The MGA With An Attitude
FLASH TO PASS (headlight flasher) Switch - AT-113

Momentary bat handle switch This is an original optional accessory part for the MGA. Bump that bat handle 10 degrees in any direction and this switch makes a ground contact. This was used with a relay and a small wiring sub harness to allow the MGA headlights to "flash to pass". For factory installation the dash was commonly drilled to mount this switch just above the turn signal switch. For aftermarket owner or dealer installation it could have been located just about anywhere in the dash, but commonly a finger's reach from the steering wheel.
Momentary bat handle switch
Since original style Lucas bat switches are now so rare, you can use the one shown here. It is spring loaded one direction; mount it either side up. See Holden part no. 020.145

The related relay may have been screwed to the firewall or high on the kick panel behind the dash. Flash to pass relay 3H1911 installed If you believe the SPL illustration (below), the factory would have mounted it on the heater shelf near where the harness branch goes to the foot switch. This picture shows it mounted in front of the heater box on a Twin Cam car. Interesting to see it has a plastic cover (not listed separately in the SPL) to protect most of the hot terminal bars on the relay.

The original type switch is Lucas 23A, part number 31898 or 31898A.
The relay is Lucas SB40/1, part number 33135 or 33169.
There must be a small wiring sub-harness as part of the installation kit (as in the photo above), but this is not listed in the SPL, and the assembly is not listed as a kit.

From the Lucas parts list: Model SB40/1 = Single bobbin, 2-hole strap fixing. All listed here are 12-volt, 4-terminal (but not sure terminal arrangement is the same). PART SPECIAL FEATURES USE WITH 33135B 4-terminals W.T. horns 33157A 4-terminals marked C1, C2, W1 and W2 Cover marked 33229A 4-terminals

The relay needs an always hot power wire from control box A1 so it can work when headlamps are off. The relay needs a wire going to the high beam circuit (probably on the foot switch high beam terminal). It needs a wire going to the dash switch to be switched to ground to actuate the relay. The fourth wire is a low power feed from the fuse box A4 terminal, so the circuit will only work when the ignition switch is on.
flash to pass illustration from MGA 1600 SPL
        The SPL has these part numbers listed (all items marked "Optional Extra"):
45 - BHA4116 - Switch, headlamp flasher (N. America)
46 - 3H1911   - Relay, headlamp flasher (N. America)
47 - PMZ0308 - Screw fixing relay (2 req'd)
48 - FNZ103  - Nut for Screw
49 - LWZ203   - Washer for nut (spring)

flash to pass switch mounted flash to pass relay
These pictures show the FTP switch and relay used in the PJB147 (prototype Twin Cam) car as factory issue. Note difference in relay from the SPL illustration. This particular relay may have been something (anything handy) the prototype builders picked up from local stock, not necessarily same as production parts, or it may have been a field replacement.
flash to pass circuit diagram
This is a composite of the Flash To Pass circuit diagram as shown in the 1600 and Twin Cam workshop manuals. The primary power for this circuit is a 16 gauge brown/blue wire from control box A1 to relay C1. The reason it draws power from the control box terminal is because it will pull an extra 10 amps to light the high beams while the low beams are still on, not wanting to add this load to the same wire that is supplying the ignition switch and lighting switch.

Relay power output from C2 goes to the high beam circuit. This can connect at either end of the blue/white wire running from the dipper switch to the twin tube snap connector at RF corner of the car. Relay W1 gets a green wire from the fused ignition circuit. This is a low current wire, so it can connect to fuse block A3 or to the back of the fuel gauge (if relay is mounted behind the dash). The grounding signal wire from relay W2 must run to the new dash switch. Being switched to ground it should be black with a stripe. A blue stripe would be appropriate (although it is not labeled on the schematic). If the dash switch is not grounded directly on the dash panel, then it will need a black grounding wire from the switch to the back of one of the instruments (where it can connect to another black harness wire).

Addendum November 2011:
On 09 Nov, 2011, the two pieces shown below sold on for 74.99-BPS ($119.47 USD). In Aug and Sep prior, identical relays described as "Austin Healey overdrive relay" sold on for 24.50 and 23.50. (The original switch then might be worth about 50.00 these days).
flash to pass switch flash to pass relay
The auction description was as follows:
MGA FLASH TO PASS (headlight flasher) Switch + Relay
This is an original extremly rare optional accessory part for all MGA models including the Twin -Cam. The part no. is marked as 31898A and is in extremly good condition. Included is a period Lucas SB 40 relay date stamped 1-58. I have not tested this relay but I obtained it from Peter Wood, so have no reason to believe that it will not work. The switch, ordered as a Factory option, will have been mounted just above the turn signal switch. Such as on the Twin Cam prototype PJB 147. A really nice job of installing this optional accessory will be achived by having a small wiring sub-harness cloth covered braiding made up. Or you could use contemporary wiring!

Below left, thought to be an original relay from a late MGA 1600-MK-II, July 1961 production date. This is the first one I've see with the plastic case.
Below right, a recent production relay being sold via eBay. Close, but not quite concours, missing the small numbers near the flange.
flash to pass relay, original flash to pass relay, reproduction

Now brace yourself. In March 2012 this New Old Stock, original Lucas 23A (31898A) switch sold for a whopping 201.53-GBP, $319.70-USD. For that it merrits a larger picture.
flash to pass switch, original
flash to pass switch
flash to pass switch
This FTP switch with screw post terminals may be a Wipac switch.

On 7/23/2017, Dominic Clancy wrote:
"A result of rewiring and sorting out a friend's car and trying to get the flash to pass system working again after the original relay fell apart. The relay has to be one with C1/C2/W1/W2 markings. The image below is taken from the Lucas manual and correctly lists the configuration of each of the relay types. There are some 6RA relays that have the correct pin configuration, but the later 6RA numbering scheme is different and under the new numbering schemes the relevant suitable relays appear to be SRB121, SRB111, SRB113.
lucas relays The only available reproduction SB40/1 relays are all C2/C3 and don't work with the standard wiring setup placing the C1 connection at C3. The only source I found listing the correct terminals delivered me a C2/C3 item which we have tried but doesn't work. It looks as if I am going to have to use 6RA innards and epoxy them into the casing of the old SB40.
The correctly wrapped and coloured harness is available from Autosparks in the UK -- HEADLAMP FLASH HARNESS - MGA 1600 -- SKU: 5951

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