Bosch dishwasher mystery

[RustyShackleford]

So my girlfriend knows I'm the clever sort, and asks me to fix her
dishwasher - it's a several year old Bosch, and there's absolutely
no sign of life (of any electrical power getting to it). Power to it is
switched by a SPST wall switch (weird, I think, mine is just wired
straight in, maybe some new code thing ??) and there's another SPST
wall switch in the SAME box, on the SAME breaker, controlling some
lites. The dishwasher switch appears to be wired as a "switch leg".

So I pull the switch plate, and measure 120 VAC across the switch terminals
(with it turned off); seems like it ought to be full line 130 VAC or so. Sure
'nuff, the other switch (on the same breaker) has 128 VAC or so across it
(when it's off). So I pull the cover on the junction box on the dishwasher,
where the AC is wired in, measure the voltage there, and again it's 120
(with the switch turned on, of course).

It the process of doing all this, I have her go out and switch the breaker on
and off a few times - for safety while I mess with the switch, etc.
Then, suddenly, the dishwasher has power and works fine. Voltage is now 128.
I jiggle lots of stuff around, trying to find evidence of a loose connection, but it
seems rock solid. So now she has a working dishwasher, with absolutely no
confidence that it won't stop working at any point.

Ideas for what was wrong and/or how to further diagnose if/when it stops again ?
Thanks.

 

[fbelec]

if the switch is old, just replace it. but put in a 20 amp switch not a 15 amp. it takes the motor load way better. also check all your wire nuts and make sure that they are not only tight but pull them off and look at the wires to see if there is corrosion and if you find nothing and you feel ok going into the main service panel go in and check out the white wire connection and the (if you have one) circuit breaker connection. if you have a rusty panel, water might be finding it way into the panel and rotting the breakers out. if you pull the breaker out and see the rivets that hold the breaker together are rusted it might be time for a new breaker. if it is rusty look closely at the breakers side that faces the top of the panel. if you see a water stain it's definitely got wet and you should be looking outside at the meter setup to see where the water is coming in from. once it's water proof replace every breaker that has a water stain.

 

[RustyShackleford]

if the switch is old, just replace it. but put in a 20 amp switch not a 15 amp. it takes the motor load way better. also check all your wire nuts and make sure that they are not only tight but pull them off and look at the wires to see if there is corrosion and if you find nothing and you feel ok going into the main service panel go in and check out the white wire connection and the (if you have one) circuit breaker connection. if you have a rusty panel, water might be finding it way into the panel and rotting the breakers out. if you pull the breaker out and see the rivets that hold the breaker together are rusted it might be time for a new breaker. if it is rusty look closely at the breakers side that faces the top of the panel. if you see a water stain it's definitely got wet and you should be looking outside at the meter setup to see where the water is coming in from. once it's water proof replace every breaker that has a water stain.

Ok thanks. So it's your guess that there's simply a poor connection in there
somewhere (a bad switch, breaker, or wire connection), and that's why the
weird low voltage was seen across the switch leg ?

 

[Valhalla]

Great advice from fbelec!

Yes, a voltage drop can cause strange and
even unusual events on AC and DC devices.
Time permitted, one could measure the voltage
drop across each connector and component
to determine the actual drops and the cause.

Glad that it is solved. As a sidebar, my wife
loves her Bosch dishwasher! It is very quiet
and has reduced electric and water consumption.

 

[fbelec]

i also forgot to say that everything on the electric supply side could be ok. what i have seen is the plug that attaches to the timer in the door could be starting to fry. also check underneath where the wire comes out of the power junction box follow that wire and see if it goes to a connection that you can see, and if you can see it check for corrosion. even the best of appliances can have problems. that bosch dishwasher has to be one of the quietest if not the most quiet dishwasher i never heard. i've hooked up a few, and a couple were so quiet i didn't know it was running. i heard it fill up then nothing. i had to open the door to make sure it was running.

 

[semipro]

I'm not sure I'd call this one fixed yet. Bad connections are a fire hazard.

Aren't dishwashers supposed to be on dedicated circuits?

I've seen bad connections where the push-in connection on the back of a switch was used instead of the screw terminals to wire high load devices. Not all switches have the push in option though.

Adding to what fbelec said and I'd particularly look for any splices between copper and aluminum wires. You can also use an IR thermometer with the circuit under load to identify bad connections.

 

[Corey]

IMHO, if you are measuring the voltage "across" the switch (presumably at the screw terminals) you aren't actually measuring the switch terminals - you're just measuring what is coming into the switch and going out. As pointed out above the switch could go bad, but generally, that would take a lot of switching while the dishwasher is under load to get a really nice arc across the terminals. If you switch the dishwasher off while it's just setting there idling, that would be an even easier switch than say a 100W light bulb.

So back to the original question - why the low voltage. Well, first of all, voltage isn't a really good indicator of what is wrong with the circuit. Especially modern digital volt meters. They can give a true reading with microamps of current. If the DW is getting 120V @ .00005 amps - it's not going to run. I would say you should be looking for an intermittent contact upstream of the switch. You mention they are on the same breaker, but presumably there are two 'hot' wires coming into the box? That would suggest a junction box somewhere between the breaker and the switch. If there is only one hot to the switch box, then I'd start looking at ground / neutral connections toward the dishwasher side.

One additional test may be to reverse the test and measure the 'line' from the light switch to the 'load' of the dishwasher and line of the DW to load of the lights. If you have 120 from the DW line regardless of which load you use, the problem is most likely upstream. If you get 128 from both lines to the light load and 120 from both to the DW load, the problem is most likely down stream toward the DW.

 

[RustyShackleford]

I'm not sure I'd call this one fixed yet. Bad connections are a fire hazard.

Aren't dishwashers supposed to be on dedicated circuits?

Tract house in rural area of county. Lame-ass inspectors and/or codes.

I've seen bad connections where the push-in connection on the back of a switch was used instead of the screw terminals to wire high load devices.

Oh yes, it was push-in connections, I'll fix that soon. Of course, the low voltage I reported was across
the wires going into the switch, so bad switch connection wasn't the cause.

Thanks for further ideas.