Why Won't She Turn Over and Start

Technical MGB discussion
Madge79
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Why Won't She Turn Over and Start

Post by Madge79 » Fri May 15, 2020 12:32 pm

Electrical woes, 79 Roadster, when you try to turn her over i hear the starter solenoid buzz,, it is like the battery is flat, but i have changed the battery, starter motor, and both the relays i am at a loss now ! Any suggestions as to what else it could be ? what about the ignition switch? HEELLP

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Charles Farran
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Re: Why Won't She Turn Over and Start

Post by Charles Farran » Fri May 15, 2020 3:56 pm

Hi,
I presume that when you turn the ignition on all you hear is the solenoid clicking / buzzing & the starter motor doesn't turn at all? If everything worked before you changed the starter motor ,then look at what you have disturbed / changed in the process. Is the earth lead from the battery to the side of the battery compartment in good condition & the connections on the leads that connect to the battery . They can look OK from the outside , but under load during starting, due to resistance from corrosion are unable to carry the high current. (I assume you changed the starter motor because it was faulty itself rather than the electrical supply to it). Is the earthing strap from the back of the engine block to the chassis in place & in good condition.The starter motor gets its' earthing via this strap. When you turn the ignition switch to the start position , with the headlights , do the headlights dim as they should when the starter should be cranking, which would indicate the high current been drawn by the starter motor?
I take it you have reconnected the two browns & the main positive feed from the back of the car to the back of the starter motor solenoid (easily done - i should know). You say you have replaced the starter & ignition relays that are on the inside wing - were the originals faulty / replacements faulty?
(The noise you here suggests the starter relay is working , but check that the black wires on both the relays are making good contact on the relay terminals (& indeed all the other connections to the relays). A bit more info may help either i or other members to help further.
Cheers,Charles
1980 Roadster

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Charles Farran
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Re: Why Won't She Turn Over and Start

Post by Charles Farran » Fri May 15, 2020 4:19 pm

Have you reconnected the wires to the relay(s) to the correct terminals ?
1980 Roadster

Vic Butler
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Re: Why Won't She Turn Over and Start

Post by Vic Butler » Fri May 15, 2020 4:51 pm

Is the replacement identical to the one you replaced or is it one of the modern small high torque type starters? Some don't have all the connections for the bypass for the ballast resistor circuit.
It does sound like the starter motor is not being activated by the relevant solenoid. One is activated only when the ignition key is in the start position and this one supplies a 12v supply to the coil and the other is the "run" one which runs the coil supply via the ballast resistor when the engine is running. The solenoids are interchangeable.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Madge79
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Re: Why Won't She Turn Over and Start

Post by Madge79 » Fri May 15, 2020 8:39 pm

Hi

Thanks for the replies, when I took The starter motor out I decided as it was 40 years old I may as well replace it. The connections are fine cleaned and they are nice and tight. The more I think about it the more the only part net checked or renewed is the ignition switch, tomorrow I will try and hot wire the car and see if by passing the ignition works.

Vic Butler
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Re: Why Won't She Turn Over and Start

Post by Vic Butler » Fri May 15, 2020 9:07 pm

Does everything else work through the ignition switch, indicators, wipers, fuel pump etc? It's unlikely to be the switch if the solenoid buzzes.
The starter motor on my 1977 BGT is nearly 43 years old but it still works fine and although I have been tempted to replace it with a high torque one like on my 1975 Land Rover I haven't, going by the old saying "if it ain't broke don't fix it."
You've replaced a number of parts so it's likely to be a wiring fault. It would be helpful to know if the new starter motor is the standard MGB large one or a modern type like a high torque or similar.
Are the relays the original round ones or after market square ones? I had to replace the " run" relay a few years ago as the engine would start but not run. As Charles asked, are they wired up correctly?
Was the fault there before you changed everything? If you can give me all the information then I might be able to offer some assistance.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Charles Farran
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Re: Why Won't She Turn Over and Start

Post by Charles Farran » Fri May 15, 2020 10:13 pm

Hi,
I would check all the other issues first before attempting to change the ignition switch which will involve removing the mounting bolts under the steering column to drop the column down a bit to give access to the lower fixing screws for the cowling together with loosening the mounting bolts where the column goes through the floor by the pedals. You have to remove the cowling to just to get access to the ignition switch.......
If the ignition switch worked before you made changes elsewhere i doubt if that is the cause of the problem you have at the moment.
Cheers,Charles
1980 Roadster

Vic Butler
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Re: Why Won't She Turn Over and Start

Post by Vic Butler » Sat May 16, 2020 10:26 am

As Charles says removing the ignition switch is quite involved and when you come to undo it you'll see there are no bolt heads. This is deliberate. To remove the bolts you have to tap the remains with a centre punch.
The bolts are designed so that when tightened the heads shear off. I had to fit a replacement switch ages ago because the key wouldn't come out. The switch was the early type with a release button. Before I sheared the bolts I checked everything worked.
The "run" relay is the front one and the starter the rear one.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Charles Farran
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Re: Why Won't She Turn Over and Start

Post by Charles Farran » Sat May 16, 2020 12:42 pm

Hi,
As your car is a later car, the ignition switch is probably the same as mine which is as fitted to my 1980 car.
If the ignition switch is faulty, but the key turns & frees the steering wheel then you can just change the electrical part that circled the key barrel.
You do not have to punch out the security bolts. All you need is a watch makers * driver (or similar) & remove the tiny grub * underneath the electrical part that fixes it to the barrel. Then simply disconnect the wiring at the connectors.(You will have to cut the cable ties & replace then with something suitable to keep the wiring of the replacement electrical part tidy & safe. Do disconnect the battery before you start as one or more of the wires will be permanently live & unfused. Before you do any of this , i would double check that the ignition switch is not the source of the problem you currently (no pun intended) have!
Cheers,Charles
1980 Roadster

Vic Butler
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hi Re: Why Won't She Turn Over and Start

Post by Vic Butler » Sat May 16, 2020 10:54 pm

Madge 79.
I've had a look at the wiring diagram and the parts list and there are 3 spade terminals on the starter solenoid on the motor. One wire is white with a red trace and this is the one that activates the relay. There is a brown wire that goes to the ignition switch which activates the solenoid and the white wire with a light green trace is the one that goes straight to the coil bypassing the ballast resistor. Did you make a note of which connectors these 3 wires went to when you disconnected the old starter and fitted the replacement? Did the replacement have 3 spade terminals on it? It's very likely these wires could be incorrectly connected. I can't look at the starter on my MG and tell you where they should go because there's a large rubber boot over the terminals.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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