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MG MGB Technical - slow cranking

battery checked out as ok,some days it will turn over at correct speed,next day to slow to start,following day no problem!Very frustrating,do I replace starter motor,where is engine earth lead located? Any ideas or suggestions much appeciated.
g swash

The engine earth lead attaches the gearbox to the crossmenber. They are flexible and eventually strands break affecting efficiency. The cure is to replace - the old lags put an earth strap somewhere they can see it in the engine bay. HTH
R Walker

On many MGBs there is an earth strap around the front engine mounting. The earth strap is usually a bare (not insulated) braided piece of cable, so its pretty obvious if you look for it. It doesn't hurt to have two, even if there is one on the gearbox. It must be bolted tightly to the body at a place where the paint has been removed for a good connection.

Slow cranking usually points to a drop in voltage and this is often caused by dirty or poorly made connections somewhere. I would go through all the high load connections. Start at the battery posts. Are they clean, are the connectors really tight. If you still have the original type that enclose the battery post with a screw through the top, change them for the clamp type with a nut and bolt.

Battery earth lead - is it on to a clean (not painted) area of bodywork and is it tight. Move forward to the starter solenoid. Are the large cables clean, and nice and tight. Engine earth lead - the same, clean and tight. Do all this before you start wondering about changing the starter motor.
Mike Howlett

Take some jumper cables and go straight from the battery to the starter terminal and the block. If it goes, then you have bad connections in the car wiring.
Art Pearse

As others have pointed out, check the grounds. Also check the battery cut-off switch if you have one connected to ground.

Tom
Tom Custer

Take voltage measurements with (and ideally without) the problem first.

With a voltmeter right on the battery posts you should see about 10v with a decent battery and engine that isn't overly stiff i.e. after a rebuild. Less than 9v indicates a weak battery or stiff engine. Measure again on the battery cable stud on the solenoid and the starter body and compare. Any difference is down to losses in the cables and any bad conenctions. Ideally the starter voltage should only be 0.5v or less lower than the battery voltage, and more indicates bad connections. If you have twin 6v batteries than you also need to measure between the battery posts that carry the link cable, this should only show a tenth or two of a volt when cranking. If the volt-drop between battery and starter is a volt or more you can break it down by measuring between the 12v battery post and the solenoid stud, and the ground battery post and the starter body, to determine whether they are in the 12v run or the earth run or both.

If you see more than 10v at the starter but the engine is still sluggish then it points to problems inside the solenoid or starter. Could be burnt contacts on the solenoid, worn brushes, faulty segments and all sorts. Some starters have an accessible copper link between the two so it is possible to diagnose further.
Paul (too many Pauls own MGBs) Hunt

This thread was discussed between 07/03/2010 and 08/03/2010

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