HVBJB recall: what breaks? and how can it be fixed?

[RobbertPatrison]

Last year I posted a detailed technical analysis of the HVBJB problem that has affected many. I suspected that software cannot fix a hardware design problem.

Let's look at the schematic of the HVB Junction Box in our MME:

The current flows from the positive battery connection (+380V) though Main Contactor+ and then through both motors and then through Main Contactor-
back to the battery (0V).

The recent recall notice for 30K Mach-e EVs gives more information about the possible cause and fix for the HVBJB proglem. This is the text:

Ford's software remedy 22S41 did not fix the root problem of Main Contactor+ or Main Contactor- overheating. Rather, it attempts to detect the deterioration of the contactors by measuring increased resistance as a sign of trouble. When higher resistance is detected, the MME will go into a 50% low-power mode and the MIL light will switch on. The car remains drivable as long as the contactors still work normally. If a contactor is welded shut or is stuck open, the MME will be bricked and needs to be towed. That is the dangerous case as the car suddenly loses propulsion.

The resistance is measured by tracking the voltage drop between A and B and between C and D while the vehicle is pulling heavy power. The normal contactor resistance is 0.0005 Ohm according to the datasheet. At 500 Amps that would only be a small 0.25 Volt drop, while any sign of decay should give more drop and a lot more heat. They hoped to detect this and then dial down the maximum power by 2X to reduce heat. Notice that at 150kW charging power the heat in each contactor is a hot 80 Watt per contactor. If the heat cannot be hauled away this will cause problems.

From the new text, it is clear that Ford admits that this strategy does not work well. Too many vehicles still fry their contactors without the software detecting increased resistance beforehand. This is not surprising IMHO. Ford does think that the regular battery with its lower current rating reduces the probability of a fried HVBJB.

This is the jampot-shaped contactor, of which there are 4 identical ones in the HVBJB:

and this is what it looks like when it welds itself shut:

I am a little puzzled about what the recall fix for this is,. The replacement HVBJB looks the same as the old one and does not have temperature sensors either. It might use some upgraded contactor. Within the same form factor, the same problem is likely to happen again. What it needs is a design change with better thermal properties.

 

[Guss-E 2021]

I thought I read that Tesla experience a similar issue in the past. I have to imagine a better engineered contactor solved the problem. My MME is, at this moment, having the HVBJB replaced. Based on the diagram above and a photo posted in the primary HVBJB failure thread, that would be a replacement of the assembly that includes the faulty contactors.

Is your photo of a 1st generation contactor or the part used after May 2022?

 

[RobbertPatrison]

I thought I read that Tesla experience a similar issue in the past. I have to imagine a better engineered contactor solved the problem. My MME is, at this moment, having the HVBJB replaced. Based on the diagram above and a photo posted in the primary HVBJB failure thread, that would be a replacement of the assembly that includes the faulty contactors.

Is your photo of a 1st generation contactor or the part used after May 2022?

This is of the 1st generation, but the second generation contactor looks exactly the same. The issue is to extract the inevitable heat energy that occurs with high current. If it has nowhere to go due to a lack of heat conductivity, it will fail again. As far as I can see from the teardown by Weber Auto, there is no cooling in this part of the battery box. If that issues remains unaddressed, I am afraid that the HVBJB will continue to fail in all MMEs.
The failure probability depends on how hard the car is driven.

 

[AKgrampy]

Last year I posted a detailed technical analysis of the HVBJB problem that has affected many. I suspected that software cannot fix a hardware design problem.

Let's look at the schematic of the HVB Junction Box in our MME:

The current flows from the positive battery connection (+380V) though Main Contactor+ and then through both motors and then through Main Contactor-
back to the battery (0V).

The recent recall notice for 30K Mach-e EVs gives more information about the possible cause and fix for the HVBJB proglem. This is the text:

Ford's software remedy 22S41 did not fix the root problem of Main Contactor+ or Main Contactor- overheating. Rather, it attempts to detect the deterioration of the contactors by measuring increased resistance as a sign of trouble. When higher resistance is detected, the MME will go into a 50% low-power mode and the MIL light will switch on. The car remains drivable as long as the contactors still work normally. If a contactor is welded shut or is stuck open, the MME will be bricked and needs to be towed. That is the dangerous case as the car suddenly loses propulsion.

The resistance is measured by tracking the voltage drop between A and B and between C and D while the vehicle is pulling heavy power. The normal contactor resistance is 0.0005 Ohm according to the datasheet. At 500 Amps that would only be a small 0.25 Volt drop, while any sign of decay should give more drop and a lot more heat. They hoped to detect this and then dial down the maximum power by 2X to reduce heat. Notice that at 150kW charging power the heat in each contactor is a hot 80 Watt per contactor. If the heat cannot be hauled away this will cause problems.

From the new text, it is clear that Ford admits that this strategy does not work well. Too many vehicles still fry their contactors without the software detecting increased resistance beforehand. This is not surprising IMHO. Ford does think that the regular battery with its lower current rating reduces the probability of a fried HVBJB.

This is the jampot-shaped contactor, of which there are 4 identical ones in the HVBJB:

and this is what it looks like when it welds itself shut:

I am a little puzzled about what the recall fix for this is,. The replacement HVBJB looks the same as the old one and does not have temperature sensors either. It might use some upgraded contactor. Within the same form factor, the same problem is likely to happen again. What it needs is a design change with better thermal properties.

I am not saying it is a perfect fix but my understanding is Ford did replace the contactors with a more robust contactor. I also understand many now are just rebuilds of the old parts. Also everyone realized that the recall was not a fix of the problem but that Ford thought that it would provide a soft landing and allow owners to continue driving and then service the vehicle. In the vast majority of issues reported on the forums that was the case. Obviously that was not the case for all issues and some continued to fail as a SSN vs the soft landing SVS. The new recall appears to contain an update to the software as well as a proactive change out of the HVBJB. My guess is that will significantly reduce if not eliminate the HVBJB issue.

 

[AKgrampy]

This is of the 1st generation, but the second generation contactor looks exactly the same. The issue is to extract the inevitable heat energy that occurs with high current. If it has nowhere to go due to a lack of heat conductivity, it will fail again. As far as I can see from the teardown by Weber Auto, there is no cooling in this part of the battery box. If that issues remains unaddressed, I am afraid that the HVBJB will continue to fail in all MMEs.
The failure probability depends on how hard the car is driven.

Heat extraction would solve the issue for sure. The issue, if you look at the tech data for the contactor, was that the contactor barely meets the continuous current requirements. Ford mentioned, not saying it was true, that there was a manufacturing tolerance issue with the contactor. So they have replaced them with a better unit. I do agree that active cooling of some sort should be added in the future but as long as the contactors are rated for the application (and not just barely rated) it should be fine. I guess we shall see over the next couple of years. As for myself I have a GT, 15,000 miles, original HVBJB, not worried but glad for the upcoming recall.

 

[Guss-E 2021]

The failure probability depends on how hard the car is driven.

I wonder what the definition of "hard" is. I accelerated from ~70 to ~80 to pass with an ambient temp of around 60F when my warning triggered (at 22,000 miles). Prior to that, I've pushed the car much harder on hotter days. Feels like the degradation occurred over time.

Just spoke with the dealership. HVBJB has been replaced, battery is being re-attached but clearing one of the codes is giving the tech trouble; call into Ford Engineering so no car today. Sigh.

 

[BMT1071]

The photo quality of the failed contactor isn't great, but it looks like there was arcing going on that caused the welding. Typically this is due to poor contact. I also find it curious that the contact pattern on the two posts is significantly different. I have always leaned toward a manufacturing defect being the root cause of this issue, rather than a design/engineering problem.
While heat removal is great, there are countless high voltage and high current electrical applications in use every day that have no active cooling.

 

[Mach-Lee]

This is of the 1st generation, but the second generation contactor looks exactly the same. The issue is to extract the inevitable heat energy that occurs with high current. If it has nowhere to go due to a lack of heat conductivity, it will fail again. As far as I can see from the teardown by Weber Auto, there is no cooling in this part of the battery box. If that issues remains unaddressed, I am afraid that the HVBJB will continue to fail in all MMEs.
The failure probability depends on how hard the car is driven.

The heat is extracted through the contact terminals. The bus bars in the HVBJB act like heat sinks. This was discussed recently on another topic, but it’s my opinion that the bus bar near the contactors is not thick enough to remove enough heat. It’s only 3 mm thick and I think it should be 5+ mm thick to provide enough thermal mass. Tesla puts very thick busbars on their contactors (6+mm?) for comparison:

Tesla Model Y contactors, note thick busbars:

I’ve measured a failed Mach-E contactor at 5+ mΩ, which is very high and would generate a lot of heat. If anything, the recall software was not sensitive enough, and could only detect a failing contactor under a high amp draw, such as full throttle acceleration for several seconds. It may not detect a failing contactor if you never floor the vehicle or only do so briefly.

I think another factor besides the HVBJB design was the part quality. Some of these original contactors just decide they’re going to stick even though they’ve lived an easy life and haven’t necessarily ever been overheated. No software will detect that ahead of time.

It’s my theory the new contactors switched to using Inconel or other high heat resistant contact material, and were hopefully subjected to more rigorous initial quality testing to exclude defects.

I would like to see a new HVBJB with thicker bus plates and a switch to a different contactor manufacturer such as Gigavac instead of TE.

 

[Logal727]

I wonder what the definition of "hard" is. I accelerated from ~70 to ~80 to pass with an ambient temp of around 60F when my warning triggered (at 22,000 miles). Prior to that, I've pushed the car much harder on hotter days. Feels like the degradation occurred over time.

Just spoke with the dealership. HVBJB has been replaced, battery is being re-attached but clearing one of the codes is giving the tech trouble; call into Ford Engineering so no car today. Sigh.

ooh boy I'm almost at 22k lol

 

[Logal727]

I have always leaned toward a manufacturing defect being the root cause of this issue

That's been the company line the whole time and I agree as well, since there are some who haven't had the issue at all.

 

[doogie63]

Right now most people get replaced under warranty. When people have to pay for the replacement is when Ford going to hear it loud and clear. Lawsuit will follow with all the documents it will be an easy win for owners. I have a select SR and drive easy so hopefully I never have this problem.

 

[Maquis]

Right now most people get replaced under warranty. When people have to pay for the replacement is when Ford going to hear it loud and clear. Lawsuit will follow with all the documents it will be an easy win for owners. I have a select SR and drive easy so hopefully I never have this problem.

Since this an EV component, warranty is 8 years/100,000 miles. It will be some time before there are significant post-warranty failures.

 

[doogie63]

Since this an EV component, warranty is 8 years/100,000 miles. It will be some time before there are significant post-warranty failures.

That is good to know. Now maybe I will go over 45 mph next drive. LOL

 

[hack-e]

Mine got SVS at 30,899 this past Friday the 13th just after I passed a vehicle. Luckily it's still drivable, but does not seem to have the mentioned power reduction. Have appointment tomorrow. It's been through 2 terribly hot summers and decides to fail on a cooler day. I agree that it seems like a cumulative problem.

 

[BMT1071]

Mine got SVS at 30,899 this past Friday the 13th just after I passed a vehicle. Luckily it's still drivable, but does not seem to have the mentioned power reduction. Have appointment tomorrow. It's been through 2 terribly hot summers and decides to fail on a cooler day. I agree that it seems like a cumulative problem.

Are you sure the SVS is due to the HVBJB code(s)?

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