The output of the alternator is just so much as is needed, so with a full battery and little load of lights, wipers, fans etc it may not reach 20A loading current. Try again with all the ancillaries working. The fuse will probably blow instantly. Kees Oudesluijs NL [demime 1.01d removed an attachment of type text/x-vcard which had a name of coudesluijs.vcf]