Just a follow-up to say that I've resolved my problem and now both the top and bottom oven will function in the self-clean mode. The advice I had that the problem is probably a failed "Pulser" wasn't really so. Turned out to be a bad sail switch (also referred to as an air switch by Thermador). I didn't even have to buy a new switch. The one in the oven had terminals that were coated with burnt grease, so all I did was remove, clean and reinstall it.
Well, the guy at another web site who led me to believe that the pulser in the wall oven circuit was the culprit might have had good intentions, but now I found out that wasn't the problem after all. As a matter of fact, it was the sail switch, also referred to as the air switch in the Thermador wiring diagram. It's a microswitch connected to a small paddle that is in the airsteam from the blower, and when the circuit is open the oven won't go into the self-clean mode. The logic behind that is that if the fan is not functioning properly, it would be dangerous to have the 1000-degree temperature that develop during the cleaning cycle. So I removed the sail switch with the idea of getting a new one, but once I had in my hand and checked the microswitch operation, the only thing wrong was that the terminals were so covered with baked grease that they didn't conduct current. I cleaned the terminals and re-installed the sail switch, and that solved my problem. Now both the self-clean feature works for both the top and lower oven.