HV Contactor
Contactors are the heavy-duty relays that connect and disconnect the traction pack from the rest of the vehicle.
By Anthony LimpertASE Master Automobile Technician- Contactors are the pack's disconnect. Stuck open = no HV. Stuck closed = HV present when it shouldn't be — a safety hazard.
They are the gatekeepers of high voltage, and their two failure modes sit at opposite ends of the severity scale.
Stuck Open vs. Stuck Closed
The contactors inside the pack are its disconnect: when the vehicle powers up, the BMS closes them to feed HV to the inverter, and when it shuts down (or a fault occurs), it opens them to isolate the pack. A contactor stuck open means no high voltage ever reaches the motor — the vehicle simply cannot move, typically presenting as a no-Ready condition with pack-side DTCs.
A contactor stuck closed is the far more dangerous failure: high voltage remains present on the vehicle side when it should be isolated. That defeats the fundamental safety assumption that shutting down the vehicle de-energizes the HV bus. This is exactly why de-energization procedures require verifying zero volts with a meter rather than trusting that the disconnect worked — a welded contactor looks identical to a healthy one until you measure. Contactors can weld from switching under load or from repeated inrush current, which is also why packs use pre-charge circuits to soften the initial connection.
| State | Symptom | Meter/data finding | Action |
|---|---|---|---|
| Stuck open | No Ready mode, vehicle will not move | 0V on the bus with closure commanded; pack-side DTCs | Diagnose contactor, driver circuit, and BMS command |
| Stuck closed (welded) | Looks normal — that is the danger | Bus voltage still present after shutdown | Stop; treat entire HV system as live; follow OE isolation procedure |
| Pre-charge failure | Clunk/click cycling, intermittent no-Ready | Bus never rises to pack voltage before main closure aborts | Test pre-charge resistor and relay |
| Healthy shutdown | Normal power-down | Bus voltage collapses to ~0V within the OE bleed-down time | Baseline behavior — know it so deviations stand out |
Contactors weld from switching under load and repeated inrush — which is exactly why pre-charge circuits exist and why zero volts must be verified by meter, never assumed.
- Shut the vehicle down per the OE procedure and pull the service disconnect. This commands the contactors open — commands, not guarantees.
- Wait the full OE bleed-down interval so capacitor discharge cannot mask a live bus.
- Measure bus voltage at the designated inverter test points with the live-dead-live meter procedure. A healthy system reads essentially zero.
- Any persistent voltage after the wait means a welded contactor until proven otherwise: stop work, keep everyone clear, and follow the OE procedure for a stuck-closed condition. Do not open HV connections on a live bus.
- On intermittent no-Ready complaints, listen and scan during power-up: contactor cycling sounds plus bus voltage that starts rising and collapses points at the pre-charge circuit, not the main contactors.
- Trusting shutdown to de-energize the bus. A welded contactor leaves full pack voltage on the vehicle side and is invisible from the outside — this single failure mode is why verified measurement is non-negotiable.
- Overlooking the pre-charge circuit on intermittent no-start complaints. A failed pre-charge resistor aborts the closure sequence and mimics contactor or pack faults; test it before condemning bigger components.
- Bypassing or jumpering contactors to 'test' the system. You are defeating the pack's only disconnect with your hands on it — there is no diagnostic result worth that.
Why do packs use a pre-charge circuit at all, and what does its failure look like?
Closing a contactor straight onto discharged inverter capacitors draws a massive inrush current that arcs and eventually welds the contacts. The pre-charge circuit charges the bus through a resistor first, then closes the main contactor gently. When pre-charge fails, the bus never reaches pack voltage in time, the closure aborts, and you get clicking, cycling, and intermittent no-Ready — often misdiagnosed as a bad pack.
You measure 380V on the inverter bus ten minutes after shutdown with the disconnect pulled. What do you conclude and do?
A contactor is welded closed — the pack is feeding the bus when everything should be isolated. Stop immediately, keep the area clear, and follow the OE stuck-closed procedure. Every safety assumption based on 'the car is off' is now false, and the system stays live until the pack-internal fault is addressed with the appropriate procedure and PPE.
Why do packs use a pre-charge circuit at all, and what does its failure look like?
Missed one? The reasoning above comes straight from the EVH exam bank, so this is the standard you will be held to.