T-Series 74 CB MGB Ignition Switch

[Sebring222]

Group- Has anyone had an ignition switch failure? I have had a condition with no power to the starter relay for several days. I have jumped the R/W wire to the Y/K wire at the seatbelt relay with no luck. I have replaced the starter relay.
New battery. I'm about out of ideas. It was running just fine....then no power to the starter. Push starts and runs just fine.

Jim D

 

[DrEntropy]

Jim, have you got a good schematic? IIRC the e-brake wiring is also involved in that circuit.

 

[Sebring222]

The schematic I have is Auto Wire diagram for my model. Of course I also have the Haynes as well. My e-brake has not been in use for the past year.

 

[Sebring222]

The previous owner installed a Bosch replacement starter that looks like a small "high torque" unit. In fact it has always seemed to spin more slowly than I would expect. I pulled the unit just as a precaution and tested it. Unfortunately it's fine.

 

[DrEntropy]

hmmm... you'll have to start chasin' electrons thru each wire then. From the description of "sudden" no power it sounds like possible discontinuity in the "trigger" wiring... VOM and head scratchin' with that AutoWire schematic as your map.

A thought: Have you tried removing the steering column cowl and slipping the electrical part of the switch out? The plastic "socket" the mechanical rod from the tumbler fits in can become worn to a point the rod no longer rotates it far enuff to make contact with the starter contacts... and sometimes the internal switch contacts corrode and no longer pass current. I'd ohm out the switch isolated from the rest to test it.

 

[Sebring222]

I got to the point where I checked the continuity at each connection on the ignition switch with the key turned to the "start" position. I was able to get 12 volts at each one, particularly the R/W wire. My next step is to remove the two bolts that hold the assembly together and examine the mechanical part of the switch. Thanks for your help

 

[BoneIdle]

I just went through this.

The steering lock has a metal shaft with flanges(?) which mesh with the *plastic* ignition switch. When I removed the cowl from the column, I discovered that the plastic had, for lack of a better term, become wallowed out.

This meant that the key wasn't turning the switch.

You should be able to touch the lower ( white or yellow, I think) wire from you relay to the lowest connection on the fuse box and the starter should turn. In fact you can start the vehicle that way if you get tired of pushing. John Twist has a video describing the process in detail.

You should also be able to hotwire the car using a screwdriver on the back side of the ignition switch. I did it using my patented fumbling around technique.

If you can't get the starter to turn using the above, it may not be the switch, but rather a wiring issue between the switch and relay.

If it is the switch, I suggest you replace the '74 ignition switch and steering lock with an assembly for a 70-72. Not only are the mating surfaces *metal*, but it's about half the price of the same thing for a '74.

If you go that route, the purple wire and the slate wire will not be used. Purple is seatbelt, I believe and slate is for the anti-running on valve. If all of your emission control devices are intact and required, you may not want to go with this solution.

It worked for me.

Good luck.

 

[Sebring222]

Gary- Thanks for the suggestions. I ordered a new ignition switch a couple of days ago. This has really been an annoying problem particularly having to deal with the seat belt module issue. What's odd is that I had no warning at all. It went from running great to not working at all in 30 seconds. Luckily my GT is a June build car rather than having the even more bizarre electrical configuration of later '74 cars. I will post my eventual resolution of this issue. Thanks again!

Jim D

 

[Sebring222]

Garry- I just took a look at the UM video and I was easily able to start the GT using the same technique.
Thanks, again!

Jim D

 

[Sebring222]

The good news is that I was able to get the GT started and running, the bad news is that I lost the fuel gauge, temp gauge, tach, turn signals, etc. Fuses are all fine, maybe its the voltage stabilizer?

 

[JPSmit]

did you check the spade connection itself

 

[Sebring222]

John- The spade connection on the voltage stabilizer? I didn't. Virtually the second that I touched the B/W wire from the starter relay to the bottom of the fuse box and the starter turned over the tach, fuel gauge, temp gauge, etc. went out. The first thing I did was check the fuses and no luck. This is why up until last year the newest MG I own has been a '48.

 

[JPSmit]

Just looked at a wiring schematic and can almost guarantee it is not your voltage stabilizer as your turn signals bypass it. As I look, there is only two places it can be.

First of all, it is the green wires that you are looking for.

1. Check where they enter the fusebox. Clean the contacts - check tightness and clean.

2. There should be three green wires coming from the fuse. one goes to the brake light switch, on goes to the reverse light switch and the third goes to a connector. The third one is the one you want. The connector looks like it should have the one coming in and two coming out. all green - check this connection.

Don't really see what else it could be. Good luck!

 

[Sebring222]

John-Peter- You are absolutely 100% correct! I went back to the fuse block for the 7th or 8th time after carefully cleaning each connection. I pulled the green wire spades again and replaced them. I jumped the b/w wire again to start the GT and the gauges are working perfectly.

Many thanks!

Jim D

 

[JPSmit]

woo hoo - now get out and drive!!!

 

[Sebring222]

It is frustrating to report that after having replaced the complete ignition switch assembly with harness, it had no effect on the starting problem. The key does not start the car. I have to jump the wires at the fuse box.
At this point I am out of ideas.

Jim D

 

[Silverghost]

Jim, you replaced the complete switch WITH harness.....did you triple check all your connections are correct and solid? The P.O.D (Prince of Darkness) sometimes sends his minions to meddle with things when your guard is down! :wink:

Also, did you turn the switch at the switch rather than turning it with the key? It may be possible that something upstream from the switch has failed? May be the rod has gone soft or broken and it only feels like it is working? Food for thought......

 

[DrEntropy]

May be the rod has gone soft or broken and it only feels like it is working?

Won't be the rod, rather the plastic "socket" the rod fits into in the switch. An easy check is to pull the plastic switch out of the mechanical bit and turn it to the "start" position with a screwdriver.