Tips, Tricks and Common Problems with the R129 SL

Posted by Carl Burgess on 28th Jan 2020

How to fix some common R129 SL problems

1. Door Mirrors Not Working (Or 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 the memory functions.


2. Wheel Bolt Snapped

Early models had 95 and 85 mm 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 16-18 mm of thread protruding beyond the back face of the wheel.


3. Low Battery Condition & Jump Starting

Although your 129 may appear to be cranking well on start up, 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, particular 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.

4. 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.


4. Roll Bar & Roof ECU

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!


5. Seat 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.


6. Seat 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.


7. Central Locking Pump Pipe Connections

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

- ZV : Central locking system vacuum and pressure
- PU : Vacuum for auxiliary demand (pump will activate if vacuum drops below 400 mbar)
- PU : Pressure for orthopaedic backrests (pump will activate if pressure below 200mbar)
- IZV : Connection for internal central locking system vacuum and pressure


8. Folding Roof 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. We can help - visit us in-store (in Kent, United Kingdom) to have your roof diagnosed and repaired. We also have our soft-top issues guide for a more in-depth look at this issue.


9. Multi Function Relay Faults

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.


10. Heater Blower Motor Issues

There are generally 3 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 (1298300103).


11. Single Button Remote Control & Infra Red Control Unit Not Working

Early R129’s, with the single button remotes, had an infra red 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.


12. Remote Key Fob Transponder for the R129 1997-2002

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 that 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 authorization 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. Since late August 2017 Mercedes has been unable to supply replacement keys with immobilizer chips.

This means if you have one of these affected models and your key is lost or damaged it’s game over. The car becomes an expensive and oversized paperweight.

Until that changes, we have a limited supply of Key Remotes, RCL control module and coded Engine Control Modules sold as coded set. Search for lock sets in our store.


13. Soft Top Colours - Which Colour?

Originally these were supplied in 3 colours:
- Code 740 Black 9001
- Code 744 Blue 5010
- Code 746 Brown 8023


14. Overhead Dome Light Sticky Finish

Principally, blue dome light units suffer from paint degradation and are impossible to clean. We have the chemicals and paint technology to refurbish these to the correct finish - book a visit to have us do this for you. The unit contains an internal temperature sensor (fed by a very small blower fan) for the Automatic Temperature control.


Service Manual

If you really want to understand your R129 we have a limited supply of the 400 page “Introduction Into Service” R129 SL Workshop Manual. This is principally for M103 M104 and M119 engined cars: the 300SL, SL300-24 and the 500SL - that's 129060, 129061 and 129066. Importantly this does still include most of the body, soft top workings and electrical detail related to all R129 model years.

The book includes complete engine specifications and a mass of information on the workings and design of the running gear, body, braking and electrical components. It makes for incredible reading. The best route to problem diagnosis is to use a manual to understand the functionality of the relevant problem part - and how it relates to the different elements within that system.

DVD’s and PDF files are an inefficient way to research the information contained within this book, and are no match for the real thing. Take a look inside.


See also:

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