Tips, Tricks & Common Problems

Tips, Tricks & Common Problems

Keep on top of the latest R129 issues and how to fix them with our common problems guide. This list is updated monthly. Keep in the loop by signing up to The R129 Co monthly newsletter.
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Battery (low battery condition / jump starting)

Although your 129 may appear to be cranking well on startup, you shouldn't assume that the voltage is sufficient to satisfy the various control units within the car.

Low voltage or flat batteries can set up fault codes, particularly within roof control units.

Jump starting the R129: 

  1. Never connect jump leads with the key in the ignition and avoid any flashing of the terminals.

  2. Leave the cables in place until the alternator has had sufficient time to put some charge in the resident battery.

  3. If the battery was very low, early exit of the charging cables can often damage the base module due to the surge when the external power supply is removed.

Central Locking Pump (early model - removal + pipe connection codes)


The early central locking pumps have 2 distributor blocks that clip on top. They can only go in correctly as they have locating slots. 

To remove these you need a small flat screwdriver. Insert the screwdriver into the side of the distributor cap and pull the tab out. This is the same on both caps. 

By pulling the caps off you won't need to remember which pipes go where. See next for what the codes on the pump mean.

Pipe Connection Codes

Early 3 and 4 port pumps have the following outlet nozzles:

  • ZV: Central Locking: Central locking system vacuum and pressure
  • PU: Vacuum for auxiliary demand (pump will activate if vacuum drops below 400 mbar). This goes to a manifold in the engine bay, passenger side (UK) near the ECU's. This helps control things like the heater matrix vacuum elements. You need to keep an eye on this as they are prone to leaking at the bottom.
  •  PU (with dots above): Pressure for orthopaedic backrests (pump will activate if pressure below 200mbar). These can be retrofitted without too much hassle.
  • IZV: Internal Central Locking. Connection for internal central locking system vacuum and pressure. This locks and unlocks the following: spectacle box, glovebox, armrest, door cards, and the rear bench (if jump seats were not fitted). 

It is common that, when removing the door card, the pipe running up to the door lock becomes detached. This causes the pump to continually run and then burn out - a new pump is needed.

Dome Light (sticky aesthetic finish)

Principally, blue dome light units suffer from paint degradation and are impossible to clean. 

You will need the chemicals and paint technology to refurbish these to the correct finish.

The unit contains an internal temperature sensor (fed by a very small blower fan) for the Automatic Temperature control.

Door Mirrors (background info / not working or blowing fuses)

Background Info

Commonly known as wing mirrors (wing mirrors left the planet along with the Vauxhall Viva), door mirrors were fitted with or without the following options:

  • Power Folding Code 500. (Very rare. The mirrors will fold back towards the door when parking up)
  • Non powerfold mirrors. (They have a large spring to return the mirror after impact with a bit of luck)
  • Heated.
  • Memory Code 246/501. They will go to a preset position as part of your seat position memory system)
  • Auto Dim Code 249.

There were three UK generations (with a few exceptions):

  • Up to ident. no.: F 140421 
  • F 140421 to ident. no.: F 155734
  • As of ident. no.: F 155735

RHD (right hand drive) mirrors are NOT the same as LHD (left hand drive) versions, as the limits of the sweep of the mirror adjustment are different. 

Check your car's data card as well as the number of plugs/pins when looking at a replacement mirror.

Not Working / Blowing Fuses

This can often result from faulty insulation beneath the shell casing.

Also, check for bare insulation where the harness passes through the A pillar. Typically, steering column adjustment and interior electrical rear view mirror performance will be affected, especially on vehicles with memory functions.

Heater Blower Motor (general issues)

There are generally three ports of call here - the heater control panel, the blower motor or the blower motor resistor. 

If the motor operates at a slow speed only, then a simple test would be to unplug the resistor and supply the motor with 12 volts, and see if it runs at full speed happily. If so, then meter the resistor for different voltages as you vary the blower speed via the control panel. 

Take great care in around the blower housing. The spring clip locating areas are famous for snapping off, and that would need replacing.

Heater Control Unit (removal)

To remove the heater control (non-digital version), you need to remove the radio first, so that the two tabs holding the heater in can move downwards. Without removing the radio the tabs have nowhere to go.

Infrared Control Unit + Single Button Remote (not working)

Early R129’s - with the single button remotes - had an infrared remote control unit (N24) which is paired to the transmitter key. 

The signal comes via the receiver unit on one or both doors, and at the rear of the car below the boot lid. The receiver itself is not coded. 

The red and green lights often fail to light up, so don’t assume they are not working. The key fob transmitter can be tested by simply pushing the button whilst pointing it at a camera phone. The red flash will show on the camera screen.

Lights - Rear (changing/upgrading to facelift)

Did you know? The R129's rear light units are easily changed to later facelift versions

Multi Function Relay 1298201026 (faults + removal and installation + wiring chart)

Typical faults indicating a failed relay are when one or both indicators are stuck on. The part number for the relay on pre-1996 vehicles is 1298201026, and thereafter is 1408207126.

Removal & Installation
One question we get asked quite often is: "Do you have a plug that goes onto the multi function relay? Mine has exploded or fallen apart."

Thankfully Mercedes parts are beautifully engineered to be easily replaced in a methodical way. 

How to remove the multi function relay: 

  1. Remove the fuse cover on drivers side (UK) - 6 Philips head machine screws

  2. Locate the relay (roughly 5" x 1" black box)

  3. Pull relay upwards and out of holder (sometimes cables are a bit tight underneath). Tease the relay out of the fuse box as much as you can.

  4. Pull out small plug (6 cables usually). Now this is the bit where people get it wrong:

  5. With the first plug out of the way, now slide the connector along the relay and the plug will detach the relay (so you're pulling out the smaller plug and sliding across the retaining carriage before pulling out the larger plug). Do NOT PULL the cables. 

To install, do in reverse order.

Wiring Chart

Between you and me.. if one or both of the plugs happen to fall from your hand into your minestrone then here is the wiring chart.

Remote Key Fob (general info for the R129 '97 - '02)

The Mercedes-Benz keys for the R129 generally come in the form of a switchblade remote style. 

Keys were also made that did not include the remote functionality and others that had limited key functionality, known as valet keys, would not unlock the glovebox or trunk. The keys for these models use two layers of security to start the engine. 

The first layer is the physical key itself, a metal blade component which moves pins in a physical lock and allows the ignition switch to rotate. 

There is a second component inside the key, a transponder or immobilizer chip that functions in combination with an antenna/activation ring that surrounds the ignition switch. When one turns the key to the start position the ring around the ignition switch energizes the chip inside the key. 

If the frequency and code emitted from the immobilizer chip in the key matches what the car expects it to be, the AAM (All Activity Module) is authorized to enable fuel and spark to start the engine. 

Without authorisation, only the starter will spin and the engine won’t stay running, or start at all, depending on the model - Start Error will be displayed on the odometer screen. 

Mercedes is the only outlet that can supply replacement keys with immobilizer chips.

Roll Bar & Roof ECU (safety warning)

Later cars have a tumbler within the roof control unit. 

Never disturb the ECU (located under the rear seat or bench) without first disconnecting the 2 multiplugs. 

If the unit is handled without isolating, the tumbler may well set off the roll bar release. Risk of injury.

Roof - Soft Top (colour codes + folding hydraulic system problems)

Colour Codes

Originally these were supplied in 3 colours:

  • Code 740 Black 9001
  • Code 744 Blue 5010
  • Code 746 Brown 8023 

Folding Hydraulic System Problems

You should know about the electrohydraulic system. It has 15 hydraulic cylinders, 11 solenoid valves, 5 connectors and 45 lines and cables. When at rest, there is no pressure in the system and all latches are mechanically locked. 

The operation is sequential, and the control unit must complete each step in its activation before proceeding. 

It is therefore reasonably logical that if the folding roof fails to operate on command, or stops mid-flow, then the preceding step in the sequence has not been completed successfully. This can be an aid to diagnostics but experience is needed to put this information to good use - especially on early cars with limited diagnostics. 

Visit our soft-top issues guide for a more in-depth look at this issue.

Seats (backrest will not lock)

If this fails to engage correctly there will be an audible or visible warning. 

There are two microswitches located on the left and right in the locking mechanism that may need adjustment via the cables that run from the release handle. 

The folding top will fail to operate in this event until the seat is locked back correctly.

Seats (will not go back)

In addition to a faulty seat switch - and sometimes control unit, located beneath the seat - there is a proximity sensor (left - 1298210151 / right - 1298210251) located on the B pillar under the carpeted wall panel. 

It senses the seat magnetically and will stop it going back further than allowed. If faulty, it may over-restrict backward movement.

Wiper Mechanism (different finishes)

There were two finishes on the main wiper mechanism. The main cover was made of metal, and the other plastic. 

You can probably tell which you have by looking at it. If it is plastic it will more than likely be faded.

Wheel Bolts (snapped)

Early models had 95mm  and 85mm wheel bolts with the flange located part-way down the shaft.

This was a weak design and are no longer available new. They were replaced on later models.

When finding replacements, the R129 bolts generally had 16-18mm of thread protruding beyond the back face of the wheel.

Thanks for reading. Check out more below:

23rd Apr 2021 Carl Burgess

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