have known main lead to starter have poor connection between lead and crimped lug where it fastens to solenoid,if it is,you could either fit new lead or clean it up and solder the joint [the latter worked in my case].
k
October 2008
If you are talking about the duratorq engine then you need to take another look at the new starter motor as it fills with clutch dust from wearing dual mass flywheel. If you see no signs of these iron filings then check the temperature of the positive lead on the battery after cranking as they break down and you then loose power.
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