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MG MGB Technical - Starter Relay Tests

I am still having intermittent problems with starting my 1972 MGB. On average it turns over 75% of the time and fires up easily. When it doesn't, I rotate the key a few times (up to 5) then it catches. All connections to the battery, solenoid, and relay have been carefully cleaned. I don't hear the solenoid engaging when I turn the key, the red light doesn't dim when it does catch. Are there any simple tests I can do with a multimeter to test the starter relay? Is there a resistance spec. that it should read? I don't have a remote starter button to try that test.
Joe

I would suspect the ignition switch itself if you do not even hear a click. Use a probe light and follow from the ignition switch. I believe that the starter wire at the ign switch is white with a red tracer. One problem I have had is the plug in from the switch to wiring harness not making good contact. be certain to check both side of that plug.
Sandy,
conrad sanders

If you can't hear the relay click either then the problem lies with the ignition switch, relay, or their connections.

With intermittent problems like this I have wired a meter in the cabin to various points in the circuit so I can see the voltage instantly the problem arises, rather than hoping it stays on while I get the meter, open the bonnet and fiddle around with the wiring which is quite likely to distrurb the problem and 'fix' it until the next time.

You need to check that you have 12v on the white/red terminal at the starter relay when you turn the key and nothing happens, and have *no* voltage on the black terminal of the starter relay. If you have no voltage on the white/red then you need to check back towards the ignition switch, particularly the multi plug Conrad mentions. Sometimes a pin can get partially pushed out the back of the block and only the tip is just touching. If you see 12v, or any voltage really, on the black then there is a bad ground connection. This is usually close by the relay and fusebox, two or three blacks in a single bolt-through terminal.

If you have 12v on the white/red and 0v on the black and the relay still doesn't click, then the relay is faulty.

If you get the relay click but no solenoid click then the tests are similar, but using the brown and the brown/white at the relay and solenoid.
Paul Hunt

Paul,

The starting problem is no longer intermittent. I am measuring 12V when the key is turned at the red/white wire at the relay, I can feel and hear the relay clicking, no sound from the solenoid. Does this mean the solenoid is bad?
Joe

You need to test the voltages at the brown and brown/white to check the wiring and the relay contact, test the wire connectors and the spades in both cases.

The brown should show 12v at all times. If it drops when the key is turned and the relay clicks there is a bad connection in the brown somewhere.

The brown/white should show 12v when the key is operated and the relay clicks. If it doesn't the relay contact is faulty, if it does you need to do the same test down at the solenoid. If you have 12v on the solenoid spade the brown/white is connected to, but the solenoid doesn't clonk, then the solenoid winding is bad.

Be mindful that poking around could disturb the fault and cause it to suddenly start working again, particularly startling if you are underneat and close to the starter at the time. Also be absolutely sure it is out of gear and firmly braked/supported.

Paul Hunt

Thanks for you help Paul, I have 12V @ the brown at all times, 12V on the brown/white when the key is turned, and nothing indicated on the black. I believe it is narrowed to the solenoid. I have previously checked the connections there, and no they are clean and tight. On a 1972B, can I replace the solenoid, or would it be easier to get a rebuilt starter? I see the solenoid is available through Moss and others for around $45, I have never swapped one in a MGB starter, not sure how difficult that is. I have seen other postings on this site that indicate it is better to exchange the starter than try to switch the solenoid.
Joe

I did opt to swap the whole starter when the solenoid on my V8 started playing up, seemed the easier way to go.
Paul Hunt

Joe,

If the solenoyd is suspect, take out the starter, give it a good clean up and check the two soldered points close to the terminals. Sometimes they have to be resoldered to cure the problem.
You can also check the solenoid on the bench with a battery, conecting the body to (-) and put 12 Volt (+) to the spade connetor for the white/brown wire.
Check whether there is a switched connection between both the big threaded terminals when the solenoid is engerized.
If there is no satisfying connection at the large terminals then, you should change the solenoid.
If your car would be a V8, I also would change the entire starter motor as Paul did because it is exposed to the heat from the rh side manifold of the V8. With the B 1.8 an easy cange of the solenoid should be ok.

Ralph
Ralph

This thread was discussed between 19/04/2009 and 26/04/2009

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