Re: (ET) Varistor keeps burning up

["Harold Zimmerman - Clean Power Supply" <cleanpower mailbug com>]

 Ben,
 Without thinking the whole thing through I did come up with a couple of 
things to look at. 
 The tractor will work fine without the varistor, it is hooked up so that 
it is directly across the positive and negative voltage and is meant to 
absorb voltage spikes. I think you are on to something when you realize 
that there may be current flowing through the varistor and it is not meant 
for that. Yes, there is a good chance that it was open all these years and 
now since you replaced it the problems start. The 2nd thing is, is your 
varistor rated properly?
 I never recommend mixing the old style and new style F/R contactors. 
Unless you are on a strict budget I would suggest replacing the odd 
contactor and then replacing all the suspect diodes and the varistor.
 Hope my ramblings are of some assistance.
 Sincerely,
 Harold Zimmerman
 Clean Power Supply
 Ephrata PA
 717-859-4234

On May 23, 2020 at 3:14pm -0400, you wrote:

>I've got an E20-DA that I've been using for mowing and snow blowing for 
>the
>past 11 years with relatively few problems.
>Last weekend after a couple laps around the lawn I smelled the 
>unmistakable
>hot electronics smell. I looked behind the panel under the hood and saw
>that the varistor was just two little legs and on card 4 the CR413 diode
>was scorched. I tested the other diodes and found that CR414 was open and
>CR404 was shorted. I figured the varistor died, as they apparently do, and
>its absence caused the diodes to die too.
>This morning I replaced all 4 components. I tested it out in neutral with
>the panel still open and when I let off the pedal there was a pop and a
>flash. The trace between terminal 7 and the varistor on card 3 had burned
>up. The one in the other side looks like it's not far behind.
>Rather than try to patch the traces I decided to pull wires 1 & 7 off of
>the card and solder the varistor to them. I tested again by just turning 
>on
>the motor for about a second two times.  I noted that the varistor already
>felt pretty warm. I left the shed for about a minute to grab my service
>manual and the moment I walked back in the tractor motor started running 
>on
>its own (fast) and smoke started streaming off of the now swollen 
>varistor!
>I turned off the main disconnect and it stopped.
>
>Before I go any further I should explain a repair I made in 2013 because I
>think it could be relevant. One of the 1F/2R contactors burned out and I
>replaced the pair with a newer style contactor from a pile of parts 
>someone
>had given me.  1R/2F is still the old style tandem contactor.  Here is how
>I did it, from an email I wrote at the time:
>
>"
>I attached the new contactor to the top part of the bracket that the 
>double
>contactor had been on, so wires 11 and 14 would reach the coil lugs.  Wire
>6 from the 2F contactor was a little bit too short but I swapped in a
>slightly longer one that came attached to one of the new style contactors.
>I left wire 14 disconnected (capped and tied out of the way).  The only
>difference I can see is that the contacts that replaced 2R will be closed
>when the motor isn't running.  I thought that might cause problems with 
>the
>wires that come off between the contactors (to the varistor, motor field
>windings, and switches 1A,2A,3A) but it looks like that's how the newer
>tractors are wired too.
>"
>
>I've been mowing and snow blowing with no problems for 7 years since that
>repair, yet that "only difference" I wrote about seems mildly concerning
>given what's going on now.
>From looking at the wiring diagram it seems to me that current is flowing
>through the toaster resistors, the contacts that replaced 2R, the motor,
>and then the varistor. The varistor isn't meant for that so it goes poof.
>But why is this happening now when it's been working fine for 7 years?
>Maybe the old varistor was failed open the whole time? Visually it looked
>fine every time I opened the panel until last weekend.
>Was I just lucky that my repair worked for as long as it did?
>
>Thanks,
>Ben Griffith
>
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