Relative Compression

Fredrick Owner Georgia Posted   Latest   Edited  

This is probably one of my most used tests. It's amazing how often I get a mobile call for a misfire that turns out to be mechanical. I think too many techs get caught up in electrical and forget to check the basics. youtu​.​be/qG6ZGLkrROE

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Victor Owner/Technician
Florida
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I openly challenge anyone here on DN Network to place an amp clamp over a positive battery cable during engine cranking and tell me how much inrush current is recorded during the event. Clue: approximately 45% of total rated alternator output. Go figure!

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Ray Diagnostician
Ontario
Ray Default
 

On this 2011 Toyota Corolla 1.8, just before the starter motor begins to spin, the starter draws 600 amps. On start up, the alternator is charging at near 100 amps.

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Fredrick Owner
Georgia
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When an electric motor is not spinning, it has almost no resistance, so you get nearly the full capability of the battery for an instant. Then as the motor starts to turn CEMF is generated which pushes back against the incoming current. The faster it spins, the less current it draws and visa versa.

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James Owner/Technician
Saskatchewan
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I'm always amazed that technicians can't hear bad compression when cranking the engine.

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Justin Mobile Technician
Utah
Justin Default
 

Not everyone has their hearing. I personally am deaf.

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Fredrick Owner
Georgia
Fredrick Default
 

Agreed James. I was actually hoping to get a car with low compression while making this video so I could record the sound. I waited a couple of weeks and then finally decided to go ahead and finish the video without it. You know that means I will get one today :) lol...

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James Owner/Technician
Saskatchewan
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I have a 14 5.0 F150 in the shop today...Cylinder 4 misfire, cold. I could hear the problem before I checked with the scan tool. I could record it, if you want. Just in case anybody is unfamiliar, early 5.0's have soft intake valves, and it always seems to start in cylinder #4.

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Fredrick Owner
Georgia
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Bad intake valves are more interesting and rare than exhaust valves. Can you get it with intake vacuum on a transducer? I did a Jeep 3.7 liter yesterday with a bad head. All 3 cylinders down on compression on the drivers side but no timing issue. I took a video of cranking, then slowed that video down to half speed to make it easier to pick out the "every other" cylinder with low compression. So…

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Scott Technician
California
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Same here. i can audibly hear a low compression issue on the first crank, it blows my coworkers away when i say. its gonna have a valve issue. they go what? it has low compression.. they cant hear it.

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Steven Instructor
Georgia
Steven Default
 

Fredrick, Well done!

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Stephen Educator
Ohio
Stephen Default
 

I use relative compression test on any driveabilty problem I get. What we have to be careful with is the term relative. The pattern seem is a comparison with all the rest of the cylinders. When one cylinder is slightly lower than the next, the test may not show a noticeable difference. This could cause use to overlook an issue with a cylinder. I had this happen on some Honda s in the past. If we…

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Ray Diagnostician
Ontario
Ray Default
 

Whenever I do a relative compression test, I always use a pressure transducer in the intake and or in the tailpipe. I find that the starter amp draw only compares each cylinder's compression strokes. You can have good starter amperage relative compression strokes but it may not show some worn valve train components. The pressure transducer in the intake will compare each cylinder's intake…

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Stephen Educator
Ohio
Stephen Default
 

Ray, that is exactly my point. Your correct. I have seen cases where a relative compression test looked okay, when in fact there was a problem. I was just recommending a volumetric efficiency test as a quick test that can be done from the drivers seat.

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