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DonD  
#1 Posted : Sunday, October 17, 2010 8:53:15 AM(UTC)
DonD

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We've had this problem for years but have managed to work around it. With Thanksgiving around the corner it's tiime to take of it once and for all.

The range is a Thermador PRDS364GL that's about 10 years old. The oven does not heat up. It used to be intermitant but seems to be getting worse and now is useless. Doing my research it look like the most likly candidate is the thermostat (Part Details - BOSCH Thermostat, part number: AP2836675). True? Does the control unit (Part Details - BOSCH Control unit, part number: AP3672574) have anything to do with the oven? Doesn't seem like it from the wiring diagram.

How do I go about diagnosing either of them? I have no problem removing/replacing them but would like to be able to diagnose at least the thermostat before buying parts, especially since they apparently stopped making the thermostat. Is there a way to diagnose the thermostat on the bench (or in the oven)?

If the thremostat is good, what else could be the problem? Is there any diagnosis for the control unit?

Any help would be appreciated!
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DonD  
#2 Posted : Monday, October 18, 2010 12:12:05 PM(UTC)
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Could this be as simple as a bad element? What kind of resistance should I be measuring across the element? Would a bad element cause an intermitant problem where the oven heats but doesn't get all the way up to the temperature set on the thermostat?
DonD  
#3 Posted : Monday, October 18, 2010 3:57:07 PM(UTC)
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OK - the elements resitance measures 11.6 ohms. Is that normal?
Servicer  
#4 Posted : Tuesday, October 19, 2010 10:20:08 AM(UTC)
Servicer

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Hello,
A few questions for you:

Does the heat light turn on?
Will it broil?
Did you self clean it before it stopped heating?

KC
DonD  
#5 Posted : Wednesday, October 20, 2010 10:36:30 AM(UTC)
DonD

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Originally Posted by: Servicer Go to Quoted Post
Hello,
A few questions for you:

Does the heat light turn on?
Will it broil?
Did you self clean it before it stopped heating?

KC


Thanks for the reply. The answer to your first 2 questions, to the best of my recollection, is yes. I'll double check tonight.

This definitely didn't happen after a self clean. It's been more of an intermittent ongoing issue that's getting worse as time goes by.

Is the 11 ohm element reading OK?

Is there anything else I should check besides the thermostat? I see lots of people having similar problems with other brands and see they have all sorts of control boards and things like that. I don't see anything like that in my wiring diagrams, or is there something I'm missing?
Servicer  
#6 Posted : Wednesday, October 20, 2010 11:16:35 AM(UTC)
Servicer

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Hello,
Yes, that resistance is almost perfect. The element puts out 5000w @ 240v, which gives us a resistance of 11.52 ohms.

If the heat light is on, your thermostat is working. Power leaves out of the t-stat and energizes the light, as well as the hot wire relay (HWR).

Being that it's intermittent, it could be a problem within said relay, because, aside from the t-stat, the relay is the only device that will cycle during bake. The other components in the circuit are the selector switch and high-temp cutout (HTC). It's not likely that the contacts within the selector switch will be intermittent because they are forced closed when the button is pressed, and the HTC can not be intermittent, nor will it reset itself if it were to trip.

The only way to confirm if it's the HWR is to get to the relay, which is located behind the toe kit, remove the wire that located on terminal H1 and set the unit up for a bake cycle. Within a few seconds, the relay should close and you should find 120vac from the terminal to chassis. (Be careful of the wire, because it should have 120v on it.) If you don't find 120 on the terminal, replace the relay, which is part number 414589.

If you find that there's power on the relay, but not on the wire, the problem will be in the wiring to either side of the element, or in the selector switch (415047).


KC
DonD  
#7 Posted : Wednesday, October 20, 2010 4:09:15 PM(UTC)
DonD

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That, sir, is the most useful and brilliant troubleshooting advise I have ever gotten. Thank you!

Confirmed - broiler works
Confirmed - heat light stays on

I set the oven to 350 for a while. Came back and it was only at 200 and the light still on. Turned thermostat down to 200 and light went out. Now that I know it's not the thermostat I'm very happy since Thermador stopped making the thermostat and couldn't have cared less when I called them to see what I should do if I needed a new one.

Off to check the relay. Thank you again.

Don
DonD  
#8 Posted : Wednesday, October 20, 2010 4:48:21 PM(UTC)
DonD

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OK - this isn't what I expected. I disconnected the wire at H1. It took a few seconds like you said but H1 had 120 vac (117 actually) but there was 120 vac on the wire too!

:confused:

To make matters more confusing the oven is now up to temperature. I wonder if I knocked something in the relay or if the wire wasn't making good contact?
Servicer  
#9 Posted : Wednesday, October 20, 2010 5:16:34 PM(UTC)
Servicer

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Originally Posted by: DonD Go to Quoted Post


To make matters more confusing the oven is now up to temperature. I wonder if I knocked something in the relay or if the wire wasn't making good contact?


That happens sometimes. If it does stop working again, it's most likely you've got an intermittent hot wire relay.

The fact that it heated initially helps to prove that it's the relay just not wanting to close the contacts after a while.

Hopefully it continues to work, but, if not, you now know how the system works.

Good job.

KC
rogerh113  
#10 Posted : Thursday, October 28, 2010 7:03:26 AM(UTC)
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Hello,

My thermador has exhibited the same issue twice before, and it is always the same thing. Mine is not the same model, but of the same vintage. There is a microswitch adjacent to the fan that tells the oven that the fan is working. The element will not heat up if the fan is not working. According to the tech that just repaired mine (again), the fan never fails but the microswitch does..... On my single oven, the two top covers have to be removed, a single screw removed, and the switch replaced. The oven is back to service. I would not be surprised if yours is not the same problem.

Regards -- Roger
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