Problem with Bosch relay.

[On_the_road]

I have been chasing a problem with my remote lock/unlock system.   Perhaps one in ten times the driver side lock fuse would blow.   All the other 9 times it worked OK.   Today I figured I would chase the problem down.

It turned out to be the Bosch relay that controlled the unlock circuit.   Looking at the schematic, it seemed unlikely to that a relay could cause the fuse to blow.   I unplugged all the lock actuator motors in the basement doors, leaving only the relay and the wires as possible sources of the problem.  But the failure persisted.   Here is what I found:

This is the bad relay, a genuine Bosch!

 

There is a piece of metal wedged between the contacts.   The relay still worked 9 out of 10 times in this condition.   The reason the fuse would blow is that the normally open contact is connected to 12 volts and the normally closed contact is connected to ground.   When the fuse would blow, the particle must have shifted such that the normally open and normally closed contact would both connect to the armature, resulting in a dead short and a blown fuse.   I have pointed to the metal particle.

 

This shows the contact of the armature with a hole in its surface.   I have pointed to the actual metal particle.   The particle is almost certainly the metal from the hole in the contact.   The contacts are made from silver.   The plastic cover of the relay fits very tightly, so it is unlikely that the particle came from the outside.

 

This is a microscopic image of the contact that shows a hole in its surface.   The hole is quite deep.   I have never seen anything like this.

 

 

Edited September 18, 2022 by On_the_road
Correction

[RNMCBR]

Nice teardown analysis. I always do a teardown and failure analysis to understand what caused it. Many times I am able to fix the failure and then have a spare. 

[Frank McElroy]

4 hours ago, On_the_road said:

I have been chasing a problem with my remote lock/unlock system.   Perhaps one in ten times the driver side lock fuse would blow.   All the other 9 times it worked OK.   Today I figured I would chase the problem down.

It turned out to be the Bosch relay that controlled the unlock circuit.   Looking at the schematic, it seemed unlikely to that a relay could cause the fuse to blow.   I unplugged all the lock actuator motors in the basement doors, leaving only the relay and the wires as possible sources of the problem.  But the failure persisted.   Here is what I found:

This is the bad relay, a genuine Bosch!

 

There is a piece of metal wedged between the contacts.   The relay still worked 9 out of 10 times in this condition.   The reason the fuse would blow is that the normally open contact is connected to 12 volts and the normally closed contact is connected to ground.   When the fuse would blow, the particle must have shifted such that the normally open and normally closed contact would both connect to the armature, resulting in a dead short and a blown fuse.   I have pointed to the metal particle.

 

This shows the contact of the armature with a hole in its surface.   I have pointed to the actual metal particle.   The particle is almost certainly the metal from the hole in the contact.   The contacts are made from silver.   The plastic cover of the relay fits very tightly, so it is unlikely that the particle came from the outside.

 

This is a microscopic image of the contact that shows a hole in its surface.   The hole is quite deep.   I have never seen anything like this.

 

 

That pitting reminds me of the good old days when a capacitor would go bad and the ignition points under the distributor would pit just like that.  Good analysis for why the fuses kept burning out.

[Paul A.]

13 hours ago, Frank McElroy said:

That pitting reminds me of the good old days when a capacitor would go bad and the ignition points under the distributor would pit just like that.  Good analysis for why the fuses kept burning out.

Well now, isn't that the PITS!

[TimSpencer]

On 9/18/2022 at 6:07 PM, On_the_road said:

I have been chasing a problem with my remote lock/unlock system.   Perhaps one in ten times the driver side lock fuse would blow.   All the other 9 times it worked OK.   Today I figured I would chase the problem down.

It turned out to be the Bosch relay that controlled the unlock circuit.   Looking at the schematic, it seemed unlikely to that a relay could cause the fuse to blow.   I unplugged all the lock actuator motors in the basement doors, leaving only the relay and the wires as possible sources of the problem.  But the failure persisted.   Here is what I found:

This is the bad relay, a genuine Bosch!

 

There is a piece of metal wedged between the contacts.   The relay still worked 9 out of 10 times in this condition.   The reason the fuse would blow is that the normally open contact is connected to 12 volts and the normally closed contact is connected to ground.   When the fuse would blow, the particle must have shifted such that the normally open and normally closed contact would both connect to the armature, resulting in a dead short and a blown fuse.   I have pointed to the metal particle.

 

This shows the contact of the armature with a hole in its surface.   I have pointed to the actual metal particle.   The particle is almost certainly the metal from the hole in the contact.   The contacts are made from silver.   The plastic cover of the relay fits very tightly, so it is unlikely that the particle came from the outside.

 

This is a microscopic image of the contact that shows a hole in its surface.   The hole is quite deep.   I have never seen anything like this.

 

 

It looks like normal wear. Unless this relay is only been in use for a short time. If relatively new increase the amp rating of the new relay. Every time it opens/closes there is a bit of lightning bolt as the contacts approach each other. A bit of a welding arc. Eventually the contacts pit any can shed tiny metal droplets. Nice work tracking it down. I always have a couple of those relays in my spares. 

Edited September 20, 2022 by TimSpencer

[On_the_road]

I have seen lots of worn out relays over the years, both DC and AC.   This particular failure is unusual.   Mostly the contact surfaces wear down, sometimes very unevenly, but not with a hole blown in a contact surface that was otherwise in good shape.   The metal from the hole then formed a single particle that got wedged between the contacts.   The other common relay failure mode is for the contacts to weld together.

I suspect that the contact had a crack in the silver from when it was manufactured.   Likely, the crack occurred during the riveting process.

 

 

[TimSpencer]

Definitely could have happened. Relays and any set of contacts can fail in a million ways. From the contact melting, dislodging from the holder, bug between the two contacts, moisture, dirt,(I have  experienced all of these and a couple other odd failures over the 3 decades of work with this type of relay and many larger types).
Great job tracking it down especially since it was intermittent.  These things wear out so good to have some good spares.