The Rolls-Royce and Bentley Technical Library
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MT11 Replacement Diodes for All Controls - Aircon - 1N400x and EM51x-series - Any is Suitable
Click here for the Data Sheet    

Low-power silicon rectifiers (diodes) are fitted throughout SY and SZ cars.  They are used in numerous applications, from the airconditioning - climate control to interior door lamp operation.  They are also used in window lift systems modified to overcome slowness due to aged wiring  - see
Slow Windows No More.  Diodes are located in the fuse panel, and elsewhere, on dedicated diode boards and individually,.  

The minimum recommended diode rating is 0.5 Amp and a peak inverse voltage (PIV) of 20 Volts.  Higher ratings are equally compatible but unnecessary.

The genuine Crewe diodes leave something to be desired in terms of quality and life.  They are rather expensive when bought from Crewe reflecting the very low sales volume.  The 1N4001-1N4007 / EM513 - EM518 series diodes are ideally suited in these applications.  They have a nominal current rating of 1 Amp, and all have a more than sufficient voltage rating.  They offer greatly improved life and reliability, cost just a few cents each, and are available anywhere from Dick Smith to electrical wholesalers.  The 1N400x-series diodes are slightly smaller and more convenient to fit than the originals despite their improved ratings.

Click the link above for the 1N400x, EM51x Diode Data Sheets here.  Any diode quoted on the sheets is equally compatible.   The most common diode available is the 1N4007. 

Note that SZ cars from 1987MY chassis 20,000 onwards have a diode board located in the fuse compartment.  All diodes on the board are 1N4007 etc except for one physically larger one, a Zener diode, which is a 1N5341B 6.2V 200mA 5W.  It may be replaced for good measure too.  See the 1N5341B Data sheet here.