Buying 1st oscilloscope - can i test outboards with it?

Thread Starter

CamaroMan

Joined Nov 21, 2016
8
Hi all - I have a small marine repair shop and once again I have an old 70s 2 stroke kicking my rear end.. Im considering buying a handheld oscilloscope to help id and narrow down faulty wires - its common for the insulation to break up and allow cross arcing which is what im thinking is happening on this one..

Would a 40hmz handheld be enough as a starter scope just to start messing around with and hopefully expose weak/bad wiring? Im not trying to probe space equipment here, more like tractor engine mechanics if i could put it bluntly.

Also - the stators can put out around 300v AC easy, maybe a touch over so i guess id prob need stht hat can manage 350-400v AC , and lastly not sure what a " non-ground referenced (differential) scope " means, but is that something to look into?

Thanks
 
Last edited:
Hi all - I have a small marine repair shop and once again I have an old 70s 2 stroke kicking my rear end.. Im considering buying a handheld oscilloscope to help id and narrow down faulty wires - its common for the insulation to break up and allow cross arcing which is what im thinking is happening on this one..

Would a 40hmz handheld be enough as a starter scope just to start messing around with and hopefully expose weak/bad wiring? Im not trying to probe space equipment here, more like tractor engine mechanics if i could put it bluntly.

Also - the stators can put out around 300v AC easy, maybe a touch over so i guess id prob need stht hat can manage 350-400v AC , and lastly not sure what a " non-ground referenced (differential) scope " means, but is that something to look into?

Thanks
Hmm interesting stuff you speak of, I unfortunately don’t have the oscilloscope answer you’re looking for. but I’m curious what said boat is doing or not doing to lead you to that possible diag? Not familiar with older boats or boats in general but is there not some form of alternator for electronic components ie lights radio etc, I guess my question is how many AC wires can there be? Are the wires always exposed to water? As in is the boat in a shop or the water when being worked on?
 

DickCappels

Joined Aug 21, 2008
10,153
A few MHz would probably be plenty. Do not be tempted to connect a ‘scope to any wires that might have spark voltage on then lest you destroy the scope and/or probe.
 

Danko

Joined Nov 22, 2017
1,829
Is that link ok? I cannot connect
It works for me. Below is some info from site:
- - - - - - - - - - - - - -
"Features:
8 channels oscilloscope for vehicle testing. Over 80 types of automotive diagnostic function (Ignition/Sensor/Bus detection/Performe/Starter and charging circuits etc.).
Video Help Function, provides the video help of diagnostic which can be watching online,
The diagnostic result can be generated diagnostic report., printed or taken screenshot by pushing one key, many methods to save the data.
Supported to record waveform for a long time. It can simulate camshaft and crankshaft signal.
Transmit signals through USB 2.0 interface plug and play, and no need extra power supply;

1679763566358.png"
 

Thread Starter

CamaroMan

Joined Nov 21, 2016
8
I actually own a gtc505 https://www.amazon.com/General-Technologies-Corp-GTC505-Ignition/dp/B001TD5JYE

I find it more informative than the DVA tests sometimes.. Its a universal tool but actually quite incredible. I set it to rpm and graph mode and can pick up dead drops instantly - it has about a 30s graph time frame, plenty to see issues...

Most older outboards are just like motorbikes - magnets in flywheel, wired coils = AC output. The stator will have 2 windings, one pair for charging batteries (usually about 25-35 AC that gets converted to 14.3v), the 2nd to charge the capacitor inside the ignition box (250-400v AC). Underneath the stator coil is a trigger coil (usually smaller), and it sends out trigger signal ac pulses (low V, maybe around 1-5v AC) to "release the energy stored in the capacitor to a given cylinder - ie the trigger actually is the timing.

Usually a DVA adapter will show a drop in v, or a severe drop after connecting a faulty component- and thats measurable.

However I am now working on a 1975 johnson thats passes all tests - but still has a kind of a cough or drop at idle.. just rebuilt carbs, fuel is filtered and newly mixed. I noticed shocks coming off the cdi at the rear (the screw terminals are exposed in the older motors up to mid 80s fyi).. I initially thought it was leaking plug wires (those were new too) so tested with some good used ones, and still got some shocks back there . so this led me to suspect the high v output from the stator - from the insulation breaking down - so yesterday i glue gunned the wires coming out of the molded pack and today will wrap ins tape around both and retry - id really love to figure this out because it costs me hours of non billable time as im trying to get better at diagnosing these things..

I told the customer we are replacing the entire ignition component set - however it REALLY bugs me that I dont have a smoking gun and i want to find it!

Here is a video of me measuring the DVA readings: Turn on subtitles so u can get an idea of the readings) - you can hear how its dropping like a plug wire leak or sth.. i think the charge volts are shorting to ground.. as u can see its kind of an open system at the back.

 

debe

Joined Sep 21, 2010
1,389
This is the circuit of a 70s 3 cyl Johnson outboard. They are very simple, this ob had one cylinder not firing. Due to the large price of a new unit we managed to get into it & replace one shorted SCR. Ocilloscope not realy needed.3CYL OB CDI.jpg
 

debe

Joined Sep 21, 2010
1,389
If you have a gtc505 tester you should be able to test each cylinder using the capacitance test lead on each individual plug lead. Hopefully identifying if its one cylinder that missing (may be faulty coil on that cylinder internaly arcing to gnd.)
 

tautech

Joined Oct 8, 2019
383
Hi all - I have a small marine repair shop and once again I have an old 70s 2 stroke kicking my rear end.. Im considering buying a handheld oscilloscope to help id and narrow down faulty wires - its common for the insulation to break up and allow cross arcing which is what im thinking is happening on this one..

Would a 40hmz handheld be enough as a starter scope just to start messing around with and hopefully expose weak/bad wiring? Im not trying to probe space equipment here, more like tractor engine mechanics if i could put it bluntly.

Also - the stators can put out around 300v AC easy, maybe a touch over so i guess id prob need stht hat can manage 350-400v AC , and lastly not sure what a " non-ground referenced (differential) scope " means, but is that something to look into?

Thanks
A scope can certainly help not only identifying which cylinder isn't performing as it should but also exactly where a issue might be but..... a lot is about the skill of the operator and his knowledge of things electrical and ignition systems and how to keep yourself and equipment from being fried !

Typically for old school ignition coils a good ohm meter is all you need and made much easier when you have multiple coils to compare measurements against where if they all read similar then the pulse or energising coils could be weak or if points a dud cap can rob energy required to provide a healthy spark.
Where we can get to each coil and particularly its LV supply testing while running is certainly possible with multiple scope channels to compare driving waveforms and their amplitudes where if low won't provide the required ignition to keep things firing smoothly when under load.
ICE ignition requirements are quite basic however are at levels that can certainly damage a scope if not used with care so having just one 100x probe to check your 10x probes are not to be subjected to damaging voltages is a good idea.
200-400V into coil primaries are not unheard of for auto coils and I imagine marine motors are similar so once the offending cylinder is identified the search for the exact issue is much reduced where we might have vibration induced conductor breakages or insulation failures both often buried in a loom or against some grounded part of the motor.

Best advice is to not overthink ignition issues as its just invisible electricity after all and so nothing special or magical and work through carefully to identify and fix the fault...... the dumbest little things can catch you like corrosion, missing star washers or a broken coil grounding wire or broken conductors inside insulated wires.
Keeping you eyes open and thinking cap on normally finds it.
 

DickCappels

Joined Aug 21, 2008
10,153
I've glanced through the manual but looks strange to me. The electrical specs are good for troubleshooting circuit boards, but can it accept a typical 1 mega ohm ±20 pf probe? I don't see any BNC connectors for a trigger input of probes for the four vertical channels.
 
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