Here is an old post that I wrote to explain about volt drop testing. (no piss taking please! )

Old post....

..................................................................Volt Drop Testing

Checking circuits, especially high current starter & alternator circuits for high resistance with an ohmmeter is not always an effective test because the load applied to the circuit by the ohmmeter is very small & will not simulate the much higher load that a starter motor or alternator will place upon the wiring & connections. Instead a method of testing these circuits under the same loads that they normally are subjected to is necessary. I am by no means an electrician but I find that the Volt Drop Test is an relatively simple way to check for these bad connections that very often cause lazy or none start problems. I am sure we all know how embarrassing it can be when a new starter motor is fitted & it turns out that an earth strap was to blame. If you don’t already know how to test using this method, then it will be well worth a read & hopefully it will take the “guesswork” out of starter fault diagnosis.





In the starter circuit pictured above, we can see that the battery feed wire to the solenoid has deteriorated to the point were only a few strands remains. If we were to perform a continuity test or even just test for a feed with a test light, then we could come to the conclusion that the circuit is good. It doesn’t take a great deal of imagination to work out what will happen to the remaining strands when the driver turns the key to crank the engine.

The higher power demands of the starter will highlight any sections of the circuit that limit the current flow. One of the most obvious effects of the high resistance is a build up of heat. It is not uncommon to see smoke or a glowing wire & on more than one occasion I have seen a throttle cable glow in the event of the engine earth strap braking.

It may seem complicated at first but volt drop testing is easy (must be if I can do it). When you get the hang of it, you will be able to pin point exactly the area in the circuit affected. It is very satisfying when you manage to find a bad connection & if you manage to prove that all of the feed & earth circuits are good, you will be able to state with confidence that the component itself is at fault.

It is worth pointing out that a starter motor may appear to be lazy when it could be functioning at it best. Never loose site of other influences that could cause problems, such as a tight engine or even a discharged battery.


Another really good example of how effective this test is…. the older Vauxhall 8 valve petrol engines, with the alternator mounted on rubber bushes & relying solely on a single earth lead to ground the alternator. Within seconds you can test for a bad earth by probing the alternator body & the battery negative terminal. Any significant voltage seen here will indicate an earth problem. So without any dismantling, you have proved conclusively if you have an earth fault or not.


PS feel free to correct any mistakes I have made. As I said I am not an auto electrician & I welcome any comments

End of old post.