945 seat motor fire

Today while my daughter was driving our 1993 945T began to release smoke
from under the drivers seat (memory power seat). Pulled over, shut down
engine and got out. Smoke continued and then flames rose over the front edge
of the seat. Fire was put out with extinguisher and on examination all the
electronics (motor, controller, wiring) under the seat was fried and the
carpet and seat upholsetery burnt.

I can understand electrical shorts and the resultant heat but the 30A fuse
controlling the seat had blown so why did the motor/controller burn up and
start a fire? The problem was the seat motor and not the seat heater. No
other fuses blew.
Anyone had something like this happen to them? Any thoughts why burning
continued to the point of flames when fuse had blown (oil from motor?)? Any
source for replacement parts or seat (nothing good on ebay at this time).
Thanks for any information you can provide.
Howard

 

Contact Volvo Customer Service & tell them what happened as this is a
issue that I have not run in to yet. Do not know what help they may or
may not offer you but it can not hurt to go on record with them about this
Glenn

--
"*-344-*Never Forgotten"
Is for the New York City Firemen who lost their lives on September 11,2001.
The official count is 343, but there was also a volunteer who lost his life
aiding in the initial rescue efforts. And I will never forget them as
long as I live,
nor should any American.

 

Yikes! Can you tell where the fire started? Was it the motor itself or the
wiring? It's entirely possible that a wire chafed, grounded out, and had
enough resistance that it didn't blow the fuse before heating up hot enough
to burn the insulation off, though it does surprise me that it actually
caught fire, that's kinda scary.

Best I can suggest for parts is a wrecking yard, or see if you can file an
insurance claim and get the parts from Volvo, but a wrecking yard is sure to
be far cheaper, even a nicer one with newer cars.

 

What probably happened is that the seat heater overheated and shorted to
the motor and burned out the motor fuse but continued to burn the seat.
That explains why when the fuse burned out it didn't stop the fire, it
was the wrong fuse. The seat heater can cause a fire and does when the
thermostat fails.

 

Seat heater is fine. Fire under seat centered at motor and controller unit.

Howard

 

It was probably the wire, or maybe a connector that overheated. I've shorted
a couple wires connected to high current 12v sources before and it's
shocking how quickly the insulation practically vaporizes, though I haven't
yet had one actually catch fire.

 

What is there to burn in a motor? The control unit is the biggest
candidate. I had a smoke detector that started a fire. Control
circuitry can start fires easily. When I was a physicist at AT&T, one
of my jobs was designing electronic components so they were not
flammable. The plastic parts are very flammable and once the fire is
started the fire continues even after the fuse blows. We solved the
problem by making all components connected to the power source
non-flammable. They were made of flammable resins but were designed so
they would always fail safely by cracking before they ignited. This
always prevented a fire.

 

The fuse is to keep the wire from catching on fire. The way to prevent
fires is to have a smaller section of bare wire that melts before the
insulation catches on fire (i.e., a fuse or fusible link). When I did
this type of work I had a 200 amp power supply and could make just about
anything catch on fire.

 

Reminds me that I should really get fire extinguishers to keep in my cars,
I've seen enough cars of all makes burned to a crisp in junkyards and heard
just enough stories that a $20 compact extiguisher is cheap insurance.

 

In an ideal world the fuse would always blow to prevent a fire, but it's
entirely possible for a connector to corrode enough to limit the current
just enough to burn up the wire without actually blowing the fuse.

 

This I don't believe. I spent a lot of time flame testing wires and
only wire smaller than 20 gauge would burn at less than 20 Amps. The
rating on wire is not for flammability, but voltage drop. At their
rated currents they don't even get warm.

 

I've seen heat damage in a circuit where the fuse doesn't blow, but it's
always been at some sort of connection, where I guess resistance from
poor contact can create enough heat to cause melting and fires.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
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