Does this seem like a bad blocking diode?

[sutor623]

This is a 1996 Evinrude 48hp, Esledc, it is a motor of a friend of mine. Dealer said powerpack was bad and changed it. Problem was still there. He brought the motor to me and I found that it was dropping spark completely and then picking it back up again, about mid rpms. Checked brown wires from power coil, and the DVA dropped from 280V down to 0 just before the "miss." We pulled the flywheel and changed the charge coil and timer base. I also changed the dual ignition coil just for kicks.

Anyways, I had checked the blocking diode in the engine harness, and got 636 ohms one way and infinite reading the other. I assumed that was good to go.

Problem STILL exists. The entire ignition system is brand new. I hooked up the old powerpack, and left the tan wires disconnected and the alarm in the box was buzzing. I think the pack shorted and took out the blocking diode when it initially failed. Put the new powerpack back on and alarm triggered on and off and then stayed off.

Does this sounds like a blocking diode to you guys, even though it seemed to have tested ok? Is there anything electrical in the boat side electrical system that needs to be checked? At this point I want to change the diode because it is highly suspect........

Here is a vid of her.

 

[sutor623]

Here is a video of her running on the water. At first all seems well, then the devil pokes his head out.

 

[Grandad]

I'm not an electronics expert, but have some knowledge of electrical theory. That reading of 636 ohms sounds much too high to me for a functional diode. Using simple Ohm's Law math, power lost to heat at given point is equal to current squared times resistance. So if only 1 amp was flowing through the diode, you'd have it dissipating 636 watts of heat. So ya, I'd suspect the diode, but I'm still learning. - Grandad

 

[Vic.S]

I'm not an electronics expert, but have some knowledge of electrical theory. That reading of 636 ohms sounds much too high to me for a functional diode. Using simple Ohm's Law math, power lost to heat at given point is equal to current squared times resistance. So if only 1 amp was flowing through the diode, you'd have it dissipating 636 watts of heat. So ya, I'd suspect the diode, but I'm still learning. - Grandad

If the OP checked the diode using a meter with a diode test range that figure of 636 would be the forward volts drop in mV and be normal.

Digital meters dont work too well for testing diodes on the ohms ranges . Old fashioned analog ones usually do though

 

[racerone]

Somebody needs to review OHMS LAW.---------With 12 volts and 600 ohms how can you get 1 amp ?

 

[Grandad]

Hi racerone. I could school you, but first ask you this. Between what 2 points did you figure you could measure 12 volts? You need a greater understanding before raising this challenge. - Grandad

 

[flyingscott]

Have you checked the temp sensor. If your test said you had continuity one way and not the other it is probably good.

 

[sutor623]

Thanks guys.

So, the test showed 636 ohms of resistance one way, and no reading the other way. There was no continuity in either direction. This is a digital multimeter. It does NOT have the diode function.

I ran the motor with the temp sensor disconnected, still the same issue. I also ran the motor with the tan wire disconnected from the powerpack, same issue.

Here is the odd thing. I hooked up the old powerpack and LEFT THE TAN WIRES DISCONNECTED and the alarm was sounding with the key on!!! (Keep in mind the motor is NOT overheating, and is premix from the factory.) There was a short somewhere in the powerpack that grounded out the tan wire is my guess. When I put the new pack on, again with the tan wires completely disconnected, the buzzer buzzed for a few seconds intermittently, and then stopped. At this point, if the diode is not bad, maybe there is a short in the harness.

 

[flyingscott]

check for something pulling on the wiring harness or pinched wires

 

[sutor623]

I looked around pretty good. Didn't see anything out of sorts, but you know how far that can go............ Why else would that motor drop spark on both cylinders sporadically like this? Maybe a bad connection in the kill switch? It is just odd that it only acts like this from half throttle and up. Dont have a tach but it seems to be doing it around 3000rpms or so. I think I want to fire up that motor with the boat side wire harness disconnected, to rule wiring on that side out of the equation.

 

[AlTn]

seems logical, sut....has the ignition switch been tested or replaced?

 

[sutor623]

seems logical, sut....has the ignition switch been tested or replaced?

Hey bud,

Nah hadn't fooled with the ignition switch yet. Could VERY well be intermittently grounding out the kill circuit intermittently. I am going to disconnect the wiring harness from the boat side and fire her up and see what we got. If that provides resolution I will take the switch out and check it. Dont want to disassemble the control box if I don't have to. Springs and detents flying all over the place!

 

[gm280]

sutor623, if that "blocking" diode is questionable to you, replace it. I mean how much can a simple diode cost? With that out of the way, and you still have the same issues, I would look at the timer plate movement for an answer. You stated that it runs great 'til about 3000 RPMs, and that points to something going on at that time. Well the timer plate moves to advance timing and it could be stretching or manipulating the wires going to the coils. JMHO!

 

[schematic]

sure sounds like its going into SLOW mode. Try disconnecting the the tan wire AT THE PACK connector. If it still gives trouble, look to the pack....

 

[sutor623]

sure sounds like its going into SLOW mode. Try disconnecting the the tan wire AT THE PACK connector. If it still gives trouble, look to the pack....

I disconnected the tan wire at the pack, and at the temp sensor, motor still does the same thing. The powerpack is brand new, and the old pack was also acting the exact same way. Did you see the latest video that I posted on here?

 

[Fed]

I think the tan wire at the pack should have Voltage on it (via the horn and then the diode) to work properly then grounding it via the temp senders would send it into SLOW.
I have no idea what disconnecting the tan from the pack would do?
I should add I have no idea what the diode's function is either, do you know sutor?

I'd replace the diode given the readings you got or at least re-test it with an analog meter and if you do replace it test the new one to compare before installing it.

 

[schematic]

Leave the tan disconnected at the pack. Also disconnect the black/yellow at the pack and try again. If it runs fine, the switch or harness is bad.
When doing this test, the engine will not shut off with the switch......kill it with the choke/primer

 

[sutor623]

Schem, that's a good idea about the kill wire because there are only 6 wires that go into that back. Two from the brand new timing sensor, two from the stator charge coil, one is tan and the other is kill.

fed my understanding is the diode protects bad signals from traveling backwards toward the alarm circuit, forcing it into slow.

 

[clanton]

Tan wire disconnected at the power pack, will bypass the diode. Test as Schematic said, still have problem, recheck charge coil, timing sensor, pack, and ignition coil output voltage with peakread reading meter.

 

[sutor623]

Tan wire disconnected at the power pack, will bypass the diode.

Yea thats what I was thinking..... At this point I am thinking it is a bad key switch.