1986 928s electrical smoke

[harleydog77]

I'm working on a 1986 928s. While driving the car checking the AC, Fresh Air Fan worked intermittently at different speeds. I hit a small bump about 30 MPH and smoke came from the passenger side glove box area and some smoke from the Dash Air Control Module, I turned the AC and fan off. Smoke stopped. Pulled the fuse panel cover off the passenger floor board and found the #17 fuse(30 amp) looking like is almost caught fire. Any help would be appreciated in diagnosing what caused this.

[stepson]

#17 is the fuse for the Fresh air blower fan, so obviously the fuse got hot enough to cause damage. Is/was the fuse the correct amperage fuse for that spot? Should be a 30 amp fuse there.

If that is/was correct, then either your blower motor is drawing way too many amps, or there is something else attached to this circuit drawing too many amps, or there is a bad/faulty connection that is causing resistance in this circuit.

Remove and replace the relay in spot X (5th from the left on the top row). It's a standard 53 relay so it's cheap and it also controls the fresh air blower.
Relay relay relay?

[honerboys]

+1^^^^did the bump cause something to jar loose and short - i would think the fuse would have blown long before it would get that hot (assuming it was the right amperage) - loose connections on the back of the panel? - bad fuse holder?

[stepson]

Those fuses look a little strange to me in picture 2. I see in your first post that a 30 amp fuse was used, which is the correct size, but unfortunately not all fuses are made the same. For sure, you would want the fuse to blow BEFORE the wires burn up, but some off-brand fuses are not made to the same specs as more expensive fuses which are actually tested to blow at a certain amperage.


MrMerlin will probably chime in at some point and tell you to disconnect your battery and remove and clean your CE panel and to use DeOxit on all the connection points.
I think that this might be a really good course of action for you at this point.

I would also measure the amps that your blower motor is using. It could be on it's way out and need to be replaced as well.

[Danglerb]

Poor or corroded connections make heat when combined with a longer than normal high current draw. The lube in the blower motors goes bad over time and the motor takes more and more current to run.

[harleydog77]

On this circuit there is also the fresh air fan blower resistor, heard there where a lot of problems with the resistors. How true is this? would anyone get the parts from a salvage yard?

[JK McDonald]

Hey Harley - All good suggestions. Our blower motors do accumulate a lot of mileage because they are normally left in the on position. After inspecting the blower speed control resistors - You might then consider pulling your blower motor out to inspect the rotation of the bearings, brush length and commutator surface. If the BM brushes have worn down to the point of damaging or smearing all the the commutator segments together - you can have a very large current draw that could over heat the fuse connection/wiring.

It does seem odd though that the 30 amp fuse didn't safely open early enough to protect the circuit. Perhaps the problem WAS just a loose (poor-over heated) electrical connection at the fuse holder. After verifying that the resistors, brushes and blower motor commutator looks OK - you could try using a pair of needle nose pliers to "very slightly" twist the feet of a new 30 amp fuse, install it in it's holder and briefly run the blower motor again ?

Don't forget to disconnect the battery- pull the top of the fuse panel down toward you to inspect all the wiring on the back side of the blower motor fuse circuitry. Once you are happy with the blower motor operation - for reliability you may need to do some rewiring and blower motor fuse relocation.

Good Luck - Michael

[Mrmerlin]

the reason the fuse panel melted is because the fuse did not make a solid connection to transfer the juice.

It is possible you replaced the fuse with a new part that had thinner pins or the CE panel sockets have gotten spread open.

At this point it would be wise to remove the sockets from the panel and replace them with the identical parts.

OR remove the wires from the rear of the panel and plug them into a fuse holder.
If the fuses you got are thinner then you could also solder the pins and this will make the pins thicker
Use deoxit 100 on all of the connections

[JK McDonald]

Hey Harley - Have you given your blower motor circuit another "Smoke Test".

After looking at the photos of the damage again I think Mr Merlin was right on target when he suggested that the primary problem was probably just a loose fuse socket connection. A relatively high (Blower Motor) current passing through a poor electrical contact can easily generate enough heat to melt plastic.

If the original fuse socket is too unreliable or questionable - you could easily add a new single auxiliary style fused holder, plug in a 30 amp fuse and let it sit above the fuse panel. Just solder each wire of the new fuse holder to the socket legs of the old fuse.

Good Luck- Michael

[harleydog77]

Found a burnt circuit board in the climate control unit (blower switch, AC/Heat)

[Danglerb]

Symptom, not source, again its the high current a blower motor with dry bearings has.