Ignition Slave Relay
Apr/2017 R. Kwas

When adding high current loads such as an Overdrive or Lights, etc. to a vintage Volvo, I always remind installers to consider the entire current path.  When doing that, we find that for Ignition powered Loads, if we simply take power from (for instance) Fuse 1 of a 122 (or Fuse 1 of a P1800), that Load Current would also flow through the Ignition Switch.  Not so much of an issue if this is for a modest load like a phone charger or similar.  However, if this is a high power load, like an Overdrive Solenoid or Lights, we would be well advised to spare the vintage switch that additional duty. 

I realize this was the OE arrangement, but knowing it is not a simple (or inexpensive) operation to replace the Ignition Switch as it is part of the Ignition Switch / Armored Cable / Ignition Coil Assy, anything we can do to reduce electrical wear and tear on the switch by routing the additional current through a contact external to the Ignition Switch is a good thing!

We do this by simply installing an inexpensive "Ignition Slave Relay", as shown below.  This relay is essentially slaved to the Ignition Switch, such that is closed when the Ignition Switch is ON, and the actual Ignition Switch is only subjected to the modest relay control current.  The beefy working contact of the relay switches the High Load Current, as shown here:

Ignition Slave Relay Current paths are highlighted.  Reader will notice that the additional current which the vintage Ignition Switch is subjected to, is the modest coil current only (Green).  The high Load Current (Blue) is not going through the Ignition Switch, but instead, is controlled by the high current working contact of the relay.  Ignition Switch is spared the additional high current, so its service life will not be affected.  An external Snubber Diode (A) is shown for the relay.     

Why two different Snubber Diode types?  A high current diode is required only for high inductance loads like Overdrive Solenoid or an Electric Cooling Fan or such. 

Details:  A 4 or 5 terminal relay must be used as an Ignition Slave Relay!  A 3 terminal relay internally connects the coil (relay control circuit) and working contact circuit, so is not suitable, because if the whole point here is to separate the relay coil and high current contact circuits, this cannot be done using the 3 terminal type.  

Comparing 3 and 4 and 5 terminal relays. 


Location of the Ignition Slave Relay could be under the dashboard or under the hood...installer's preference...just remember to protect any wires going through the firewall, or around other sharp metal edges, with sleeving.  Relays are marvelous things...so says your 50 year old Ignition Switch! 



Good Relay info:  https://www.linkedin.com/pulse/automotive-relays-fundamentals-testing-kiril-mucevski


This information is Copyright © 2017.  Ronald Kwas.   The terms Volvo and Bosch are used for reference only.  I have no affiliation with either company other than to try to keep its products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at around the watercooler, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future!

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor, or analogy (see above!) and probably wise-a** comment. 


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