View single post by Tom Bradley
 Posted: 03-29-2014 04:37 pm
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Tom Bradley


Joined: 07-15-2013
Posts: 178
Attached should be the circuit I used to replace my ignition switch.  This gets rid of the steering column switch entirely and replaces it with some switches installed on the dashboard.  An alternative circuit is to still use the steering column switch to control the MOSFET's.  This reduces the current through the steering column switch to a few milli-amps instead of amps, so it should last just about forever.

The transistors I used were actually IRF4905's, but these do not seem to be readily available any more, so for this drawing I put in some others which have the same characteristics.

Two of these in parallel have an on resistance of about 10 milli-ohms which gives minimal voltage drop as well as have low enough power dissipation so the transistors will not heat up much even under a fault condition before the fuses blow.  I put this circuit in my JH back in the mid 1980's and it has been working with no problems for almost 100K miles so far.

Hope this is of some use.

Attachment: JH ignition switch replacement.jpg (Downloaded 65 times)