Join Date: Feb 2011
Location: San Francisco Bay area
Thanked 76 Times in 76 Posts
Rep Power: 6
Re: Voltage Drop Problem: battery, wiring or both
I've spent quite a bit of time trouble-shooting an intermittent no-start problem, that only occured after the engine had been heat-soaked (ie, re-start an hour after shutdown). The solenoid went click, but the starter did not spin up.
There are many things that can go wrong.
1) You need a healthy voltage to activate the solenoid. This is down to the ignition switch and wiring.
2) You need a healthy voltage at the starter main terminals. This depends on the cabling, battery switch, and starter battery.
3) You need a healthy starter motor.
Firstly, you need to systematically measure the voltage drops in your system. The starter motor needs at least 9V, ideally 10. You need to measure the voltage at the starter terminals WHILE CRANKING.
For voltage drops, connect the meter between battery + and starter +. Then repeat for battery - and engine ground. Again, measure while cranking. Please note that the losses on + and - sides add up.
Good cabling and battery switch should give you a 1V drop or less. A good battery should hold up to 11V while cranking, leaving you a healthy 10V at the starter.
In my case pretty much everything was wrong. Poor battery cables, an ageing 1-2-both switch, weak battery, ageing ignition switch, poor quality wiring to the solenoid.
So I now have new 2/0 battery cables from Genuinedealz.com, a new ignition switch, new wiring to the solenoid, a new 1-2-both switch from Blue Sea Systems, and a new group 31 battery from US Battery. Very healthy voltage at the starter now.
But guess what? The intermittent start problem was still present. I replaced the starter motor, all is good so far. Will need 6 months of use before I declare it fixed!
Bristol 31.1, San Francisco Bay