I learned a cool trick as a bench tech working on variable speed DC motor
controls many, many years ago.
One of the most common failures we would see on units coming in for repairs
were shorted triacs in the output stage of the drive. Of course these would
immediately blow a fuse or trip a breaker when the controller was powered up
and activated.
Our design guy, a real savant but total nut job, set up a power cord at each
bench with a mogul base light bulb in series whose amperage draw was less than
the controller under load. If the controller was good, it would run just fine
and the bulb would never light. But - if the controller was bad, in that the
output triacs were shorted, as soon as you plugged it in the bulb would light.
Simple, yet effective. No blown fuses or tripped breakers.
I’ve used the same approach with troubleshooting DC circuits in cars for years
using a brake light bulb in place of the fuse for the affected circuit. If
there’s a short, the bulb will light. If not, it will pass enough current for
the circuit to operate normally.
-D
> On Sep 28, 2018, at 7:19 AM, Meade Dillon via Mercedes
> <mercedes@okiebenz.com> wrote:
>
> Last weekend I had a dead short that blew #5 fuse in The White Whale. Took
> me about five minutes to diagnose and fix. No white coat or blue gloves
> needed.
>
> The story: rented a u-haul trailer, when I hooked up my trailer wiring I
> managed to get the euro socket inserted 180 degrees off, and the fuse
> blew. Tried a new fuse, it blew as well. Double-checked the last thing
> that changed (the trailer electrical connector) and found and fixed the
> problem.
>
> I also have an intermittent problem with the HVAC blower motor in my
> 124.131 ('95 E300 Diesel). That took me a couple of hours to diagnose
> completely (because I had to take off all the cowling rubber and plastic to
> get down to the blower motor), but I was able to narrow the problem down to
> either the blower motor or the controller for the blower motor in about 20
> minutes using divide and conquer. I'm really thinking about putting new
> brushes in that motor instead of replacing it.
>
> Kent's stuff: I did buy his guide and special tool for R&R the squirrel
> cage fan blades on a 124 blower motor, and did that job about 12 years ago
> on The White Whale, that Bosh motor is still going strong. I think it was
> a good value, IIRC it was around $25 for the tool and guide.
> -------------
> Max
> Charleston SC
>
>
> On Fri, Sep 28, 2018 at 12:41 AM fmiser via Mercedes <mercedes@okiebenz.com>
> wrote:
>
>> I
>> guess mostly I'm trying to get a glimpse of "normal" from wherever
>> it is that I am.
>>
>>
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