bench testing Bosch 3 pin CDI ? possible ?

[ty901]

guys...ran a search, but came up empty. is it possible to bench test a Bosch CDI box ? if so, what is the procedure ?
TIA

[danco_]

Bob Ashlock rebuilt three units for me last year or so. He's the go-to guy for these. Located here in Southern California.

BOSCH CDI

[Tom F2]

Have Bob check it for you. He has a rig that simulates engine bay heat and cycles the unit long enough to get a real assessment of operability. When these things go haywire, they can be intermittent, but Bob's rig will expose an intermittent failure.

[Trackrash]

Here is me testing my 6 pin CDI. Not visible is my 12v power source, but you can see the red and black wires. I spun the distributor by hand and watched the spark.

[ischmitz]

A quick yes/no test is really easy and can be done in the car using its coil or on the bench. You need a coil and a 12V power supply to do it on the bench. A spark plug can be simulated or use an old one.

To really determine if the box works properly there are several measurements that need to be made while the box is at operating temperature. Only then can you determine if the box will work reliably. See here for my test setup:



The trigger board simulates a running engine signal


The spark plug in action

[mysocal911]

Originally Posted by danco_

He's the go-to guy for these.

Really? Seems now we find that there are others too, right?

[Trackrash]

OK to clarify, here is a shot showing how I hooked everything up. I used the SC engine wiring harness, since I had the motor out and apart. With a three pin CDI the ignition harness is separate and might be easier to use.

I attached all the components to a metal strap to act as the ground (-).

I spun the distributor by hand to see a spark. I then hooked up a drill motor to the distributor to check for spark.

I did not bother to heat up any of the components, but if you suspected an intermittent unit, this would make sense. In my case, before I committed to using an unknown CDI I felt this test was adequate.

[boyt911sc]

Gordon,

I sent you a PM earlier. Did not see this post until later. I was requesting for a schematic diagram of your set-up. Ignore my PM. This post is exactly what I need. Thanks a lot.

Tony

[ty901]

guys ... big thanks!

[Jonny H]

You don't need the distributor to test a 3 pin. Just hold the 'C' pin to ground then release to fire the spark.

https://www.dropbox.com/s/cq9igspoi79qo7z/IMG_5704.m4v?dl=0

[Targalid]

Ingo, that is a superb photo of a firing spark plug! Did you trip the camera with the signal to the spark plug?

[ischmitz]

Nope - ambient light was low enough so that the camera shutter time was long enough to capture multiple sparks.

In this picture there were 300 sparks per second as I simulate 6000 RPM engine speed. That’s how it ended up looking so nice. Sheer luck with a cheap Canon Ixis.

[dicklague]

You can use a square wave generator like this to simulate the points.

CDI TESTER SQUARE WAVE GENERATOR - IG3TECH

[ischmitz]

That’s essentially what the PCB in my setup does: it’s a simple NE555 acting as a switch (3-pin) or generating a VR signal (6-pin) to simulate trigger signals. Less than 5$ in parts....

[dicklague]

Originally Posted by ischmitz

That’s essentially what the PCB in my setup does: it’s a simple NE555 acting as a switch (3-pin) or generating a VR signal (6-pin) to simulate trigger signals. Less than 5$ in parts....

and.....how many on here can build one???

[dicklague]

take a look at this tester from Parts Klassic. I have one of these and it is a great tool.

https://www.youtube.com/watch?v=Y34mFbFEclo

[ischmitz]

I’m just offering an alternative and agree there are many ways to accomplish the trigger. As we see in this thread anything from simply touching a wire to GND to building a trigger from a distributor attached to a power drill can trigger a 3-pin. That being said I like the small black box.

However the last device shown in the video and the explanation in there don’t make sense (to me). It doesn’t test the box according to the Bosch test/repair plan for CDI boxes either. A “voltage drop in the power supply” is not a CDI box failure. And it’s not an indicator for an intermittent failure either. The statements made are very misleading.

These LED are nonsense in my opinion. The Bosch manual is clear in what is tested. The one thing Bosch missed is testing for the resistive component the internal capacitor while at operating temperature. An ideal capacitor has an infinite DC resistance. As these units age that changes and puts additional stress (read: heat) onto the box. This eventually leads to failure.

The internal DC resistance of the capacitor is a good indicator on how far away a box is from intermittent and total failure.

[Jonny H]

Originally Posted by ischmitz

A “voltage drop in the power supply” is not a CDI box failure. And it’s not an indicator for an intermittent failure either. The statements made are very misleading.

I was going to comment Ingo but didn't want to start another 'CDI war' but since you mention it...

A black box tester can't 'see inside' the CDI box so this could only be testing input voltage or output (primary coil voltage). As you correctly state, input voltage is nothing to do with the CDI - it's down to the charging system. When cranking, you can get down to 9 volts and when charging, as high as 15, failed regulator not withstanding...

The primary coil 'A' output is proportional to battery voltage and inversely proportional to RPM. In order to do a proper test, you'd have to compare the output spike amplitude (and duration) and cross check that against input voltage and RPM. This is most certainly in the 'non-trivial' category, considering the short duration of the pulse.

Go/no go testing for missed distributor signals is also not that easy. It's the same challenge facing RPM sampling to apply an ignition timing curve and is a complex problem. Also, as the CDI box may fail in a mode where it 'holds' the distributor signal, the test may be inconclusive.

[Quickstep192]

Originally Posted by ischmitz

A quick yes/no test is really easy and can be done in the car using its coil or on the bench. You need a coil and a 12V power supply to do it on the bench. A spark plug can be simulated or use an old one.

To really determine if the box works properly there are several measurements that need to be made while the box is at operating temperature. Only then can you determine if the box will work reliably. See here for my test setup:



The trigger board simulates a running engine signal


The spark plug in action

Do you like those plugs with the three electrodes?

[mysocal911]

Originally Posted by Jonny H

When cranking, you can get down to 9 volts and when charging, as high as 15, failed regulator not withstanding...

So what! The Bosch output spark voltage range is more than adequate at those input voltages. You have a problem with the
Bosch CDI design that has reliably provided continued service for many 911 owners for almost 50 years? Surely, you have
extrapolated long term reliability data on your CDI unit for comparison, right?

Originally Posted by Jonny H

Go/no go testing for missed distributor signals is also not that easy. It's the same challenge facing RPM sampling to apply an ignition timing curve and is a complex problem.

Then the design engineer is lacking in automotive electronics knowledge and an ability to translate
the system design into a simple digital design. M/B had no problem designing a digital ignition system
using three trigger points off the flywheel about 35 years ago. Now days, a digital ignition system with
timing maps can be easily designed with as few as 40-50 lines of C code using a 8 bit processor running
at 16 MHz.

Originally Posted by Jonny H

Also, as the CDI box may fail in a mode where it 'holds' the distributor signal, the test may be inconclusive.

Not realistic!

Bottom line: The very simple test setups shown in this thread are MORE than adequate for testing the Bosch CDI.