[Tigers] Light Switch Outputs

Mon Dec 1 21:53:22 MST 2025

Hi Steve, 
Graham got his issue resolved (at least for now) but here's more or less what I sent. 

Firstly, the wire gauges used in the Tiger wiring harness tend toward the small end of comfort zone, especially when many of us end up putting in more powerful accessories. 
55W halogen headlights draw about five amps each; an aftermarket ignition system might draw ten amps. Stereos can draw a LOT. The horns are supposed to be adjusted for about 3.5 amps each (if I recall correctly) but if they are not in adjustment you might see five amps each. 
All that adds up. Another problem can be that the lights are not fused from the factory; we can inspect the factory wiring diagram (online link: [ http://tigersunited.com/resources/wsm/tt-images/MKIBIGwiring.jpg | http://tigersunited.com/resources/wsm/tt-images/MKIBIGwiring.jpg ] ) to get a sense of where there might be inter-related problems, and what switches will take a big hit when the various electrical accessories are used. 

This chart shows the recommended current limits for various gauge and length of wire. 
[ https://wiringproducts.com/pages/wire-amperage-capacity-chart | https://wiringproducts.com/pages/wire-amperage-capacity-chart ] 
The Alpine and Tiger harnesses are pretty skimpy, especially if (over time) some of the wire strands end up breaking near connection points due to vibration and other wear and tear. 

If I was wiring stuff from scratch I would use 12 gauge for the headlights where the wires are serving both lights. Where the run splits, you could go to 14 or even 16 gauge if the run is short - that would depend on how you are running the light wiring. I re-routed the headlight and marker light wiring down the right fender, branch to the right headlight and marker assembly, and then I ran the left headlight, horn, and marker light wiring along the lower radiator support. 

If you are going to do relays (and add fuses) there are many options. But, the most effective and least invasive thing to do would be to add a three-relay block such as this one from Painless: [ https://www.summitracing.com/parts/prf-30107?seid=srese1 | https://www.summitracing.com/parts/prf-30107?seid=srese1 ] 

One relay serves the headlights, one for the horns and the last one can be used for the ignition circuit (basically, it serves everything coming off terminal 2 of the ignition switch). 

The wiring instructions for this relay block are here: [ https://static.summitracing.com/global/images/instructions/prf-30107-30108instruction.pdf | prf-30107-30108instruction.pdf ] 
With reference to those instructions, the ignition and the headlights are wired according to the Figure 1 "Power activated Circuit" schematic, while the horns are wired according to the "Ground activated Circuit" (figure 2) except that the left side (RED) battery connection (for this relay only) must be rewired so that it connects to a good chassis ground. 

More specifically: 
Ignition Relay: 
The activation switch is the ignition key switch. 
Remove the #2 terminal connections (all the white wires) and connect them to the (solid color) wire of the relay harness (middle right wire in the Figure 1 drawing). 
The upper-right RED/striped trigger wire connects to the #2 terminal of the ignition switch. 
The middle-left RED wire should connect to the supplied circuit breaker and it, in turn, should connect to a good 12V source. 
If you have an ammeter, it connects there; otherwise you could use the battery terminal of the starter relay, or the 12V (brown wire) B terminal on the generator voltage regulator. 
The black/stripe wire connects to a good ground near where you mounted the relays. 

Headlight relay: 
The activation switch is the headlight switch. 
Remove the BLUE wire that runs from the headlight switch to the high/low beam footswitch, and connect it to the (solid color) wire of the relay harness. 
The upper-right RED/striped trigger wire connects to the terminal of the headlight switch from which you removed the BLUE wire. 
Black/stripe wire connects to ground 
Middle-left RED wire connects to the circuit breaker just as the ignition switch did. 

Horn relay: 
This is a ground switched, ground switching circuit so it is not the same as the previous two. 
The activation switch is the horn ring. 
The upper-right (in the Figure 2 drawing) RED/stripe wire connects to the circuit breaker output (where the RED wires from the other relays are connected). This ensures the horns will work irrespective of whether the ignition is on. 
The black/stripe wire is your trigger wire and it connects to the purple/black wire that leads to the horn push in the steering column. You need to disconnect the wire at a convenient point there. 
The RED wire (left side of diagram) may be pre-wired so that it connects to the RED wires of the other relays. If so, it needs to be disconnected from that circuit, and instead connected to a good chassis ground. You can usually use a small screwdriver to undo the push tab on the spade connector in the relay socket, then push the connector out; after that you can undo the crimp and either use a new spade connector or re-crimp the spade lug onto a new piece of (black) wire. Make sure you cut off or otherwise insulate the cut-off portion of the old red wire since it will have 12 volts on it when the battery is re-connected. Re-install the spade lug into the proper slot of the relay socket and attach the other end of that wire to a good chassis ground. 
The RED/stripe output wire now connects to the other part of the purple/black wire that leads to the horns. 

The above description applies to the Painless kit and the instructions they have posted... there are lots of other possible relays and kits that could be used for this purpose. If you have questions about a particular kit you are working with, let me know. 
In general, don't forget to practice electrical safety. Disconnect the battery before starting. Insulate all connections and make sure connections are crimped tight or bolted down. I put pieces of rubber vacuum hose over the ends of those circuit breaker screw terminals, to make it less likely a stray wrench or screwdriver touches a hot circuit. That circuit breaker (in the Painless kit) is rated at 50A, but the wiring at the relays and downstream is not, so you may want to add lower amperage inline fuses to protect the wiring. The headlights could be fused at 20A and the horns at 15 amps; this is about double the expected current in normal operation, but if something causes a short then it will blow that fuse before the wiring overheats. 

Cheers, 
Theo 

From: "STEVE MURPHY via Tigers" <tigers at autox.team.net> 
To: "tigers" <tigers at autox.team.net> 
Sent: Monday, December 1, 2025 12:46:39 PM 
Subject: [Tigers] Light Switch Outputs 

I'd be interested in the diagram for adding a relay to the headlights. Horn too, if you have that. 

Thank you, 

Steve 
> On 12/01/2025 2:00 PM EST tigers-request at autox.team.net wrote: 
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> 1. Light Switch Outputs (gharlowe at comcast.net) 
> 2. Re: Light Switch Outputs (Theo Smit) 
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> Long story short, I decided to take an inventory of how my Tiger’s lights are reacting in response to the dash light switch. In mid-position (marker lights only?), the two front white marker lights come on and the bottom tail light lenses illuminate. Brake pedal application doubles the brightness of the bottom lens. 
> 
> 
> 
> With the dash light switch in the fully ON position, I get headlights, but nothing in the yellow/white lenses in the front; however, I also get nothing in the rear lights (whatsoever!). This doesn’t seem right or safe, but I’ve forgotten after so many years of daylight-only driving which lights are supposed to be illuminated with the switch in mid-position vs full position. 
> 
> 
> 
> To aid in troubleshooting, what should happen front/back in mid-switch position vs full position? 
> 
> 
> 
> Thanks and Happy Holidays to you! 
> 
> 
> 
> Graham Harlowe 
> 
> B382001466 
> 
> 
> 
> That sounds like your switch is not working properly anymore. If you go under the dash and switch the position of the headlight wire and the taillight/marker light wires, you should find that the behavior is reversed. Or maybe the switch will just fall apart... they are not especially sturdy, and they can get distorted because of heating due to all the light current flowing through the switch. 
> 
> If/when you replace the switch, you should consider wiring in a relay to power the headlights. That reduces the current through the switch from 10 amps to less than one amp. 
> 
> If you have questions about that I can draw up a diagram. 
> 
> While you are at it, also make sure that the ground connections to all the lights are good. 
> 
> Cheers, 
> Theo 
> 
> 
> > On Nov 30, 2025, at 4:14 PM, gharlowe--- via Tigers <tigers at autox.team.net> wrote: 
> > 
> > 
> 
> > 
> > 
> > 
> > 
> > All – 
> > 
> > 
> > 
> > Long story short, I decided to take an inventory of how my Tiger’s lights are reacting in response to the dash light switch. In mid-position (marker lights only?), the two front white marker lights come on and the bottom tail light lenses illuminate. Brake pedal application doubles the brightness of the bottom lens. 
> > 
> > 
> > 
> > With the dash light switch in the fully ON position, I get headlights, but nothing in the yellow/white lenses in the front; however, I also get nothing in the rear lights (whatsoever!). This doesn’t seem right or safe, but I’ve forgotten after so many years of daylight-only driving which lights are supposed to be illuminated with the switch in mid-position vs full position. 
> > 
> > 
> > 
> > To aid in troubleshooting, what should happen front/back in mid-switch position vs full position? 
> > 
> > 
> > 
> > Thanks and Happy Holidays to you! 
> > 
> > 
> > 
> > Graham Harlowe 
> > 
> > B382001466 
> > 
> > 
> > 
> > 
> > 
> > _______________________________________________ 
> > 
> > tigers at autox.team.net 
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