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Old 10-30-2019, 03:18 PM
Makus Makus is offline
Join Date: Jan 2019
Location: UK
Posts: 10
Makus is on a distinguished road
Originally Posted by andrewwynn View Post
If the current draw drops to 40ma after it goes to sleep you don't have a parasitic draw.

Check the vBat at the battery when you attempt to start. If vBat stays high the problem is likely either the B+ terminal loose or a ground or a connection like the starter.

You can measure vBat at the cig lighter as that is always live and takes a different path to the front of the car than the start/alternator cable.

The other thing you should check is the current draw of the starter which should be about 200-250A. If you have a peak reading multimeter you can measure the voltage drop from the B+ under the hood to the pos of the sig lighter. IIRC you multiply the voltage drop by 300 to get amps to the starter eg. 0.75v= 225A.

When a starter begins to fail it will take more and more current up to double. When that happens you lose double the voltage on the wiring and you get slow turning starter it will "feel like" the battery is low. I've seen a dozen cases of somebody replace both battery and alternator when the starter was the actual problem.

A loose ground strap will have the same problem. If it's loose it can have a 4v drop across the connection and suddenly 12v is 8 and a car won't start
it might make sense, while I was charging the battery and it had 13V and I decided to try to start the car and the value dropped to 12.5V or even lower and there was no power to start the car, only at about 13.4V the starter managed to start without a problem
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