Current ramp over an extended period of time

Brandon Diagnostician Reading , Pennsylvania Posted   Latest   Edited  
Tech Tip
Trained By Techs
Electrical
2008 Buick Lucerne CXL 3.8L (2) 4-spd (4T65-E)
Intermittently Blowing Fuse

Just a tip...

When analyzing an inductive device for current flow like a coil, be sure to monitor over time. The subject vehicle came to me with inop AC clutch coil, due to blown fuse. Multiple current ramps were acquired and appeared perfectly normal with a current ramp of about 3A and a pintle bump @ about 2A. This would lead you to believe the health of the coil was satisfactory.

While monitoring the same circuit over a period of about 5 minutes, current slowly increased until exceedeing the rated capacity of the circuit (10A). Always keep in mind that an energized device like a dwelling coil produces heat. This can cause the device to flex and allow coils to short. I have a capture displaying current over time of about 100 seconds. It initiated at clutch engagement and ended when fuse overloaded/opened. Unfortunately , I didn’t save it...

I hope the tip is beneficial, Happy-Hunting👍

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Mario Diagnostician
Weston, Florida
Mario Default
 

Great tip Brandon. We all forget to use the scope to grab the bigger picture, I think we all just love to zoom in on the fine details:) at least I do.

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Brandon Diagnostician
Reading , Pennsylvania
Brandon Default
 

Yes,Quite a common practice, Mario. Thanks for the response

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Randy Curriculum Developer
Raleigh, North Carolina
Randy Default
 

Good tip Brandon. We all need to remember that circuits are a dynamic environment, not a static one time measurement.

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Jim Mobile Technician
Southampton, Pennsylvania
Jim Default
 

EXTREMELY good tip, Brandon. I can't begin to tell you how many problem vehicles I've been called in to look at and hear from the driver that the problem happens about 15 or 20 minutes after starting up, then I talk to the techs and hear one of two things...…They only looked at it for 2 or 3 minutes and couldn't find anything wrong OR they put it outside and let it run for a half an hour and

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Good tip Brandon. This is not only true for current ramping, a circuit like you described, but also for things like fuel pumps too. Goofy things can happen over time. Another great place to use time is when doing relative compression cranking tests. I always like to crank for at least 20 seconds to get a good stress test on both the electrical, and also to watch for intermittent valve leakage.

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James Diagnostician
Bishop, Georgia
James Default
 

Good one

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Eric Owner/Technician
Edgerton, Wisconsin
Eric Default
 

Brandon, I read the title, then the first sentence. I was having a real hard time visualizing how you could monitor a current ramp and not do it over time. I was envisioning some kind of flat two dimensional graph that you were adding time to. Once I read the entire post I understood, but I sure was confused for a few seconds ;-) Maybe the title should have had the words Long Enough in it. :-)

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Brandon Diagnostician
Reading , Pennsylvania
Brandon Default
 

Thanks Eric, fair enough… Perhaps I can edit the title

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Robert Diagnostician
Ballston, New York
Robert Default
 

Brandon that’s a great tip. In the interest of time in a busy techs life it’s easy just to take a quick glimpse at an image and assume it’s alwlays ok. I know I’m guilty of that. I am finally trainig myself to slow down and actually measure patterns instead of assuming they look good without paying close attention to time and actual voltage.

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Chris Diagnostician
Commack, New York
Chris Default
 

I was getting all excited......and then you didn't save it. Awesome tip though. Thanks Brandon!

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Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default
 

Hey Chris, if there is no video can we really say it happened? :-)

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Brandon Diagnostician
Reading , Pennsylvania
Brandon Default
 

😆😆😆...next time...I PROMISE!

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Eric Owner/Technician
Edgerton, Wisconsin
Eric Default
 

Great tip. I do something similar for relays. I have a variable speed timer I use to turn the relay on and off. Connect the scope to the relay, either use the OEM load or connect one of your own, set the timer to the speed you want, turn the timer on and go. It helps find intermittent issues.

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Thomas Owner/Technician
Rudyard, Michigan
Thomas Default
 

I have also had A/C clutch coils open circuit after 30 minutes or so of operation. We get the complaint of “no A/C “ and find normal operation in the bay. Fortunately this car also needed an alignment and balance. Near the end of the test drive the A/C finally failed. A quick test finds the clutch coil opens when hot and works fine after a cool down. I often admonish my techs for staring at a

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