Smart Internal Magnetic Switch (IMS) starters include protective features that can be mistaken for faults. When diagnosing a vehicle with a Smart IMS starter, follow the standard procedure but take these built-in protections into account.
Diagnostic Procedure
1. Identify the type of complaint: slow cranking, click/no-crank or no-click/no-crank.
2. Check the battery state. Charge batteries that are below 65% state-of-charge before testing — discharged batteries will produce false failures.
3. Conduct a starter voltage drop test on both the positive and negative cables. On a 12-volt heavy-duty system the voltage drop should not exceed 0.5 V at 500 amps. Replace or repair cables that exceed the limit.
4. Test the control circuit. The Smart IMS “S” terminal should have at least 8 V when the key is in the start position. Low voltage points to wiring or switch problems. If voltage is above 8 V and the cables and batteries have passed testing, replace the starter.
Smart IMS Protection Features (Normal Behaviour)
- Limits cranking time to 20 seconds with a 10-second rest
- Introduces a three-second delay between start attempts
- Prevents engagement if battery voltage is below 11.75 V or above 13.75 V
These protections are normal and prevent damage — they should not be misinterpreted as faults.