 123GT shaft, if you can find one. I had been told that a shaft from a 122 with AT would work, but it was about 3/4" too long for use in our '65 122 two-door.), 2) a transmission top cover plate which has an overdrive lock-out switch (mine is set up as in a 123GT or early 140 with OD using the original long direct shift lever), 3) a switch within easy reach of the driver, 4) a relay between the driver's switch and the transmission top switch, and 5) a longer speedometer cable. All the necessary parts could come from a wrecked 123 GT, if you could find one; otherwise, you can obtain the necessary components from other sources.

2. The 123GT OD indicator light is red. This is OK when you get used to it, but red is counter-intuitive, looking more like danger than relaxed cruising. Later models used green for the OD indicator light.

3. I mounted mine to a small bracket attached with a wing nut on top of one of the existing steering column bracket bolts.

4. The circuit board I installed uses three 10 ohm, 1 watt resistors. If one 30 ohm resistor is used instead, as suggested by the diagram in the article, it should be 3 watt. The diodes used are 1 amp each.

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