Bernard's 175 Page

Experience from over 50 years of KR175 ownership

KR175 Ignition Switch Repair

Bernard Beeston's KR175 ignition switchBernard Beeston's KR175 ignition switch

purchased my KR175 second-hand in 1958 and as it is now 55 years old it is not surprising that from time to time something fails and a few months ago the HELLA combined ignition and lighting switch ceased to control the lights; in rotation the ignition key was not limited to its three positions but just kept turning, with the lights doing silly things. So out it had to come and hopefully be repaired.

Ignition switch and keyIgnition switch and keyBefore removing the switch I disconnected the battery as there is always a danger of shorting something while working on the electrics. I then removed the three screws surrounding the keyhole and worked the switch out from under the dash. I had to disconnect the lead to terminal 56 first as (in my case at least) it was too short to allow the switch out from under the dash. As I removed each lead I labelled it as I didn't know how long it would be before I would be ready to refit the switch.

Switch disassembled: with the replacement insulator in the centreSwitch disassembled: with the replacement insulator in the centreHaving removed the switch the next step was to open it up. First I removed the wire link from between terminals 51 and 30/51 as this was in the way. I then lifted the four tags of the metal cover using a screwdriver and pliers and parted the cover from the plastic body, being careful as there is a strong spring pushing them apart.

On inspecting the inner workings I found that the lights are controlled by a brass contactor that is rotated by the key holder. But the key holder is metal and needs insulating from the contactor; this is accomplished with a moulded plastic insulator and that had disintegrated. It was obvious that all the stress of rotation was taken through the insulator's rim in the hole of the contactor, so I made a larger insulator where the stress was also taken by the contactor's raised edges.

My choice of material for the new insulator was determined by what I had available that was both electrically and mechanically suitable. Choosing 3mm Perspex for the main part as there was room to accommodate the extra thickness, but had it been available 2mm Perspex would have done just as well.

Original insulator: Completely disintegrated.Original insulator: Completely disintegrated.Replacement insulator: In position on the contactor with both on the switch body.Replacement insulator: In position on the contactor with both on the switch body.Repaired unit ready for refitting: Note the red link wire repositioned with neater sleevingRepaired unit ready for refitting: Note the red link wire repositioned with neater sleeving

The rim to fit in the contactor's hole was cut from a piece of 1.5mm thick fibreglass sheet (as used for high quality printed circuit boards). Mine had a copper coating which I had to remove to maintain electrical insulation. If available 1.5mm Perspex could be used, in which case Perspex cement could be used to bond the two parts. However, using dissimilar materials I chose UniBond Repair Extreme Power Glue as this will bond almost anything. See fig 2 for dimensions of replacement insulator. And for comparison, details of the failed part.

Both parts were cut very slightly over size. After gluing together the complete item was carefully filed for snug fits for the key holder and into the contactor's hole and also the outside fit against the inside of the contactor's raised edges.

Before reassembly I smeared the contacts with a thin film of grease suitable for electrical contacts, and made sure that the four tags on the metal cover were free of kinks and correctly aligned to enter their slots on the plastic body.

Switch mechanism insulatorSwitch mechanism insulatorI then assembled the parts in order, making sure that the key holder correctly entered the new insulator and that the tags of the cover entered their slots in the plastic body — not the easiest of operations as this has to be done against the spring's pressure. Note that the cover can fit in four possible positions and only one is correct, this being determined by the position of the earth terminal on the cover.

With the parts held firmly together the tags were bent down using mole grips and finally a hammer and a blunt punch — nothing brutal, just enough pressure with the grips to bend the tags over and gentle hammering on the punch to bed the tips of the tags down. I then refitted the wire link, but for a neater finish, repositioned it to enter terminal 30/51 on the terminal 51 side. There should be a barrier around terminal 30/51 on this side preventing the wire's entry, but in my case this was missing and as it serves no purpose anyway can be removed. I also used a neater piece of plastic sleeving to cover the link.

The switch was now ready for refitting, but first I extended the lead for terminal 56 by a few inches so that it reached out from under the dash. I did this by soldering a piece of insulated wire on to the existing lead and covered the joint with a tight fitting rubber sleeve. All the wires were then refitted in their respective terminals and the switch screwed back into place, the battery reconnected and the switch tested.

One thing I discovered was that the ignition coil terminal (15/54) is earthed when the key is removed. This serves no purpose for coil ignition and is a side effect of terminal 2 being earthed and I assume terminal 2 is used to earth the contact breaker (to stop the engine) when this switch is used in magneto ignition systems.

Published in Take Off, May 2011, Volume 2011, No. 2