Without looking up why the MOV is there.... The voltage rating for that part is too low. The maximum continuous DC voltage allowed on that part is 28V. It begins to conduct at a little over 32V. It will trip even when the batteries are drained as low as they should be allowed to be.
Any MOV that sees the full pack voltage has to be rated above the maximum charging voltage.
The number after the V in the part number is not the standoff voltage.
Dave
On 5/23/2020 4:55 PM, Ben Griffith wrote:
The more I look at the wiring diagrams in the homeowners service manual, the more I think my repair 7 years ago should be fine. It should work the same as the E20-EA.
Now I'm starting to suspect that the varistor I got isn't the right one. I got a V36ZA80P. I chose that one based on some ET related post I found online but can't seem to find now.
On Sat, May 23, 2020, 3:15 PM Ben Griffith <griffithba+ET@gmail.com> 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.Â
"
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.Â
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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