🚗 Classic Fault — Mercedes-Benz W115 (1968–1976)
Ballast resistor failure — no start or weak spark
💰 Estimated Repair Cost
$10 – $25 USD
Parts and labour estimate
🔧 Tools Required
Multimeter
📋 Overview
The ballast resistor reduces voltage to the coil during normal running. A failed resistor causes no spark or excessive voltage burning out points quickly.
🩺 Symptoms
- No start
- very weak spark
- points burning quickly after replacement
- coil overheating
🔍 Possible Causes
- Failed ballast resistor
- broken resistor wire
- open or short circuit
🛠️ Diagnosis Steps
Measure resistance of ballast resistor — should be approximately 1.5-2 ohms. Check voltage at coil positive terminal: should be 7-8V running, 12V cranking.
✅ Most Confirmed Fix
Replace ballast resistor with correct value unit.
82% of users confirm this fix worked (22 reports)
🏷️ Parts Needed
Ballast resistor — correct value for M115
🔗 Other Mercedes-Benz W115 Faults
Own a Mercedes-Benz W115?
Share your experience with this fault — or report a different issue. Your knowledge helps other classic car owners worldwide.