Bringing a classic Mercedes-Benz back to life is a rewarding experience, but it often comes with its share of head-scratching moments. Recently, a fellow enthusiast encountered a starting issue with their beautifully restored 1969 Mercedes 280 SL (USA version), and documented the troubleshooting process. This article delves into the steps taken to diagnose a no-start condition, offering insights that can be helpful for anyone facing similar challenges with a classic Mercedes.
The project began after a meticulous restoration, with the engine, a rebuilt and detailed unit from Buds Benz, finally installed. Wiring was completed – seemingly – and a fresh fuel system, featuring stainless steel and new rubber lines, was in place. Ten gallons of high-test gasoline were added, ready for the first start.
Initial attempts to fire up the 280 SL revealed it would only run briefly on a prime, hinting at a fuel delivery problem. The initial fixes were classic DIY: cleaning fuse #4, addressing a ground connection behind the driver’s seat, and even a gentle tap on the fuel pump after sending it direct 12V power. This “boost” got the pump running with the key, a small victory in the troubleshooting process.
A key question arose: does the fuel pump on a 1969 280 Mercedes run continuously, or does it shut off once pressure is built? In this case, the pump was running constantly, maintaining a healthy 10-15 psi of fuel pressure at the cold start injector. This eliminated the fuel pump itself as the primary culprit for the no-start issue.
With the fuel delivery seemingly addressed, the engine would now run at higher RPMs, but stubbornly refused to idle. The venturi was closing completely, pointing towards the need to install and adjust the gas pedal linkage – a task for later, once the starting issue was resolved.
The core problem remained: the engine wouldn’t start on its own. Further investigation revealed that the cold start injector, crucial for initial engine firing, wasn’t receiving power during cranking. Continuity tests confirmed the black wire to the injector was connected to pole #87 on the CSV relay harness and fuse #6, suggesting the wiring path was intact.
To isolate the issue, a direct jumper lead to the cold start injector was used during cranking. Tapping the injector a couple of times coaxed the engine to start and run smoothly (albeit with the idle issue). This confirmed the engine itself was sound, and the problem likely lay within the cold start injector circuit. Neither the CSV relay nor the cold start injector were audibly “clicking” during normal cranking, indicating a lack of power to these components.
Suspecting the CSV relay, a test was performed based on online resources. Applying 12V to prong #86 and ground to #85 resulted in a “click” and continuity between prongs #87 and #30, confirming the relay itself was functional. Furthermore, prong #30 was receiving 10.6 volts during cranking.
However, the relay wasn’t “clicking” during cranking under normal conditions, meaning prong #86 likely wasn’t receiving power. Continuity tests showed prong #86 connected to “W” on the thermo time switch, and #85 to “G”. While the thermo time switch’s functionality was unconfirmed, disconnecting it and even bypassing it with a copper wire yielded no change, suggesting it wasn’t the immediate problem.
The next logical step? Investigating the wiring from the CSV relay to the ignition switch. This classic Mercedes 280 SL starting puzzle highlights the methodical approach needed when diagnosing vintage car issues. Tracing the electrical path and systematically testing components are key to uncovering the root cause and getting these beautiful machines back on the road. Stay tuned as we delve deeper into the ignition switch wiring to hopefully solve this 1969 Mercedes starting mystery.
