Scoping starter cranking circuit?
Vehicle was brought to the shop for an intermittent “no crank” condition. Could not get precise details but restarting was accomplished by letting it sit awhile. Battery & charging system were checked - OK. Test drive and hot soak did not produce the no crank. I decided it would be a good time to use the Picoscope and do some checks. First Pico file is measuring starter current and battery voltage @ the battery (note starter in-rush current is over 700a). Second file is current & voltage measured from battery through starter relay to starter solenoid. Final Pico file is starter relay coil current & voltage.
I have only used the Pico to measure starter current draw / voltage and want to know if the second 2 tests have an validity or should different tests be done to analyze the cranking circuit? Changing the starter is a little involved on the 4wd colorado. Thanks
*Battery was weak during testing hence the lower starting voltage.
The validity of your test is somewhat doubtful since you mention a no- crank condition. I’d say to test for presence of voltages at starter , starter relay and also for signals from neutral safety switch, ECM, BCM, immobilizer, ignition switch when the symptom is present. It can be tedious to wait till symptom shows up but that is the best time to run your tests . HT H.
Yeah battery validity but more of a start / charge test if you ask me .. What is that reading ?? are you on the alternator output cable 100 amps & that battery voltage drop is hooked on the battery ? I dont think your checking Ignition switch to BCM start 5v start request and BCM checking immobilizer then another digital start request to PCM and PNP switch , IMS and TCM Ground definitely on…
Those are valid tests if you believe that the starter circuits are pulling high amps or some other measurable parameter for a cranking issue. But those tests are only valid if the system has failed, as far as determining what's going wrong with the system and the accompanying symptoms. It can be helpful insofar as finding a commutator that might have a ‘dead spot’. Unfortunately, the testing…
ZEB- As Timothy pointed out, your testing is only going to point out a failing commutator(if there is one) otherwise there is not much else it will show in regards to the starter itself, if it isnt acting up. Other than Vdropping the starting circuit and ensuring wiring integrity, there isnt much more you can do. Personally I wouldn't toss a starter at it until you can confirm the fault, or…
Zeb; Good morning Sir, i hope all is well. I recommend you zoom into the waveform right where the Starter Solenoid engages. Sometimes you can see issues with the internal contacts which could cause an “Intermittent Click” instead of an actual “Cranking Event”. I've attached a waveform capture for your reference.
Zeb Here is a Zoomed in capture of your waveform. You can clearly see where there was an intermittent release of the solenoid contacts.
Your captures indicate a … enough motor, but that is rarely the fault. The thing to focus on is the quality of connection in the solenoid contacts. If there are two or more “bounces”, it is likely to cause an intermittent no-crank. The best view is obtained by scoping the power supply and the motor power, then comparing the two; this will clearly show the condition of the contacts.
You can scope the starter solenoid amps to see the contacts arcing. In this capture, CH A is the batt volts, CH B is the sol amps, CH C is the starter amps. Because the no crank is intermittent and if you don't hear the starter solenoid “clicking”, you could connect a horn, which draws 4 amps, to the starter solenoid terminal. When the starter won't crank, the horn will tell you if there is a…
I can address the starter change,,, maybe. IIRC, SI says to pull the intake. The starter can be pulled out thru the driver’s wheel well. It’s tricky but not terrible. It’s been years so, I’ll have to do some research to remember how we did it. Lemme know if you decide to go that route. Mine is an 05 with 3.7L. youtu.be/A3cspJA5D1U
We have 4wd's if that makes a difference. One of our very capable tech's did attempt to wrestle the starter out and succumbed to removing the intake. Not saying it can't be done but some corner cutting takes more time than additional parts removal. Thanks.