Hi Folks,

Could use some help trying to diagnose a very odd intermittent starting problem.

First though, a bit of a backstory. The car with the problem is an immaculately restored, all numbers matching '69 Z-28. The car was the prime possession of one of my dearest friends who passed away a few years ago. My buddy lovingly restored this car to a supremely high level before he died, so believe me, there isn't a nut or screw out of place. The car remains with his family in revered status as a family heirloom in remembrance of him.

As a close friend of the family, I've been asked to help get the car into a reliably drivable condition, so it can be driven occasionally and taken to local shows. I've worked on the car a few times trying to isolate the condition, but so far nothing conclusive has emerged.

The starter and starter circuit appears to be functioning reliably. There's nothing that provides any predictor of when the condition will occur. For example, his wife\widow, told me she tried to start it a few weeks ago, turned the key and nothing. No clicks, just a faint red light on the dash board. Battery is fine and always left disconnected when not in use.

When I got over there last week, I connected the battery and it started right up with no issue. I proceeded to stop and start it at least a half dozen times in succession with no issue. While I was looking under the hood for anything that might be suspect, she decided to give it another try herself, but this time nothing but the faint red light on the dash. Of course, I had to try it again, but no go.

I suspect the problem may be in the ignition switch circuitry, but I'm hesitant to start taking things apart without a clear idea that's where the problem is, so I'd like to hear some ideas before I start digging in. BTW, I'm no slouch mechanic when it comes to diagnosing/troubleshooting and resolving problems, but this one has me baffled. My buddy had a nice library of service manuals and books for the car, so I have access to the wiring schematics and nothing presents itself as an obvious fault point to start at.

Any help at all would be greatly appreciated!

Thx, Mike

A few pics of the car.

 

When no start, does the solenoid kick in?
I would wire brush battery post clamps and posts and same for other end of cable.
Solenoid power is on the purple wire and it goes from ign switch to clutch pedal switch to starter.
Either one of these two switches could have corroded contacts.
Solenoid could also have corroded disc and/or bolts

 

When no crank/no start condition occurs, starter solenoid makes no noise. The red light on the dash board dims when the key is turned to start, but nothing else happens. Believe me when I tell you, there is absolutely no corrosion on any of the terminals, or anywhere else on this car. It’s a true show queen that rarely has ever left the garage on anything other than a sunny Florida day.

When this condition doesn’t occur, the car starts and runs as it should.

Btw, I’ve had the starter out and tested it thoroughly on the bench. It’s a fully restored factory original starter and it functions perfectly.

 

More often than not it's an invisible film of corrosion on the battery posts and clamps,I agree with Everett.
Typical when a vehicle sits more than it's driven as well so that's where I'd start.
A little baking soda and water paste on it for a few minutes and then rinse off with water will take it away without a lot of wire brushing so I usually do that first but I still like to use a small wire brush to make everything shiny metal before putting it back together.
This is very common with motorcycles I see it all the time since at least in Northern states like mine they usually sit all winter,same goes for cars that don't get driven in the winter .
That's a real fine looking Z28 by the way!

 

I'm not a noob at this stuff, things like that were checked and eliminated at the very start of my investigation. Been through all of the low hanging fruit more than once, hence my query for a more expansive short list of things to target. My best guess right now are the two electrical contacts (possibly defective or out of position?) at the ignition look cylinder inside the column. Anyone with experiential knowledge and insight before I start disassembling? Any other ideas of things to look at?

Thx

 

Could also be in the solenoid even if it's new.
When it fails to start what are you doing to get it to start?

 

The problem s not in the solenoid, that’s been thoroughly checked out and eliminated.

Thx.

 

You didn't answer my question though.
When it fails what does it take to get it to start?

 

I’ve just quit at that point, figured I’d try to diagnose it before tearing into the circuitry. That’s why I’m asking now.

I’m plannng to go work on the car sometime this week to do more extensive diagnosis. I’ll be starting at the ignition switch, that’s likely where the breakdown is occurring. The run position of the ignition cylinder apparently applies power through the purple wire, through the clutch safety switch (which is bypassed via jumper wire) and goes directly to the starter solenoid. The problem has to exist in that simple circuit, no?

 

All you need is a test light on wire the energizes the solenoid. If there is no power being fed to the solenoid you need to back up to the switch. If it lights up you can assume the switch is working.

The diming light indicates a draw on the system. The dummy light is showing an imbalance between the charging side and the running voltage. If the power is lower on the charging side than the running side the light is powered. The only way to dim the light increase the voltage on the charging side or decrease the voltage on the running said. Since increasing the voltage on the charging side of a car that is not running is not possible the dimming of the charging light indicates there is a large draw on the running side voltage. It is possible but pretty rare to cause a drop in voltage more than once. They tend to melt. Normally it take a pretty large draw to get the light to fade.

If I played the odds I would say the solenoid is trying to power up but there is resistance that is stopping the electrical flow somewhere. This could be a ground issue or a positive cable issue. An AMP meter would show you real quick if you saw a 20 amp draw and you had lights dimming then you know there is an issue with flow. You could also feel around for hot connector. Old school ignition switches are very basic, intermittences are very rare. Normally they just fall apart internally or something burns/melts. Its not the type of thing that goes bad then works then goes bad again. There are a lot of places that could have bad connections but bad enough to drop the running voltage on a 650+ amp battery without catching on fire? It is possible but unlikely and it should be getting really hot. .