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do voltage drop test.
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Posted by JohnB (more from JohnB) on Sat, 27 Nov 2004 13:54:31 Share Post by Email
In Reply to: Charging System, joe c. 85 900 T, Sat, 27 Nov 2004 13:43:24
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Often cables or terminals will heat up or resistance changes as the system warms, especially on older systems.

You need a high internal resistance voltmeter, like a dvom. Set the volt range to several volts (you don't want more than a volt or two drop but you can use a 12v scale too) and put one terminal on one end of the cable and the other probe terminal on the other end. I use the battery end and the ground end of a negative ground cable, for example.

Run the starter with the secondary wire from the coil grounded or otherwise disable engine starting. The idea is to get maximum current flow through the cable...V=IR so you'll be measuring voltage drop across the cable as the product of the almost immeasurably low cable resistance and the high current flowing through it. You don't want more than a volt or so drop...a new cable will have about .5v drop or less on shorter cables.

Do the same with the cable from the alternator to the battery...only with the engine running and max load on the system. It would be nice to do it with a cold system and monitor it as the system warms up if it only takes ten minutes. I use a dvom with a min/max reading capability so I don't have to watch it constantly.

Lastly, it's possible you have a heat sensitive diode in the diode bridge internal to the alternator. Have the alternator tested in the car.

PS: I don't like the stock bosch VR...I use an adjustable one I bought from IPDUSA.com (works on all bosch alternators, including volvos) and set it to 14.2v. I'd set it higher like 14.5 but so far it's worked out well with my stock 1990 volvo 740T alternator (100a?) and the big 100/130 euro H4 bulbs and reflectors.

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