71 Merc 650 runs only in START poosition

[arydant]

I did a bad thing.

I started my outboard with one battery and tried swapping the leads to another while still running. Actually, the motor continued to run for a few seconds but stalled before I could get the leads connected to the other battery.

I reconnected the leads to the start battery again and attempted another start. I was getting ignition as long as I held it in "Start" position, but would cut off when I tried to go to "RUN".

Engine Tests:
Compression OK,
Spark is present during "START".
Fuel pump is OK.
Didn't check timing, but have no reason to think it has failed.

I am now thinking about the mystery "switchbox". I think I may have damaged something in the ignition system, but can't pinpoint it at the moment. I would like to see some circuit diagrams of the switchbox. (I design circuits for a living).

Thank you for your time.

Engine Ser: 3155373
Switchbox #332-2986A8

 

[Faztbullet]

Re: 71 Merc 650 runs only in START poosition

You killed the trigger....check ebay or pull off and send to CDI for repair.

 

[dwco5051]

Re: 71 Merc 650 runs only in START poosition

Might be possible that your ignition switch picked this time to die. Check for 12v with the key on at what I remember to be the white wire that goes from your ignition switch to the switchbox. I could be wrong on the colors since it has been at least ten years since I worked on one of these. I don't have a wiring diagram for your year motor but that was the way they were a few years later. There are letters on the back of the switch and A should be 12v from the battery, B 12v to the starter solenoid, and F 12v to the switch box. If you can locate the motor wiring diagram you should with your knowledge find where the voltages should appear. Before I would blame the switch box I would be certain all the external wiring is working right.

edit - FB I think gave you the right info. I was typing when he answered. My initial response comes from my natural inclination to always look for the cheapest solution to a problem

 

[dwco5051]

Re: 71 Merc 650 runs only in START poosition

You killed the trigger....check ebay or pull off and send to CDI for repair.

At my age the memory isn't really the first thing to go but weren't 1971 Mercs still distributor ignitions?

 

[arydant]

Re: 71 Merc 650 runs only in START poosition

You killed the trigger....check ebay or pull off and send to CDI for repair.

The 71 has a distributor with a trigger ring. It's a 4 cylinder unit.

If I have spark, and the trigger coil is dead then the timing IS the problem in that there is NO timing.

I have a service manual for the 75 thru 78 outboards. I am assuming I have the battery powered CD Ignition system on the '71 engine as well. It explains the operation.

The trigger ring is a double coil unit around a distributor shaft rotor disc that creates a pulse triggering the SCR in the switchbox.

If a trigger ring coil is burnt out then that would be an open and there would be no signal to trigger the SCR.

If this is the case, then I would not have spark when I spin the engine with the starter.

Am I correct?

 

[dwco5051]

Re: 71 Merc 650 runs only in START poosition

I dug into my files and the oldest manual I have is the 75-75 factory one. I am assuming that the this is the cd system the sames as the 650 3 cylinder in operation. ( I know what they say about assuming anything) A quick study of the operation makes me want to agree that the spark should not be dependent on the position of the key switch if it is operating correctly. However I never claim to be close to an expert on these older Mercs.

 

[arydant]

Re: 71 Merc 650 runs only in START poosition

I dug into my files and the oldest manual I have is the 75-75 factory one. I am assuming that the this is the cd system the sames as the 650 3 cylinder in operation. ( I know what they say about assuming anything) A quick study of the operation makes me want to agree that the spark should not be dependent on the position of the key switch if it is operating correctly. However I never claim to be close to an expert on these older Mercs.

Thanks for your diligent research.

My service manual does not have the correct diagram, but I do have one for my engine that I downloaded some time ago.

"Start" appears to essentially be the same as "RUN" but energizes the starter. Once the engine is running, the starter is disengaged. I can see spark on a plug external to the cylinder when I spin it with the starter, but when installed the engine it stops when I release the key to "RUN". If the trigger is bad, I don't understand how I can get spark at all. Perhaps I don't understand how the system works entirely.

Magneto charges cap. Distributor shaft turns rotor. Rotor causes pulses in trigger coil assembly. Pulses fire SCR. SCR discharges cap into ignition coil primary. Coil secondary delivers HV to plugs. Plugs ignite fuel.

 

[Faztbullet]

Re: 71 Merc 650 runs only in START poosition

Here is the trouble shooting from CDI, notice last paragraph....

Troubleshooting
NOTICE: The trigger cannot be bench tested except with the CDI Electronics Trigger Tester P/N: 511-9710 (the tester can also be used to test the trigger while on the engine). Be sure to follow the instructions at the bottom when lining up the rotor and disk.
Dead or no spark until you let off of the key switch:
1. Clean ALL engine ground wires from the battery thru the engine cowling, splash pan, top cowl and support brackets.
2. Verify that there is at least 9-1/2 volts on the White wire going to the switch box (from the engine harness) while cranking the engine.
3. Verify that there is at least 9 volts on the Brown wire going to the trigger while cranking the engine. If the voltage is
low and the voltage on the White wire from the harness is above 9-1/2 volts, disconnect the wires from the switchbox, then check the resistance from the White terminal beside the Red terminal to the Brown terminal going to the trigger. You should read 60-76 ohms. A high reading indicates a possible bad switchbox.
4. Check resistance between the White wire and engine ground. It should read less than 1 ohm. If it reads higher, make
sure the Black ground wire is connected to engine ground.
5. Connect a secondary ground wire (12 gauge) from the negative battery cable to the base (or case) of the switch box.
6. Using the 511-9710 Trigger Tester, check the output of the trigger according to the directions for the tester.
Only has spark as long as the starter is engaged: Usually indicates a bad trigger. Test per above

 

[dwco5051]

Re: 71 Merc 650 runs only in START poosition

Thanks for the info. It is nice to learn something even if I never run into this problem. Since I was a Mechanical Eng. major I am forever mystified by what happens inside those little black boxes.

 

[arydant]

Re: 71 Merc 650 runs only in START poosition

Thanks - I found several references to this online - particularly on CDI. Only has spark as long as the starter is engaged: Usually indicates a bad trigger. Test per above is my issue so I will be ripping out the trigger. It's about $230 to have it repaired. Looks like it's potted. So is the Switch box. I am an electrical engineer and me hate potted things. Unless someone has a schematic, it is impossible to understand how it works. The Switch box provides a low level signal to energize the trigger and a high voltage signal to charge the discharge cap. It should be possible to do bench testing with standard bench top equipment as long as there are parameters to follow, but Mercury developed customized testing equipment so that technicians can diagnose without having to fully understand the circuitry or know how to use lab equipment.

 

[Faztbullet]

Re: 71 Merc 650 runs only in START poosition

Here ya go.....

View attachment IMG.pdf

 

[dwco5051]

Re: 71 Merc 650 runs only in START poosition

Now my curiosity is getting the best of me.

I am thinking that a bad trigger unit can still deliver enough signal at cranking speed but as soon as the motor revs up to idle speed or above the signal is not good enough for the switch box to function.

Anybody know if that if correct?

 

[arydant]

Re: 71 Merc 650 runs only in START poosition

Oh great! A schematic - well most of one anyway. I can see how the alternator can damage the trigger without the battery to regulate the input voltage. The diagram appears to jibe with the system description in the service manual. But it looks incorrect. It shows the switch, diode and the capacitor in the wrong configuration. The converter, or more correctly, inverter cannot charge the cap with the diode drawn and the switch grounds out the cap instead of connecting it to the coil. I still don't know how the trigger is constructed and it isn't apparent how I can get spark with the starter engaged but not in RUN. The trigger is made of two opposing coil elements. The primary is DC energized and the rotor disc cuts the field creating a pulse current in the secondary that triggers the SCR switch. A damaged trigger would mean a shorted or open primary coil. Either way, no field is present to trigger the capacitor into the main coil. That's about as much as I can figure at the moment.

dwco5051, as far as the trigger signal is concerned, it is the output frequency of the trigger that varies with engine RPM - the current would remain the same since it drives an SCR. This is a very low level signal, so I don't think you are correct.