View Full Version : TR6 '74 TR6 ignition switch woes

02-04-2007, 06:06 PM
Hello, listers, an hour before the BIG GAME (not trademarked), and I've spent all weekend trying to get my new-to-me '6 to start. Here'a a brief synopsis:
The car has started in the past. Sometimes it would start easily, sometimes jiggling the key would be needed. The back of the ignition switch was physically separating; one of the three metal tangs had disintegrated, so I ordered the electrical part of the switch from Moss, TRF was out.

Further investigation revealed a burnt white-pink wire in the car side of the electrical switch, all through the wiring loom, and dead ending by where the radio would have been. This is the power wire for the radio. I couldn't see much more damage to the wiring, except for that wire; I carefully removed the plastic wrap from around the loom, cleared as much of the burnt wiring as I could, and didn't hook up anything to that position in the ignition swith connector.

Needless to say, the car won't start... all I get is the red ignition light on. Blowing up Dan Master's nice color PDF wiring diagram, (11x17, 6.50 at Kinkos!), I checked solenoid wiring, coil wiring, etc. and I have continuity.

I next began to suspect the switch itself, again. Page 559 of the Bentley shows electrical connections when the key is in it's various positions. On the key position III, I can't get continuity from 2-1, or 3-1, on the back of the electrical switch.

So, what's next? Do I have a bad electrical part from Moss? Could the mechanical part of the switch be bad? ( I don't see how). Of course, the unknown wiring problems don't help any; has any other wire been burned/broken I don't know about. Is there a wiring work around I can do just to start the car... I haven't tackled carbueration, changing hoses, rebuilding fuel filer, etc. Would Dan Master's Electrical Bible help me any here, and Dale can answer here, how horrible is putting in a new wiring harness... Thanks for any and all suggestions, I'm thouroughly disgusted right now....

02-04-2007, 08:55 PM
Welcome to the world of the '74 TR6, where starting depends on the whims of the dreaded Seatbelt Module (SBM). To meet US regulations, Triumph inserted a Starter Relay between the IGN switch and the starter solenoid. The Starter Relay is controlled by the SBM, which used early 70's technology in electronic logic to decide it should let you start the car, depending on seat and seatbelt sensors.

Open the hood and listen/feel for a click in the starter relay as you try to start (the starter and horn relays are mounted together by the fusebox) If it does not click, the SBM may not be passing 12V to it. Using Don Master's excellent diagram, you'll see the SBM is the 12-pin device at the bottom middle. 12V from the IGN switch goes in at pin 11 on a white/red wire and hopefully out at pin 12 (white orange) and on to the starter relay.

Do not use a short piece of solid copper wire to jumper those two sockets. This would completely bypass this important safety feature (that was changed to a buzzer/light for '75). If the relay does click, check that it's sending 12V to the starter solenoid (big White/Red wire). I kept having problems with the Lucas relay so I replaced it with a headlight relay. I've had no problems since, but then I also rebuilt the starter and dismantled the solenoid and cleaned the contacts. Also, check the connections where the IGN switch connects into the main harness.

I found lots of gremlins in this area, but eventually worked them out. Is your '74 pretty original?

TR6 CF13816U
TR6 CF27006U

02-05-2007, 03:59 PM
Thanks, Jeff, for the response. Ya know, I did see the SBM on Dan Master's Wiring Schematic, but stupidly thought, since the seat belts were disconnected, I didn't need it, The last time I dealt with a seat belt interlock was on my girlfriend's '74 Chevy Vega; I loved them both!

I'm going to have to investigate this SBM; I don't think I've even seen it in the car. My '74 is pretty original, I don't have seats in it right now, but the original belts are in there, and the original seat belt wiring is curled up in the back of the cockpit. Do you think I'd have any luck by reattaching the original wiring back together, to see if it'll start? What kills me is that I've started it w/o the seatbelts; now it's just taunting me. I'll also investigate the starter relay. I only went so far as to check continuity in/out, now I'll check for 12V. Perhaps Paul (Brosky) can chime in here and tell me if he still has his interlock hooked up, or if he devised a go-around. I'll also scare up a new relay, just in case; all my wiring there seems OK,though!

Thanks again for the heads up; I'll like to fire this car up again, one day!

02-05-2007, 05:39 PM
All I ever did with my 74 is to unplug the wires going to each seat. I taped them up and that was the end of that, so unless you have a problem with another circuit that the SBM is controlling behind the scenes, as Jeff seems to be suggesting, we are the same, car for car. Quite possible on a 33 year old car.

Like Jeff, I have a locally rebuilt starter, and all new under hood relays. Additionally, I replaced both battery cables and all connections are good both electrically as well as mechanically.

The only other electrical component that I disconnected was the annoying ignition buzzer but removing the wire leading from the ignition buzzer to the left front door jamb switch.

02-05-2007, 11:03 PM
Thanks, Paul, I planned to have both the starter and alternator rebuilt; I've got a good shop nearby work. All new relays is a good idea, it's a little grimy under there. But Jeez, I'd like to have driven it more than the block and a half to fill the tires!

It seems this car is fighting me to come back to life! My '3A seemed positively helpful in every way to return from slumber. This car throws up roadblocks, and breaks at every opportunity.... I really think the 3A is a better made car; or at least, it's simpler!

02-06-2007, 12:26 AM

Don't be discouraged. Once you sort things out you'll be good to go!

02-27-2007, 10:40 PM
We have ignition! https://www.britishcarforum.com/bcforum/images/icons/default/lightbulb.gif After spending the better part of a month of Saturdays trying to get my new-to-me '74 to start, I gave up and brought in the big guns! A couple fellas from my local club recommended a guy, who graciously came to my house today and poked around the electrical system. It seems I had a fair number of problems: Bad connections on the car side of the back of the (new) switch, a bad wire from the SBModule to the starter relay, and no grounds from the engine to the body chassis. He also looked at the turn signals, and I got to see the British Wiring bullet crimper in action... COOL! I'd have been chasing those wires for quite a while longer...impressive to see a pro in action!

Finally, I can start working on this car. On a short drive for a splash of gas, it steers very lightly, compared to the 3A, shifts nicely, and stops well. I still have a leaking rear carb to sort out, and the temp sender isn't working, but I've got great oil pressure. I can't seem to close the hood; I'm sure the shut pin is misaligned. After putting in Mark Macy's very nice backup release, I find I have the opposite problem!

It's just nice not to have a LBC lump in the garage anymore!