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MG MGB Technical - Ignition switch voltage drop mystery

I have a 1971 MGB roadster (US). I have an ignition/electrical problem and this is not my forte so appreciate your patience and suggestions for areas for me to examine. My starter seems to need replacing, but I have a mysterious voltage drop at the ignition switch as well, and wonder if the two are related.
Here is what I know:
1. I turn the ignition and nothing happens. Totally dead, no starter click, no fuel pump, no warning light.
2. In addition, my lights and radio, which are independent of the ignition, do not operate.
3. I have 12.5v at the battery.
4. Ignition switch is brand new (thought this was the issue).
5. Starter relay is new
6. Earth strap is okay, battery terminals clean, battery ground is good.
7. I have thoroughly cleaned the connections on the starter
8. I have 12.5v at the brown wire at the ignition switch.
9. I have 12.5v at the nut of the starter where the brown, battery cable connect.
10. I have 12.5v at the brown wire at the fuse box.
11. I have 12.5v at the white/green wire coming out of the ignition switch when turn to first position.
12. I have only .5v (yes, half a volt) at the white wire when ignition turned to second position.
13. I have .5v on the white/red wire when ignition turned to crank position.
14. A jumper from the brown wire at fuse box to the starter relay (white/red) makes the relay click, but doesn’t start the starter.
15. A jumper from the starter nut (hot) to the starter spade where white/brown goes does nothing
16. I have nothing on the white/brown wire to the solenoid when ignition turned to crank

So, okay I can put in a new starter, but what explains the voltage drop at the ignition switch and the lack of power to the headlights and the radio??
Thank you!
DP Pyle

Hook your meter to any one of the 4 points where you read 12.5V and then turn the ignition on to crank. If you see the voltage drop to 0.5V, then connect the meter to the battery terminals and again turn the ignition to the crank position. If you see a large drop at that point, your battery is past it's pull date. If you don't get a bit voltage drop when you turn the ignition to crank, connect the meter to the clamp on the battery terminals 0V = bad terminals. No voltage drop, attach the meter to the other end of each terminal and again turn the ignition to crank - 0V indicates one or both battery cables bad, no voltage = good cables. Keep working toward the ignition switch until you find doing the same checks at each step until you find the bad link. Cheers - Dave
DW DuBois

I get the same thing from time to time. Giving the connection block where the ignition switch wires join the wiring harness a away it goes. New switch wiring and cleaned every contact more than once but still happens once in a while.
K Hawkins

Dear Dave,

This is very helpful. I'll run these tests. Much appreciated, Doug
DP Pyle

Dear Dave,

Thank you again for your help. You were right, voltage did drop to .5v on the browns when I turned the ignition to crank. I tried to recharge the battery, but it was toast and got a new one. So now with the new battery I have strong 12.5v reads on white/red at crank, but starter still doesn't start. I know the starter is fine because I'm able to hotwire start it by doing a jump between the brown and white fuses (lower left) and applying the white/brown to the lower right fuse (the John Twist technique). That starts the car no problem. So, that tells me the white/brown from the relay onwards is fine. I also get 12.5v on the white/red at the relay at crank. And the relay is new. And the brown to the relay is showing 12.5v. I'm obviously missing something obvious, any new suggestions (Dave or anyone!)?

Thanks again!

Doug
DP Pyle

When you say 'starter still doesn't start' do you mean the starter doesn't try to turn the engine? Or the engine cranks as normal but it doesn't fire up?

1971 was the year these cars gained a starter relay, even though they got the pre-engaged starter a year or to earlier.

If you have 12v on the white/red from the ignition switch but the starter doesn't turn, then you need to check the terminals on the starter relay. With the ignition key turned to START, you should have 12v on the white/red, brown and white/brown terminals of the relay, and 0v on the black terminal. All wiring connected to the relay. The brown and white/brown wires should be thicker than the other two.

White/red and black should be on terminals W1 and W2 (either way round) to operate the relay, brown and white/brown on C1 and C2 (also either way round) to extend 12v to the starter solenoid.
paulh4

This thread was discussed between 29/08/2017 and 18/09/2017

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