Starter testing with a scope
Over the years, I have seen a lot of starters that didn't function as designed. Problems like this can be caused by many things : stuck engines, poor power supply, dirty battery cables, defective battery cables, and the list could go on an on. Many times the parts cannon will be loaded with a new battery, a set of battery cables, new starter relay & finally a new starter, Why not find out what the real problem is before you start installing parts?
By using a scope, current probe and a voltage leak, you can make a few quick hookups, apply the load to the starter and get a great picture of what is going on in the starter motor.
When testing a starter motor with a scope, it is going to give you some different patterns than you will find when testing a fuel pump motor, since the fuel pump has a constant load and the starter motor has a variable load, which changes every time a piston comes up on compression. You can still zoom in and take a look at what is going on with the brushes & the armature, like we do with fuel pumps. I have found that there are very few commutator and brush failures on starters though. Most of the failures I find are either in the motor bearings being worn out, in the solenoid contacts, or in the power or ground supply to the starter.
Here is a case study about a Jeep with the 4.0L engine with a starting complaint. When starting the engine, it had a way longer crank time then is usual for this engine, and the starter sounded as if it were going to come apart. Two rebuilt starters had already been installed, and each starter would work fine for a short period of time, then the problem would come back.
Here is what I found on the first test. diag.net/file/f4a0364vp…
diag.net/file/f6gyf8wrp… With this capture, I can test the battery, right along with the starter. If you zoom in on the waveform, it is easy to see that the voltage and current dropouts do not have the rhythm of an armature problem.
diag.net/file/f4fg6qc9z… . This looks more like a connection problem to me. Keep in mind, this is the second starter replacement, which worked for a short time.
The next test went pretty quick. I cranked the engine a few times, then started feeling of the battery cables & connections. I found the problem was in the positive battery cable, which was a cast lead terminal, and it had a poor connection where the coper cable went inside of the lead terminal. I crimped a new cast coper battery terminal on the end of the coper cable, and all was well.
The last waveform diag.net/file/fef7gcctn… Confirms the fix, although, it still need a new battery. Notice the inrush voltage of 7.3V.
Hello Albin, Usually when I see any Jeep product, they seem to have one or more cable or ground issues. Ah! But the cables “looked good” right? I cringe when I hear people say that. Even a good DVOM would not catch this issue because they do not refresh quick enough.
The trusty 87 and 88V in peak detect mode would, refresh rate is 250 MICROseconds = 4000 samples/sec….but then again what is a ‘good’ meter?
Oh ya, I always love the term, I checked the grounds and they are all clean & tight. :) The big question is, “will they flow current”?
This might be a spot to use your infrared camera… Thanks for the info!
True, but I hate to use tools to go out and look for a problem. I like to start with a tool that will tell me where to go and look for that problem. I am an old guy and get lazy streaks in me from time to time.