The main contactor separates the battery from the controller and the traction motor. When it fails, the truck stops moving, or the controller reports a fault as soon as the key is turned on. This guide matches the controller messages to the causes the Curtis and Sevcon manuals give and shows what to measure before you replace a part.
How the controller monitors the main contactor
Curtis treats the main contactor as a safety feature. It disconnects the controller and motor from the battery when a controller or wiring fault would otherwise put full battery voltage on the motor. For that reason the contactor coil must be wired directly to the driver output of the controller, which is pin 17 on the Curtis 1244. If the coil is wired elsewhere, the controller cannot open the contactor in a serious fault.
The checks differ by controller.
- The Curtis 1244 tests for welded contacts when the keyswitch is first turned on and every time it commands the contactor open. A second check senses the driver pin to confirm that a coil is connected and that the contactor actually closed.
- The Curtis 1243 GEN2 first checks on every close command whether the contactor is already closed. It then closes the contactor and checks that it really pulled in.
- The Sevcon Gen4 reports a welded line contactor when the contactor is closed at power up or after the coil has been de-energized. It reports a fault when the contactor does not close although the coil is energized.
Messages and the causes the manuals give
| Finding | Curtis 1243 and 1244 | Sevcon Gen4 | Causes according to the manual |
|---|---|---|---|
| Contactor welded | LED 3,2 "welded main contactor" | 4 flashes "Line contactor welded" | Contactor stuck closed. Curtis adds a shorted driver. |
| Contactor does not close | LED 3,4 "missing contactor" or "main contactor did not close" | 4 flashes "Line contactor did not close" | Contactor missing, wire to the coil open, or coil not connected. On Gen4 check contactor and wiring. |
| Driver overcurrent | LED 3,1 "contactor driver overcurrent" (1244) | not in the table we read | Coil shorted. |
| Precharge fault | LED 3,3 "precharge fault" | 7 flashes "Precharge failure" | Curtis lists a defective controller, an external short, or a leakage path to B-. Gen4 flags capacitor voltage below 5 V after precharge and points to shorts between B+ and B-. |
After a welded contactor message the controller blocks its output. On the Curtis 1244 the block stays until the fault is removed and the keyswitch has been cycled.
Contactor does not pull in or drops out
Start by measuring the voltage at the coil terminals while the controller gives the command. For the SW200, Albright gives guideline values relative to the rated coil voltage (Us) at a coil temperature of 20 °C.
| Quantity | SW200 guideline |
|---|---|
| Maximum pull-in voltage, intermittent and prolonged coils | 60% of Us |
| Maximum pull-in voltage, continuously rated coils | 66% of Us |
| Drop-out voltage | 10 to 20% of Us |
| Typical pull-in time | 40 ms |
| Typical drop-out time, no suppression | 10 ms |
| Typical drop-out time, diode | 100 ms |
| Typical drop-out time, diode and resistor | 30 ms |
| Typical contact bounce | 3 ms |
For a 24 V coil, 60% works out to 14.4 V and 66% to 15.8 V. If less arrives, check the fuse, keyswitch, connectors and the controller driver.
Both manufacturers often run the coil with voltage control. The Curtis 1244 applies the pull-in voltage first and drops to the holding voltage after 0.1 seconds. The holding voltage can be set from 20 to 100% in 2% steps. Curtis says it must stay high enough to hold the contactor closed under shock and vibration. If the contactor drops out on rough floors, check these parameters first. At a holding voltage of 100%, the coil should be rated for continuous duty.
On the Sevcon Gen4 pull-in voltage, pull-in time and hold voltage are also configurable. The manual adds further conditions.
- Contactor coils must not be connected to the supply side of the key switch. The outputs have their own supply pins.
- Contactors with built-in economiser circuits are not allowed, because they are not compatible with the controller.
- Contactors with blow-out magnets must be connected to the power terminals with the polarity marked on the housing.
Welded contacts
Both manufacturers name a contactor that stays closed after the coil has dropped out. Curtis also lists a shorted driver. That produces the same picture, because the coil is then permanently energized. Check the contactor and the coil circuit together.
Precharge protects the contacts at switch on. Sevcon states its purpose as protecting the contact tips from inrush current. Curtis closes the contactor only after the capacitor bank of the controller has come within 10% of battery voltage, and requires precharge for battery packs of 48 V or higher. On the Gen4 the target level lies between 50 and 90% and is adjustable. A precharge fault means the contactor never closes in the first place. Look for a short between B+ and B- before you replace the contactor.
The SW200 data shows the limits of the contactor.
| Item | SW200 per data sheet |
|---|---|
| Thermal current, interrupted and uninterrupted | 250 A and 400 A |
| Fault breaking capacity, 5 ms time constant | 1500 A at 96 V |
| Contact voltage with blow-out magnet | up to 96 V |
| Contact voltage without blow-out magnet | 48 V or 60 V depending on the test standard |
| Contacts | double breaking, silver alloy |
Albright notes that frequent switching on load raises contact resistance.
Judging contact wear
For new SW200 contacts Albright quotes a typical voltage drop of 40 mV per pole at 250 A. This is a reference for new contacts, not a wear limit. We found no published limit for replacing the contacts. Ask the manufacturer before you base a decision on measured values.
Safety during testing
Curtis requires opening the battery circuit before working on the motor control circuit, wearing safety glasses and using insulated tools. The vehicle should be jacked up so the drive wheels are off the ground, or the motor disconnected. Lead acid batteries release hydrogen while charging and discharging, so follow the battery manufacturer's instructions. In its cleaning procedure Curtis discharges the controller capacitors through a load across B+ and B-.
OSHA 1910.178 requires disconnecting the battery before electrical repairs. Replacement parts must be equivalent in safety to the original. A truck in unsafe condition must be removed from service.
Matching parts from the catalog
Check fit against data plate, coil voltage and part number before ordering.
- C29508 is a single pole SW 180 contactor with a 24 V coil, C29522 is the 48 V version. Curtis names Albright SW80 or SW180 as the main contactor for the 1243 GEN2.
- C29532 is a single pole SW 200 contactor with a 48 V coil, C29509 is the 80 V version.
- For the Gen4 up to 450 A, Sevcon lists a main contactor with a 24 V coil (828/37024), a 48 V coil (828/57026) and an 80 V coil (828/67010).
- For the Gen4 up to 650 A the manual names a 24 V intermittent coil that is chopped at 17 V. The catalog lists 828/39001 for it.
- For worn contacts the catalog carries 3253582700 from Linde with four silver contacts, and the contact sets 370071 and 370072 from Crown.
FAQ
Can the truck keep running with a welded contactor message?
No. The Curtis 1244 blocks its output in this case. OSHA requires trucks in unsafe condition to be taken out of service.
What voltage must reach the contactor coil?
For the SW200 Albright gives a maximum pull-in voltage of 60 or 66% of rated voltage and a drop-out voltage of 10 to 20%. Controllers with voltage control supply a pull-in and a holding voltage that you set according to the data sheet of your contactor.
Can I fit a contactor with a different coil voltage?
The Curtis and Sevcon drivers can run coils with a lower rated voltage when pull-in and holding voltage are programmed to suit. Curtis gives a 12 V coil on a 36 V system with 34% holding voltage as an example. On the Gen4, contactors with economiser circuits are excluded. Follow the conditions of the contactor manufacturer.
Do you need one of the parts mentioned? We deliver the matching part with a quote on the same day, and express shipping is possible.