Main relay stuck on - how to diagnose?

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PaulNewall

Member
Joined
Dec 31, 2023
Messages
13
This relates to another post of mine, but I think it is a general enough question to have its own thread.
I think I may have the main relays in the battery stuck on, and I'd like to be really sure of that before I start taking the battery out of the car.
My car is a leaf 2015, the service manual (now I have the right one for the car) does say something about main relays stuck on. It suggests that shorts in the HV harness or any of the HV coponents (inverter, heater, ac compressor) could give similar DTCs.

The reasons that I think the relays are stuck on are:
1. The first problem was that the leaf would not shut down normally. Stuck relays might be a cause of that.
2. I have a DTC P317A indicating "inverter voltage too high" The manual suggests the inverter may be faulty, but I am thinking the battery voltage being present before start up might also cause this error.
3. When I remove the service plug from the battery, I measure 372V between the service plug's socket contacts. I would expect to measure 0V if either one of the relays were open. With the 12V battery disconnected, the main relays should be open, unless they are stuck.
4. (added 6/1/24) When the battery output HV connectors are unplugged, the voltage across the battery service plug socket is zero.
5. (added 6/1/24) When the battery output HV connectors are unplugged, there's no voltage at the harness side and there's a high resistance at the harness side, this is normal according to the service manual.
6. When I replace the service plug, I get a small but significant arc when it makes contact. I would not expect any current to flow if either one of the main relays were open. I am guessing that the arc is some current charging the capacitor at the inverter input.

Update 23/1/24 I replaced the main relays. They were stuck on as I expected. With the working replacement relays, I measure almost 0V at the service plug socket. So high voltage at the service plug socket, when the battery is in the car, is a good indicator that both main relays are stuck on.
 
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TLDNR: factoid about relays
Relays are an old tech. Car manufacturers used to put them in the fuse boxes as they had to be occasionally replaced. There are really old elevators from the 40’s that only use relays. You can hear them clicking as you ascend. Stuff is still used for high voltage stuff sometimes because it can handle more power than silicon.
 
This relates to another post of mine, but I think it is a general enough question to have its own thread.
I think I may have the main relays in the battery stuck on, and I'd like to be really sure of that before I start taking the battery out of the car.
My car is a leaf 2015, the service manual (now I have the right one for the car) does say something about main relays stuck on. It suggests that shorts in the HV harness or any of the HV coponents (inverter, heater, ac compressor) could give similar DTCs.

The reasons that I think the relays are stuck on are:
1. The first problem was that the leaf would not shut down normally. Stuck relays might be a cause of that.
2. I have a DTC P317A indicating "inverter voltage too high" The manual suggests the inverter may be faulty, but I am thinking the battery voltage being present before start up might also cause this error.
3. When I remove the service plug from the battery, I measure 372V between the service plug's socket contacts. I would expect to measure 0V if either one of the relays were open. With the 12V battery disconnected, the main relays should be open, unless they are stuck.
4. (added 6/1/24) When the battery output HV connectors are unplugged, the voltage across the battery service plug socket is zero.
5. (added 6/1/24) When the battery output HV connectors are unplugged, there's no voltage at the harness side and there's a high resistance at the harness side, this is normal according to the service manual.
6. When I replace the service plug, I get a small but significant arc when it makes contact. I would not expect any current to flow if either one of the main relays were open. I am guessing that the arc is some current charging the capacitor at the inverter input.

Update 23/1/24 I replaced the main relays. They were stuck on as I expected. With the working replacement relays, I measure almost 0V at the service plug socket. So high voltage at the service plug socket, when the battery is in the car, is a good indicator that both main relays are stuck on.
A very interesting & useful post.
Just goes to reinforce the absolute necessity to remove the HV disconnect before doing any work on any component of the HV system.

I'm guessing that one contactor's contacts became welded together followed shortly by the second one when it had all the load to deal with on it's own.

Would be interested to know where you were able to source replacements from.
 
3. When I remove the service plug from the battery, I measure 372V between the service plug's socket contacts. I would expect to measure 0V if either one of the relays were open. With the 12V battery disconnected, the main relays should be open, unless they are stuck.

That confirms it, you are correct, that with the 12 volt disconnected, the relays should be open, and no power should be at the disconnect or the harness connection to the PDM on the front of the HV battery.
Good diagnoses!
While inside, I would check the pre-charge resistors. Just to be safe.
 
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A very interesting & useful post.
Just goes to reinforce the absolute necessity to remove the HV disconnect before doing any work on any component of the HV system.

I'm guessing that one contactor's contacts became welded together followed shortly by the second one when it had all the load to deal with on it's own.

Would be interested to know where you were able to source replacements from.
I got replacement relays on ebay from someone who had taken a battery apart.
 
3. When I remove the service plug from the battery, I measure 372V between the service plug's socket contacts. I would expect to measure 0V if either one of the relays were open. With the 12V battery disconnected, the main relays should be open, unless they are stuck.

That confirms it, you are correct, that with the 12 volt disconnected, the relays should be open, and no power should be at the disconnect or the harness connection to the PDM on the front of the HV battery.
Good diagnoses!
While inside, I would check the pre-charge resistors. Just to be safe.
Yes, I was surprised to find that the pre-charge resistor and relay were OK, since they would be an obvious cause for the main relays welding. The car worked fine for a while after the relay replacement. (Now it has more problems, but the relays are still OK.)
 
" The car worked fine for a while after the relay replacement. (Now it has more problems, but the relays are still OK.) "

The contactors you have replaced are only supposed to operate when little current is flowing in the circuit.
Perhaps the ongoing problems with the car are causing the contactors to operate when high current is flowing causing them to get welded up.
I guess time will tell.

What are the ongoing problems ?
 
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