MGB Wierd starting problem - 77 mgb

[cfiaime]

Greetings kind people...

I'm again trying to get the 77 roadster running. When I last posted, I was attempting to rehash the fuse box and ignition relay. I think (dangerous words, those) the fuse box and ignition relay are working properly. I think.

I tried to start the car and the starter did not energize. It looks like the starter relay (aft of the fuse box on the right wing of the car) was bad. Hence, I replaced it with a generic relay from Radio Shack, powered up the electrical system, turned on the key, and the starter immediately ran. Oh, the key was NOT in the START position. This is not making my day.

Well, I figured the relay may not be appropriate for ignition use, so while I was in Kansas City last week I stopped at Victoria British to get the official relay. I put it in the car and the same thing happened. When I twist the key to START, the starter ceases to function.

The red/white wire has power when the key is on. From the schematics the red/white wire ought not to be powered unless the key is twisted to the START position.

I have not touched the ignition switch, in fact, I haven't removed the bezel from around the switch, for a long time. Last fall the car started properly.

Any ideas? Oh, I have consulted the electrical system book and the Advanced Wiring schematics. The wiring system on my car is the same as the later 1978 cars, so it seems.

Many thanks.

jeff williams - cfiaime@cfiaime.com

 

[eschneider]

what happens if you bypass the relay?

there should be a red/white wire going to the relay from the switch, and another red/white wire going from the relay to the starter solenoid. Unplug the 2 wires from the relay, connect/jumper them and see if everything goes back to normal?

I'm not sure what the lucas guys were smoking when they decided to activate a solenoid with a relay, given a circuit that has a momentary current of roughly 1 amp......

 

[Kimberly]

Look at the wiring diagram for a '78 and later from https://www.advanceautowire.com
There is a diode in the brake light circuit that connects the white/red wire to the green/orange wire. If the diode has failed then the starter will engage if the ignition switch is in the on position and the parking brake handle is pulled up. Try releasing the parking brake and see if the starter quits trying to engage when the ignition switch is in the run position.

If your car is equipped with a EGR service counter and lamp, it can also cause the same problem if the diode to the EGR service counter has failed allowing the green/pink wire to energize the white/red wire.

If the diodes are the problem, just remove them and insulate the unconnected wires so they don't short. All the diodes do is make the EGR and brake light on the dash illuminate when the ignition switch is in the start position (a bulb test feature).

You can use a generic Bosch style 30amp relay to replace the starter relay or the ignition relay. They are available at most auto parts stores for less than five dollars.

Wiring for the starter relay
Pin 85 - black wire - ground
Pin 86 - white/red wire - power from the start position of the ignition switch.
Pin 30 - brown wire - constant twelve volts
Pin 87 - white/brown wire - connects to the starter solenoid

Wiring for the ignition relay
Pin 85 - black wire - ground
Pin 86 - white wire - power from the run position of the ignition switch.
Pin 30 - brown wire - constant twelve volts
Pin 87 - white/brown wire - connects to the fuse box.

 

[cfiaime]

Many thanks for the tips. The parking brake handle was down when I tried the starter, but that doesn't mean that the various switches, etc., are properly adjusted.

Ok, where do I find the EGR counter for the car?

It looks like I may have a long Saturday of work on the old girl next week.

 

[Kimberly]

Many thanks for the tips. The parking brake handle was down when I tried the starter, but that doesn't mean that the various switches, etc., are properly adjusted.

Ok, where do I find the EGR counter for the car?

It looks like I may have a long Saturday of work on the old girl next week.

Follow the speedometer cable. If fitted, the speedometer cable from the speedometer will connect to it. You most likely don't have one, but you never know until you look.