Its hard to find a problem in a car when it is not there while you are looking for it, isn’t it? If something isn’t working all the time it is already half-way fixed – only need to find what’s broken and what’s caused it to fail. When something is only acting once in a blue moon so that the problem can not be even reproduced at will – this is where things get complicated and frustrating.
Sometimes, though, mystery cases getting solved with ease and grace.
Ford F-350 diesel comes in with complaint of intermittent no start. Of course, while I am near it starts just perfectly every time.
By the way, it helps a lot when customer knows the difference between “no crank – no start” and “crank OK – no start”. It takes completely different diagnostic paths in those two cases so if customer can tell which “no start” situation had happened it saves tremendous amount of diagnostic time. Meaning smaller repair bill.
So this one was intermittent “no crank – no start”. Starter on 6.7 diesel is not easy to get to and inspect as it is in a tight spot. Although with bright flashlight, small mirror and keen eye I noticed a tiny bit of difference in color of the starter solenoid bolts. Worth to spend a bit of time and remove the starter for closer inspection.
Ta-dam! Now the problem is obvious:
Loose contact bolt nut, poor electrical contact, local overheating and corrosion
Wire terminal and its mating surface on the contact bolt are badly burnt. No wonder it was NOT starting from time to time, I am surprised it still have been operational most of the time with almost no contact left between parts.
New starter got installed and solved the problem
Next case. 2002 Toyota Corolla doing the same thing – no crank, no start. This time starter visually looks just fine. But what if I scope the voltage on the solenoid contact bolt? It is only a matter of hooking up the crocodile clip and crank the starter for couple of seconds. Now invisible problem reveals itself!
What are those huge spikes of negative (!) voltage occurring for mere milliseconds after starter turned off? This is a combination of high inductance of starter solenoid wiring, starter motor now producing voltage as it spins to the stop and, most important, bad connection between motor brushes and commutator segments. Flat horizontal spot on the waveform right from the marker shows burnt commutator segment where contact is lost altogether. Quickly removed and disassembled the starter and verified that indeed one segment is burnt (the top most darker one).
Whenever starter motor brush stops against this burnt segment – no start situation occurs. Something like a casino roulette although its tow truck who gets the prize :-).
Again, replaced the starter and scoped the voltage again for curiosity sake
Much smoother waveform that would get even better after initial break-in. Another problem solved and another happy customer added to my circles