What is a "CD Voltmeter"?

[Anonymous]

Hi Everybody,

First let me say that I am the guy that has been known off and on here over many years as "outbdnut", with over 1000 posts, but after not using the board all last winter (many issues going on at my house last year, including my heart attack) the board would not let me log in, so I had to re-register with another name to get back on. So from now on, I’m "oldoutbdnut".

Now for my question: In a Clymer repair manual for OMCs, when troubleshooting electronic ignitions on 25-35 HP models from the 1980s, they refer to the use of a "CD voltmeter". I assume the CD means Capacitive Discharge, but how does this differ from a conventional voltmeter? Does it read the peaks of complex waveforms or pulses?….like the peak of a capacitive discharge spike? Don’t be afraid to use electronic terms – I’m a retired Electrical Engineer.
If it’s reading peaks, I can use my oscilloscope to do these tests. Anyone out there done that? My cousin is bringin’ over his 1985 25 Johnson with no spark for me to work on.-
Thx in advance….and…it’s good to be back active in the hobby again!
Dave

[fleetwin]

Dave
Glad to have you back, and am hoping you have made a full recovery from the heart attack.
In any event, I’m not educated enough to give you the explanation you are looking for, except to say that a conventional digital voltmeter won’t work, but I know they make affordable adapters that can be used with a conventional voltmeter.
But, I can speak about doing the basic stuff first with these systems before getting buried in output tests.
Your cousin’s 25hp has no spark on either cylinder, so the first basic test you would want to perform is the stop circuit elimination test. Pressing the stop button grounds the powerpack’s internal capacitor keeping it from holding a charge. Look for the two black stop switch leads coming from the engine pan. One lead will be grounded, the other stop switch lead should end up in the five pin rubber connector on top of the cylinder head. The tricky part is pushing the black stop switch lead pin out of the rubber connector, the amphenol pin tools make this easier, but I’m sure you can rig something up. Split the connector and dip the stop switch side of the connector is some rubbing alcohol to loosen the rubber’s grip on the pin. Push the stop switch pin out of the connector, then reconnect the rubber connectors and check for spark. Please do not cut the leads, and simply disconnecting the stop switch’s ground lead is not a complete elimination test. If the engine has spark with the stop circuit pin removed then you will need to look for shorted stop switch leads and/or replace the stop switch.
The next simple test is to simply look at those amphenol rubber connections, they do cause problems. Sometimes the pins/sockets get pushed back in the rubber connectors, the pins get bent/misaligned also. Gently tug on each lead in the rubber connector to make sure the wires haven’t broke free from the pins/sockets. Always use rubbing alcohol to "lubricate" the connectors to make reconnecting them easier. Never use WD40/grease, or anything else that might actually conduct electricity.
Your cousin’s engine has the primary push on coil connectors coming from the powerpack also, these connections get loose, and corroded, especially in salt water. Pull the primary connections off, check for rust/corrosion, and clean the terminals as best you can.
Then, crimp the primary connectors slightly so they will fit tightly on the coil primary studs. Go ahead and use a little grease/moly lube on the primary coil studs to help prevent further rust and corrosion. You might want to check the coil secondary connections/wires also, those spring pins get rusted and loose continuity.
Finally, check the powerpack and coil grounds with an ohm meter set on the low ohm’s scale. Checking between each coil’s ground strap
and a good engine ground should show as close to zero ohms as possible. Do the same test between the powerpack’s ground lead and a good engine ground.
Once these tests have been completed, we can move on to resistance and output tests if there is still no spark. I would be glad to send you one of my PRVs, but I am guessing you want to get the problem solved ASAP.

[orpilot]

Hi, I think what they are referring to is a kilovolt meter that measures voltage right out of plug-wre. I bought one back in the 70’s and it was $300 or $400 back then. It set up to measure ignition output while the motors running. I still have it but it was a bit sensitive and I had to send it back for repairs a couple of times.

[legendre]

CD meter is synonymous with DVA meter aka peak-reading AC/DC voltmeter. If you have a quality DVM with a peak sample-and-hold feature (like many Fluke models) that will work fine.

Likewise, if your scope can capture and measure the brief pulses, that should work just as well.

[orpilot]

I believe your repair manual is referring to one of these… http://www.stevensinstrument.com/peak_dva.htm

I have one and I believe it was the standard model that a lot of OMC serviced manuals refer to. You’ll notice they have an adapter as well.

I see they have a newer improved and much cheaper smaller KV meter… at the bottom of the page.

http://www.stevensinstrument.com/spark_checkers.htm

[r-c]

The CD ignitions need a peak volt meter, not one for the spark plug end. The most common we used in the 1980’s (when I was servicing OMC outboards) was a Stevens CD-77. It seems to be used to this day too.

[legendre]

While the Stevens unit may be a well-respected and perfectly useful industry standard, one can most likely acquire a used Fluke meter with the same capability for roughly the same cost, or even less – and the Fluke will have many other useful features that the Stevens lacks.

There’s nothing magic about a peak-hold meter.. given sufficient bandwidth, these days, it’s all done in software.

[billw]

Here is one example available for a reasonable cost:

http://www.ebay.com/itm/Digital-Multime … 43&vxp=mtr

I may be, and frequently am, wrong; but my Fluke DVM has a min-max feature. I never tried it as a peak reading volt meter but I would expect that it would read just that: The ONE peak in a long sample, as it does for conventional AC and DC readings. The meter above is nothing special but the plug-in adapter that is show with it allows you to monitor the peaks of the waves continuously.

Long live American manufacturing!

[Author]

Posts