76 Merc 500 Coil DVA?

Part of the Mercury trouble shooting manual states "Check the DVA output on the green wires from the switch box while connected to the ignition coils. Check the reading on the switch box and on the ignition coil terminal. You should have a reading of 150V or more at both terminals." I checked from the 4 green wires to ground and didn't get any reading which is odd because the motor starts and runs. The DVA readings from the stator and trigger coils were ok even though the resistance of the high speed coil was half of what is supposed to be. I am using a Fluke 87 on 120ms max reading scale. The problem with the motor is when cold it doesn't have full power, 22mph but after a minutes running WOT it kicks in another cylinder and ups to 29mph and is fine for the rest of the day and it always starts immediately. Can't figure why I can't get a reading from the coil primaries, any suggestions?
The manual also states if the reading is low disconnect green wire from coil and connect load resistor (doesn't say what size) and retest if ok bad coil if low bad power pak. The coils check out resistance wise and the motor runs so I think coils are ok. I need to figure out why I can't read the output from the switch to determine if it is bad.

Re: 76 Merc 500 Coil DVA?

Since nobody in this forum seems to know anything about reading coil DVA does anybody know of another boating forum where there might be some help with this?

Re: 76 Merc 500 Coil DVA?

Before you trash anyone on here try doing a search about your question. If you don't find anything perhaps you can get a refund for what you paid for the advice.

Re: 76 Merc 500 Coil DVA?

Have you checked that you are losing spark? Weak spark on all cylinders? Better spark the faster it runs? Your 500 has an ignition driver correct? Compression and fuel check out ok?

Re: 76 Merc 500 Coil DVA?

You need a DVA adapter to use with the meter... the meter alone will not read it correctly.

Re: 76 Merc 500 Coil DVA?

gonefishy said:

Since nobody in this forum seems to know anything about reading coil DVA does anybody know of another boating forum where there might be some help with this?

Click to expand...

Your right best to go to coil dva .com or ask your local marina to help you for free.

Re: 76 Merc 500 Coil DVA?

The Fluke is a fine meter, HOWEVER, the peak reading feature is NOT capable of reading DVA, you still need a DVA adaptor to use WITH your FLUKE.

Most any ignition questions may be answered by reading thru the excellent Ignition Troubleshooting Guide found on the Support pages of CDIElectronics

Re: 76 Merc 500 Coil DVA?

Re Wingedwheel
I was not trashing anyone, after 100 views and no reply's I assumed no one had an answer and I did try a search on "coil dva" and came up with evinrude and mercruiser and johnson posts none of which talked about coil dva or an ignition system relative to mine after reading several pages of posts I decided to post. It's also hard to get a refund on zero pieces of advice, yours was the first post and it wasn't worth 2 cents.
Thanks for all your help.

CharlieB
I did some research and you are right my Fluke 87 is not fast enough to capture the coil peak at 120ms apparently a Fluke 87V would as it is faster 250us. I ordered a DVA adapter, PVA-1 from Stevens Instruments, hope it is good quality, does anybody here have one.
Thanks, will post results with new adapter after it arrives.

Re: 76 Merc 500 Coil DVA?

The reason that you use a DVA is because it stores the voltage momentarily and the meter reads that. Unless your meter has a DVA built in, it can't react fast enough to give you an accurate reading if any reading at all. Either build a DVA yourself or buy one and that will tell you if it's a switch box problem or not. Personally, it sounds like you've got low compression in one cylinder and once it warms up and the piston swells a little it seals better and can hold enough fuel to fire.

Re: 76 Merc 500 Coil DVA?

Jlawsen said:

The reason that you use a DVA is because it stores the voltage momentarily and the meter reads that. Unless your meter has a DVA built in, it can't react fast enough to give you an accurate reading if any reading at all. Either build a DVA yourself or buy one and that will tell you if it's a switch box problem or not. Personally, it sounds like you've got low compression in one cylinder and once it warms up and the piston swells a little it seals better and can hold enough fuel to fire.

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I will check compression cold, what would be a pass for a 35 year old merc 500?

Re: 76 Merc 500 Coil DVA?

It aint a piston swelling.....:facepalm: You either have a bad switchbox or a coil grounding issue. DVA stands for DIRECT VOLTAGE ADAPTER and has nothing to do with voltages...just on capturing and reading them with meter. That why your got little response to the post:

Since nobody in this forum seems to know anything about reading coil DVA

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as there is no such thing as coil DVA.

Re: 76 Merc 500 Coil DVA?

"The problem with the motor is when cold it doesn't have full power, 22mph but after a minutes running WOT it kicks in another cylinder.............."

What i would do is find the cyl(s) that aren't firing when cold and that will help isolate (narrow) the problem(s), then go to cdi's website and systematically go through the steps:

http://issuu.com/cdielectronics/docs/troubleshootingguide

From my research coils go bad on old johnnyrudes but on mercs its a rarity

Re: 76 Merc 500 Coil DVA?

as there is no such thing as coil DVA.

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3. Check the DVA output on the green wires from the switch box while connected to the ignition coils. Check the reading on the switch box terminal AND on the ignition coil terminal. You should have a reading of at least 150V or more at both places. If the reading is low on one cylinder, disconnect the green wire from the ignition coil for that cylinder and reconnect it to a load resistor. Retest. If the reading is now good, the ignition coil is likely bad. A continued low reading indicates a bad power pack.

The above text directly quoted from a Mecury troubleshooting guide explains how to take a DVA reading of the coil primary.

Re: 76 Merc 500 Coil DVA?

You're both right, just using different words.

Dva is just a term for the measurement of voltage and current, the measurement can be done in many locations, all help to determine where any fault may be in the ignition system.

Re: 76 Merc 500 Coil DVA?

Got my Steven's DVA adapter for my Fluke today and reran the tests. Stator and trigger voltages were ok but the outputs to the coils on 1 & 2 were over 200V but 3&4 were 0V. Did the tests of swapping stator wires problem stayed on 3&4 then did same on trigger wires problem again stayed on 3&4, this would indicate a bad switch box just to make sure I swapped the green wires going to 2 and 3 and #3 coil fired so it is not a coil problem.
One odd thing though my high speed stator resistance was only 65 ohms supposed to be 125-155 but the DVA was 45V supposed to be over 25V, if the ohms are low suggesting shorted windings why is the DVA so good, maybe it won't show a problem till run at high RPM?

The decision now is with 90psi compression on all 4 cylinders how much money should I sink into this 36 year old motor, it's definitely going to need a new switch box, the bottom end needs a new water pump and oil seals and may need a new stator soon although it does run good at WOT once the switch box gets warm and it starts firing on all cylinders.

Re: 76 Merc 500 Coil DVA?

Compression will vary depending on the quality of the gauge used, as long as you get consistent readings on all cyls and the motor starts and idles, compression is not an issue.

The stator is fine, I wouldn't worry about it at all.

DVA isn't just holding the voltage long enough for a reading, it is also placing a measured load on the circuit. A broken yet abutted wire in a circuit can still carry voltage, yet fail once required to carry current/load. An ohms reading is merely an indication that a circuit exists, it cannot tell you if that circuit will hold up to any current flow.

Long before I learned of DVA an old man taught me to connect a battery and test light in series with the charge coil in question, if the light shone, the coil was good. You must be careful in testing in this manner as battery voltage must not exceed the capacity of the coil or you can burn the coil. For a stator I used a 12 V battery, for a trigger only a flashlight 1.5 V battery. Got my first DVA meter in '93