My wife and I recently moved from Los Angeles to the Pacific Northwest. Our 1991 300 SE has never seen cold and the cars heating sytem was never switched on as far as I know. In Tacoma of course a heating system is a necessity of life and after serveral attempts to have it fixed I decided to attempt the repair myself. The technical information on the 126 Model I have goes as far as 1986 and does not reflect on the changes incorporated in the 1991 Model. I am assuming that somewhere under the dash a flap is stuck since there was never any need to operate them in the past. Does anyone have a vacuum and wiring diagram that show the changes for 1991, the last year of model 126. Any hints and assistance with this problem is greatly appreciated.
Best regards

My wife and I recently moved from Los Angeles to the Pacific Northwest. Our 1991 300 SE has never seen cold and the cars heating sytem was never switched on as far as I know. In Tacoma of course a heating system is a necessity of life and after serveral attempts to have it fixed I decided to attempt the repair myself. The technical information on the 126 Model I have goes as far as 1986 and does not reflect on the changes incorporated in the 1991 Model. I am assuming that somewhere under the dash a flap is stuck since there was never any need to operate them in the past. Does anyone have a vacuum and wiring diagram that show the changes for 1991, the last year of model 126. Any hints and assistance with this problem is greatly appreciated.
Best regards

I have a 1989 300SE, and the only time I had a no-heat symptom, the cause was the monovalve. This is located in the area between the firewall and the outer firewall, if that is what it is called. As I recall, it is to the right of the battery. Anyway, there is a rubber diaphram that can spring a leak, and render the part inoperable. The repair was simple, and I was able to do it myself. I suggest running a search on "monovalve," and reading up on it. It may not be the problem, but when I had the symptom, I did some research and found that it is quite common for this part to fail after years of service. As I recall, the repair kit was not too expensive. Good luck.

Joe B.

Hi Rudolf,well first i would say is the same about the monovalve.it Should be beside the battery on the fire wall.i will let you know more wher it is if you want.And second what you should check if you auxilary waterpump is working.if it is not than ,NO HEAT FOR YOU..it is on the right side under the coolanr resevoir.To check if it works,turn on car an heat.touch the pump and check for vibration or huming.if it does not work remove the eletrical connector an ad +and - to the pump.If it just sparks ypu need a pump and you will have heat......But than again it might be the push button unit not switching the monovalve or the hear flaps,but that usally dosent happen.So check the first 2 first .....Harald