Adding Factory Comfort to a 40-Year-Old W123: Cruise Control, Armrest, and the Art of the Retrofit

There’s a moment on a chilly morning when you’re most in tune with your classic car. Those few seconds of lumpy idle before fuel catches in all cylinders and the oil ways fill. You’re hyper-alert, listening for new noises, willing the temperature gauge to register. The car is talking. It doesn’t like a cold start any more than you do.

But there’s no drama. Those days when the question of reliability destroyed the daydream of dailying a classic are over. So if you aren’t chasing that dream, you’d best ask yourself why.

And when you’ve got a battle-ready machine, one of the sweetest rewards is adding those few touches that make it your own. In this case: a driver’s armrest, cruise control, and some classic spot lamps. A retrofit comfort pack.

The Headlamp Detour

The car had been sitting with its front end only half attached. Shakedown mileage had left it sitting on the car and about half a field up in the wheel arches. So the first job was cleaning all that out and tacking the front end back on.

While the weather turned rainy and windy, the headlamps came apart in the shed. One was decidedly the worse for wear. The other looked good because it had already been given some attention — you could tell from the reflector between the two lamps. That one, apparently, came from a car originally sold in South Africa.

Spraying lamps with high-temperature paint isn’t the greatest solution. But budget restrictions meant it was the quickest fix. And that’s exactly why the car is getting auxiliary lamps later — supplementing the lighting rather than trying to fix it outright.

The lamp that had already been sprayed had been done badly. The whole unit had been sprayed with no masking, and they were never going to match in appearance. A quick dip, some thinners, and a lot of rubbing later, they were at least closer to even.

A W123 Tip Worth Knowing

If you take the bonnet off your car and the wings, put your bonnet hinges back in before you put the front wings back on. The aft bolts on the hinges are doable with the wings on, but it’s a job for a contortionist.

Suspension Geometry and a Trip to the Alignment Shop

The car had been dropped 50 millimetres on off-the-shelf springs. That meant all the suspension geometry was out, and all the steering linkage had been off. It had been set to factory baseline, but a baseline is only a baseline. You can align a car fairly well with string and patience, but this was one thing worth doing with the most modern equipment possible.

A local shop brought the car back to factory alignment — camber and caster. Worth every penny.

The Auxiliary Lamps: Lucas, Not Hella

The factory lamps in this car are made by Hella, and a set of Hella driving lamps would have been the obvious choice. But the owner fell in love with a set from Lucas — the prince of darkness.

These are great lamps because they’ve got beautiful stainless steel housings that take standard five-and-three-quarter-inch sealed beam units. The stone guards are a bit more reserved than you get on a Hella lamp.

To keep things practical and neat, two old W123 factory wiring connectors were sourced so the lamps would be easy to plug in and out for servicing. And when it comes to wiring on a car, especially exterior stuff, it’s all about sealing your wires so damp can’t get down the core.

Marine-grade heat shrink with a resin lining was used. As it shrinks and heats up, an adhesive lining inside melts and flows out to create a sealed joint. Shiny sections at either end of the heat shrink show where the resin has done its job.

Mounting brackets were made from stainless steel. And then the wiring logic: the main light switch pulls out one click to activate the fog lights. Given that all the reflective material in their bowls is gone, the factory fog lights are kind of useless anyway, so they don’t dazzle oncoming drivers.

With the column stalk, selecting high beam activates both high beam and the new auxiliary lamps. But if you don’t feel like using them, you can turn the fog lamps off on the main switch and you’re back to just regular high-low beam and driving lights.

The main switch on these Mercedes is a beautiful thing. It’s got a multi-core 12-volt feed coming into it, and the fog light terminal is directly above and behind that red 12-volt feed. That’s where the auxiliary lamp relay feed was wired, so when the fog lights come on, the new system is energised — well, put on standby at the relay.

A second 12-volt feed from the hi-lo beam switch on the column triggers the relay to complete the circuit to the lamps. The result: high-low beam with or without the auxiliary lamps. And no crappy aftermarket switch drilled into the dashboard.

chine-story">The Armrest: A Sewing Machine Story

A friend of the series sent a W123 armrest from the USA. The only trouble is, this armrest had a fight with the sunshine and lost.

Extra material was sent along, and a valiant attempt was made at recovering it. But the sewing machine — a Swiss-made beauty — just wasn’t powerful enough for this material. At some point you have to sew three layers together, and it was too much.

A sewing machine is a beautiful piece of precision engineering. It’s precisely timed so that as the needle sews down through your work piece, a mechanism pulls that work piece through the machine, offering the needle a fresh piece of material at every stroke. The thread comes from both above and below the needle. And the machine is tunable — each black disc is a different stitch, like flash remapping your ECU.

But even with a skilled sewing veteran at the controls, the material was too much. Loose stitches that pulled back through. A battle to revisit another time.

In the meantime, the armrest was mounted. Not perfect, but functional. And in a car like this, functional matters more than flawless.

Cruise Control: The Hard Part

The car was a crossover model. In 1982, a new engine appeared — the 2.3-litre straight four, which was carried on into the W124. That meant a factory-fitted VDO Tempomat system from a W124 could be sourced and retrofitted.

The actuator is just a motor with a solenoid that physically acts on the throttle. A simple 12-volt feed spins the motor. That motor is disengaged unless another 12-volt signal activates the solenoid to bring the gears into contact with the actuator arm.

Testing it was straightforward. The motor spins, the solenoid engages, the arm rotates. And the actuator worked fine.

The wiring required one modification: the speed sensor. On the W124 system, that signal is picked up by a single plug. The W123 has a double-pin connection on the speedometer. So the wire was soldered onto that double connection, with an old Mercedes wiring connector put in line.

Then came the stumbling block. The cruise control came from a W124 version of the engine, and there were supposed to be three flanges on the side of the block where the actuator would mount. They just weren’t on this car.

So a triangulation was created between a bolt that holds the transmission dipstick, the engine hoisting point on the back of the block, and a flange on the rocker cover that holds the throttle cable. A custom bracket was fabricated, with bracing to take the three metal elastic bushes that soft-mounted the actuator.

A dimple die was used for the first time — a tool donated by a friend of the series. You drill a hole the same diameter as the centre of the die, then press the die through that hole, creating a flange that lightens the piece and stiffens it.

The very last piece was to weld a ball to the throttle linkage that the cruise control actuator arm would sit on, scavenged from the original throttle body.

Up on the steering column, the cruise control stalk was mounted. Cruise control on the Mercedes W123 was a very rare option in Ireland and even in the UK, so the only switch available was an American one designed to go on the opposite side of an American steering column. The only way to mount it was pointing downwards rather than upwards. It hugs the steering wheel closely, so it’s in no danger of being hit by your knee. Instead of pushing it up to set it, you push it down. And you live with that.

The Honest Ending

After all that — testing all the wiring, the brake switch, the cruise control switch, the actuator — the cruise control still didn’t work.

The last place to turn was the cruise control amplifier, the brain of the system. All the joints were resoldered, and some capacitors were replaced like for like.

And the cruise control still isn’t working.

So that’s a problem to be solved another day, with new multimeter knowledge, resistance values, and tools. In this life or the next.

The Bigger Picture

For all the welding and rust-busting goodness done over the past ten months, the outside of this car is still majorly challenged by rust. It’s covered in crap. But the owner doesn’t care.

This is going to be a rolling resto rat rod over winter. Until there’s energy and better weather again to work on it properly. In the meantime, it’s more than enough car and more than enough comfort to get across the country to sand and surf.

Somehow this has been the most difficult episode ever produced, and the torture of winter hasn’t even kicked in yet. So the resolve is stronger than ever: a workshop is needed, and all efforts are now focused in that direction.

Because that’s what owning a W123 is really about. Not perfection. Not concours condition. Just keeping it going, adding a little comfort here and there, and driving it. The car doesn’t care if the paint is rough. It just wants to be used.

And honestly? That’s the best thing about these machines.