W116/R107 no spark? Check the TSZ-k ignition control unit
Short answer: If the starter motor turns normally but there is no spark – or the engine dies at operating temperature and starts again once it has cooled down – on W114, W116 and R107/C107 with contact-controlled transistor ignition, the ignition control unit (TSZ-k) is often defective. Solution: replacement with a genuine ignition control unit Bosch 0 227 051 024 / Mercedes A 000 545 84 32 for 189,00 €.
The car sat for a few days, the starter motor cranks vigorously, there's fuel – and still nothing happens. Or even more annoying: the engine runs flawlessly for half an hour, then dies at a traffic light and only starts again after a cigarette-length pause. Anyone facing this on a W116, W114 or an SL of the R107 model series usually looks for fuel problems first. In practice, though, the trail on these vehicles noticeably often leads to the ignition control unit of the contact-controlled transistor ignition TSZ-k – a small box with a heat sink that switches the primary current of the ignition coil.
Symptoms
The key distinction: if there's no spark, the fault lies in the ignition system. If the spark is there, keep looking on the fuel side.
| Symptom | Likely cause |
|---|---|
| Starter motor turns normally, engine doesn't start, no spark at the ignition coil's high-voltage lead | Ignition control unit without output signal, defective ignition coil, or no voltage at terminal 15 |
| Engine dies at operating temperature, starts again after cooling down | Thermal failure of the output stage in the ignition control unit – the classic pattern |
| Misfiring under load, jerking at higher RPM, sporadic stalling | Cold solder joint in the ignition control unit or corroded connector contact |
| Spark present, engine cranks noticeably long after sitting for a while | Distinction, fuel side: residual pressure drops – see Fuel accumulator for starting problems on the W116/W126 |
| Spark present, engine runs rough when cold and responds poorly to the throttle | Distinction, mixture formation: see Warm-up regulator defective – symptoms and testing (K-Jetronic) |
Causes
The ignition control unit is a semiconductor component from the seventies. Its output stage switches the full primary current of the ignition coil, converting part of the energy into heat that is dissipated via the heat sink. After four to five decades, three ageing effects add up: the thermal paste between the unit and the cooling surface dries out, so the output stage runs hotter than intended. Solder joints become brittle from thousands of heating and cooling cycles and develop fine cracks – cold, the contact still closes; warm, the material expands and breaks the connection. And the connector oxidizes in the damp engine bay.
The failure therefore rarely happens all at once: first a single misfire in a summer traffic jam, weeks later regular stalling at operating temperature, and eventually no spark at all.
Testing it yourself
- Battery and ground first. Measure the resting voltage and check the ground straps between engine, body and battery for corrosion. A bad ground point produces exactly the same sporadic misfires – and costs nothing but a wire brush.
- Visual inspection of the unit. Pull off the connector, check the contacts for verdigris and widened sockets, retighten the mounting screws on the heat sink. A loose unit no longer has thermal contact and tends to fail when warm.
- Check voltage at terminal 15. Measure at the unit's connector with the ignition switched on and during a start attempt. If the voltage drops sharply while cranking, the problem lies in the supply wiring or the ignition lock, not in the unit.
- Spark test. Pull the high-voltage lead out of the distributor cap, hold a test plug against ground, and crank the engine. No spark directly at the ignition coil's lead means: the fault lies upstream of the distributor – that is, at the ignition coil, breaker contact, or ignition control unit.
- Cross-check the ignition coil and contact, then replace. Check the coil's primary and secondary resistance against the factory specification, inspect the breaker contact and capacitor. If these are fine, only a test installation of a known-good ignition control unit will reliably narrow down the fault – a thermal misfire simply cannot be measured on a cold component.
Solution
If the suspicion is confirmed, the ignition control unit is replaced completely; repairing the potted output stage is not feasible. We stock the genuine TSZ-k ignition control unit with the Bosch number 0 227 051 024 and the Mercedes number A 000 545 84 32 at a price of 189,00 €. It is a genuine removed part with intact security varnish – never opened. According to the product data, it fits W114, W116 and R107/C107. Compare the number on your removed housing before ordering.
Ignition control unit TSZ-k – genuine
Part number: Bosch 0 227 051 024 · Mercedes A 000 545 84 32
Fits W114, W116, R107/C107 · genuine removed part, security varnish intact
189,00 €
To the ignition control unit
Bosch engine control unit M110 A0005452932
Different component, not to be confused with the ignition control unit · W116 · 329,00 €
You'll find more tested electronics in the categories Mercedes relays & control units and W116.
Compatible vehicles
The assignment is taken unchanged from the product data. A prerequisite is contact-controlled transistor ignition TSZ-k.
| Model series | Stated according to product data |
|---|---|
| W114 | fits according to product data |
| W116 | fits according to product data |
| R107 | fits according to product data; 350 SL is among the models named in the product title |
| C107 | fits according to product data |
For other model series we have no documented data. When in doubt, the number on the installed unit decides: 0 227 051 024 or A 000 545 84 32.
FAQ
How do I tell that the ignition control unit is defective and not the ignition coil?
Both components lead to the same symptom: no spark. First measure the ignition coil (primary and secondary resistance against the factory specification) and check whether terminal 15 at the ignition control unit carries battery voltage during a start attempt. If voltage is present, the coil is fine, and the breaker contact switches, then the ignition control unit remains as the cause.
What part number does the ignition control unit for W114, W116 and R107/C107 have?
The unit offered here carries the Bosch number 0 227 051 024 and the Mercedes number A 000 545 84 32. Compare this number with the print on your removed housing before buying – only this guarantees fitment.
Why does the ignition only fail when warm?
The power output stage in the ignition control unit switches the ignition coil's primary current and releases heat to the heat sink. If solder joints or semiconductors are degraded after decades, the contact only breaks at operating temperature. The engine dies and starts again once it has cooled down – a very typical pattern for this component.
Can I replace the ignition control unit myself?
Yes. The unit is bolted on and connected via a connector. Before removal, disconnect the battery, release the connector, unbolt the unit from the heat sink or its mounting plate, and fit the replacement part in the same position. Make sure the ground contact is clean and the connector sits firmly.
What does a replacement ignition control unit cost, and is it available immediately?
The genuine ignition control unit Bosch 0 227 051 024 / Mercedes A 000 545 84 32 costs 189,00 €. It's in stock as a genuine removed part with intact security varnish – currently only a single unit, so stock is limited.
KFZTeile-Thot is a specialist shop for tested and refurbished Mercedes parts with our own test benches. Every part is checked before shipping – confirmed by over 1,000 eBay reviews (almost all positive). If you're unsure whether this ignition control unit fits your vehicle, call us: 0179 8991069, Mon–Fri 8 am–6 pm. It's usually faster with the VIN and a photo of the removed unit via our contact form.