Oldie But A Goodie Sunday Throwback

Robert Diagnostician New York Posted   Latest   Edited  
Case Study
Electrical
1969 Chevrolet Nova 5.7L 4-spd (Muncie)
Stall

This very nicely restored Nova had a client complaint of stalling when the clutch was depressed while coming to a stop. A little backstory: this car has been restored back to like new condition no modifications at all. It has had the engine rebuilt and installed very nicely, but ever since the reinstall it has had the stalling issue. diag​.​net/file/f54i5104c… diag​.​net/file/f4pmox5ng…

The car has been to several other classic car shops to attempt to repair the stall. The original rebuilt factory quadrajet carburetor was removed and an aftermarket carb was installed in an attempt to cure the stall but the issue never changed. Several sets of points and almost all of the ignition system components have been replaced several times with quality parts. Still the stalling complaint continued and never changed.

When the car was dropped of the client requested that I check and reinstall the original quadrajet if it is OK. We were also instructed to do whatever it took to repair the stall. There were also a few other ancillary service requests that are not related to the case study.

Of course in upstate NY the weather is still close to winterlike and the day after the car was dropped off it was pouring rain all day and with road salt still on the roads it was not advisable to go for a road test. Any attempt at recreating the stall was going to have to happen in the shop and of course it would not stall there. So to move ahead with the project the original carb was reinstalled, the other service items were addressed and an attempt to locate a possible cause for the stall was made.

While checking the ignition system it was found the vacuum advance diaphragm was ruptured and the breaker plate was also worn and did not always return to base timing position when moved by hand. So the decision was made to install a replacement distributor assembly. The rest of the fault inspection in the shop offered no clues as to the cause of the stall. My hope was the failed distributor advance components were responsible for the issue.

A replacement distributor installed, the original quadrajet was inspected and reinstalled. The dwell and timing were set. The carb was adjusted and the engine ran fantastic in the shop.

Finally a nice weather day for the road test. This did not go well! First trip out indicated a severe hesitation and the ignition system breaking up bad under load. Upon inspection back at the shop the new points gap is way out of spec now and the dwell is now off. Replaced the points again with a quality set and a new condenser is installed. Road test resulted in the same issue. Scoped the ignition system and found only 8-10k volts from the coil when it was stressed. A new coil was ordered and during the coil install it was discovered that the original coil was wired in backwards! Replaced coil, properly connected it and road tested it again. Now no more issues under load.

However it did stall one time when slowing down for a turn. It caught me by surprise so I could not tell if it lost fuel or spark. After pulling over it cranked and fired right up like no issues. So a few more drives and the stall could be duplicated on demand. It would stall after the clutch was pressed down during a turn or fairly aggressive braking event, and it would always crank and restart. If the brakes were applied for a very light slow down chances are it would NOT stall. Then I had discovered that if it stalled and I popped the clutch while it was still rolling it would not start. Would not even attempt to fire.

The issue appears to be ignition related and not fuel as the engine starts immediately upon cranking with the starter. So a test light was attached to the power side of the coil and during the braking event the light would either become very dim or go out completely. Yeah I know should use a meter for measurements but for this issue the test light was sufficient testing device. So voltage is lost at the coil power supply during the stall. Now time to check some voltage drops, diag​.​net/file/f92n1edtw… Voltage was checked at point X (ignition switch output) and battery voltage was there and 100 mv drop to battery positive. That proves the supply from the battery to the ignition switch and the switch output to the coil is OK.

So now the harness was visually inspected thoroughly from the ignition switch to the coil. There were no issues found. The fuse box was visually inspected and voltage drop tests at the firewall bulkhead indicated there was a complete circuit from the ignition switch to the coil. So where is the coil supply voltage going during the stall?

It only stalls during a fairly aggressive braking or turning maneuver so it seems like a poor connection in the harness or a wire damaged and shorting to ground is a strong possibility, again a complete inspection of the harness is performed and absolutely no issues are found. Now what?

If the ignition switch output for the coil has battery voltage at the switch during the stall but there is no voltage available at the coil primary and there are no wiring issues found from the ignition switch to the coil why is the coil losing voltage supply? I begin to think the resistance wire may be failing internally. Voltage drop testing of the resistance wire shows no issues under load from where it begins at the bulkhead connector to the coil. Convinced there are no harness issues from the battery, through the ignition switch and the resistance wiring to the coil why is the coil losing supply voltage?

On a whim I decide to disconnect the R terminal at the starter. The function of the R terminal is to provide battery voltage to the coil during cranking. This is done to allow the engine to start quicker by applying full battery voltage to the coil positive during crank bypassing the resistance wire. With the R terminal wire removed at the starter the stalling issue is resolved.

The starter was removed and the solenoid was disassembled for inspection and it was found that the solenoid had damaged terminals causing an internal short on the R terminal. Not shorted to ground but shorted to the contact plate that provides power to the brushes. While braking the contact disc would move allowing the R terminal in the solenoid to touch the contact disc supplying the brushes with power effectively a short to ground as the starter motor has very little resistance and that coupled with the resistance wire would pull the battery voltage away from the coil. That is why it would not start when popping the clutch. The starter had to be activated to remove the internal short on the R terminal in the solenoid that was pulling the coil supply voltage to ground. 

I apologize for the lack of visual test results but this issue did not seem to be case study worthy until the end when the real issue was discovered. The client informed us the starter was replaced during the engine reinstall as preventive maintenance that is why the stall did not occur until after the engine rebuild and install. A new starter cured the stalling issue and this beautiful classic Nova is back home in safe keeping.

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Interesting
Thanks
Joel Diagnostician
California
Joel Default
 

Very interesting

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Gary Owner/Technician
Colorado
Gary Default
 

Great detective work, Robert. It's refreshing to see a ‘69 Nova restored rather than hot-rodded. The Nova was one of Chevy’s most trouble-free cars. The owner should be proud of what he owns…

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Robert Owner/Technician
New York
Robert Default
 

Thanks Gary. cool part is this guy has 10 very nice original cars like this Nova. Super bee, GTX a couple Camaros. He doesn’t care about cost just wants it done right.

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Mark Engineer
Colorado
Mark Default
 

Robert Very logical and methodical diagnostics, a lost art. It is so refreshing to see a “differential diagnosis” with a solution (resolution) and root cause (sadly this type of diagnostics is not the norm anymore). Thanks for sharing the symptoms, diagnostics, root cause, and solution.

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Jim Mobile Technician
Pennsylvania
Jim Default
 

GREAT Diagnostic Procedures applied here, Robert. You painted a PERFECT picture of the “G”s General Testing first to find the correct area of the issue (Ignition), then performed “P” Pinpoint Testing to drive down the correct Funnel locating the “ROOT CAUSE” of the issue. WELL DONE, My Friend

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Chris Owner/Technician
Montana
Chris Default
 

Robert Great example of strategic thinking and narrowing down the root cause. Some how when you said on a whim by removing the (R) wire was the actual problem that is years of listening to your gut after weighting out every thing else. Great repair and awesome explanation of your procedure.

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Robert Owner/Technician
New York
Robert Default
 

Thank you sir. I am blessed to have been able to attend training by some of the best in the industry!

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Michael Technician
Massachusetts
Michael Default
   

Thank you for an interesting case study.

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Glenn Owner/Technician
Texas
Glenn Default
 

I am glad you stuck with it and did not give up like the others. As I was reading your information I kept thinking that possibly the ignition ballast was bad possibly. I had forgotten about the R terminal on these older starters. Once electronic ignition became common, many of these vehicles no longer used the R terminal.

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Tony Owner/Technician
Missouri
Tony Default
   

Great writeup, Robert. I enjoyed progressing through your story and test steps to the conclusion. At one point it made me think outside the box: what if ground at the distributor was only being made at the cam gear (let's say the block's distributor boss and bolt hole were heavily painted/cruddy) and the gear mesh improved ground conductivity on acceleration but reduced it on decel or stall. I…

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Sean Owner/Technician
California
Sean Default
 

Great work, where did you get that great wiring diagram?? Mitchell has diagrams but not like this…

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