Case Study – 2001 Chevy Suburban
No start from the key
I’m Positive it’s a Ground!
Keeping diagnostics quick and simple are a trade mark of a
good technician.  A lot of them will “invent” their own methods
of diagnosing a problem based on either previous experiences
or an improvised method they learned years before. 

Sometimes, the testing method goes against the grain of
the mechanic in the next bay or the boss, and sometimes even
the customer may not understand what you’re doing, but if it
works… do it.

A 2001 Chevy Suburban shows up with a starting problem. 
The only way the customer has figured out how to start is how
the last shop showed him.  They bashed in the start relay
cover so you could get to the contacts inside it, and if you
left the key on while pressing the relay contacts together
it would start every time.  Probably not the best way of getting
the job done, but it did save him a tow charge.  Now, I’ve got to
figure out why it won’t start from the key.

I’m a big fan of these relay simulators and relay test connectors.  Before these were around you had to strip some wire down to a few strands and shove them into the socket while leaving enough of it expose to get your test light onto it.  Of course way back when, the underside of the relay connections weren’t molded into a permanent fuse box as they are today.  So a lot of times it was just as easy to turn the relay and it’s connector upside down to access the terminals. With today’s multi-purpose fuse boxes and relay centers these new tests connectors make the job a whole lot easier.  This particular problem lent itself as a prime candidate for a relay test connector to be put to good use. 

After installing the relay test connector and the factory relay on top of it, I turned the key to on.  I then attached the test light to a known good ground, as I touched the test light to the positive input lead which would have been the actual start signal the starter engaged and the truck was up and running.  Now hold on a second here… I didn’t turn the key to start yet; I merely touched the test light to the relay contact and then was going to turn the key to start to check the signal.  But by just touching the contact lead it caused the relay to engage and since the key was already in the run position, the truck started.  I’m sure a lot of techs out there are gathering thoughts and ideas as to what is going on, but let’s stop right there and ask ourselves, as technicians, a very simple question.  “What do you need to make a relay trip on?” 

Back to the Basics – Powers and Grounds
Checking the wiring diagram showed the four relay terminals, one going to the starter, one as primary voltage (constant), one coming from the neutral safety switch and ignition key, while the last leg of the relay was from a permanent ground lead, (G105).  But in this case the mere act of applying a test light (which was hooked to a ground source) activated the relay, which means, there had to be a positive on the ground side.  As with most relays, all it takes is a positive and a negative feed to trip the relay on.  For some reason this particular relay’s terminals are completely backwards. 

Tracking it down
Here’s where those various methods mechanics will use to diagnose this type of problem go in different directions.  Some want to start at the base of the relay and hand trace the offending lead to its eventual end.  While others start peeling wiring harnesses apart trying to find a burn spot or melted section.  Then there are the hack job repairs where instead of tracking it down at all, they’ll just cut the lead off and tag another ground lead on and call it done.  Of course, this would be totally ignoring the possibility of any future problems, or any associated systems that aren’t working correctly.

My preferred method is to work with the wiring diagrams in the manner that they are written.  Basically, every wire has two ends.  That’s the way they are drawn on the prints, that’s the way I’m going to tackle the problem.  I’ll check both ends of the leads for a problem first, then if need be, I’ll start dividing the car in sections, usually splitting the car in half.  Then half of that half, so on and so on until you’ll eventually get to that single spot that’s causing the problem.

In this instance, the wiring diagram showed the negative lead was attached way over on the other side of the engine down along the front edge.  There, broken off was a clump of several ground leads all ganged together that had been sheared off.  It didn’t take much to notice there was a brand new oil pan gasket installed either.  And it wasn’t hard to surmise how the leads got broken off. All these different ground leads were to different circuits and each one of them was lacking a solid ground.  Obviously, a few of them were activated with the key in the on position.  Having no ground signal at their post meant the leads were sending out a weak, but affective positive signal searching for ground.  Reattaching the leads back to the ground lug on the engine solved the entire issue.

The Final word
Sometimes I like to know everything that has been done to the car prior to me working on it.  Sometimes, the customer won’t tell you what’s important, they’ll end up explaining things about the day they bought it, or the time the fan belt broke, or which tire went flat. This owner didn’t tell me anything about any previous work.  I had to drag it out of him. I asked him, “Did somebody just put a pan gasket on your truck?”  In fact, that was exactly the question he wanted to hear from me.  He had a suspicion that all of his woes started after the oil pan gasket was changed but he wanted somebody else to prove it and not just blame them for something he couldn’t say for sure was the cause. 

Seems everybody has a different strategy or method when it comes to diagnosing a problem.  It really doesn’t matter how you get to the solution of a problem…just as long as you do.

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