Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-2918A6

Doug Redinger

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Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-2918A6

Does anyone know how to test these old ignition triggers. I took it to my Mercury dealer and he said he hadn't tested one in decades and he didn't remember how to test it. I thought he might be able to test it on his old Mercotronic analyzer. I followed EMckelvy's procedure for testing my switchbox and coil and both appear to be working fine. I then pulled the distributor and disassembled it and want to test the trigger. It has 3 leads one white, one brown, and one black. It appears that I have a Merc 1350 (1971) powerhead serial # 3116535 on a Merc 1000 labeled leg with a Merc 1250 Cowl wrap. The ignition appears to be from an earlier Merc (950, 1000, 1100.1250) as it is the merc distributor # 393-2799A9 . It has the merc trigger # 393-2918A6 The rotor appears to be from a merc 950 or 1100 part # 393-2921 Any help as to how to test this old trigger would be greatly appreciated!!! Thanks in advance.

Doug
 

emckelvy

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Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Doug, you should be able to test the trigger, even disassembled, by connecting the 3 wires to the switchbox, grounding the metal body of the trigger housing, and passing the copper "chopper" disc thru the trigger gap. This should make it spark.

It may be easier to install the trigger & rotor (with chopper) back into the distributor body temporarily, so you can spin the rotor, causing the chopper to spin in the trigger gap.

If the trigger fails this test, it's definitely bad.

I've bench-tested complete distributor assy's before by hooking to a switchbox, and grounding the parts together using the ground wire that's normally attached to the distributor.

Spin the shaft and they spark like mad!

It's a lot easier (and less frustrating) than reassembling everything and finding out it still doesn't spark! :grumpy:

HTH & let us know what happens...........ed
 

Doug Redinger

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Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Thank you again Ed for your great to the point advice. I ordered a new trigger from Ebay and installed it in the distributor. While I had everything apart I made a new set of plug wires using Accel Super stock copper core wires. I also tested my rectifier which as it turns out had a bad diode. I replaced the rectifier with a combination rectifier-regulator made by Trail Tech ( 7004-RR150 )which allows charging with adjustable voltage output. This should keep voltage from reaching dangerously high levels and help save my electronics like my switchbox and trigger and coil. I also repaired some of my wiring which had crumbling insulation on the stator leads and several other wires which were in poor condition. After installing the distributor and timing it with the flywheel I readied the motor for starting. It fired right up and seems to idle great. The water output from the the pee-hole seems to be very weak so I will drop the lower unit next and check the impeller and seals.

Doug
 

wheelerdealer

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Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Does anyone know how to test these old ignition triggers. I took it to my Mercury dealer and he said he hadn't tested one in decades and he didn't remember how to test it. I thought he might be able to test it on his old Mercotronic analyzer. I followed EMckelvy's procedure for testing my switchbox and coil and both appear to be working fine. I then pulled the distributor and disassembled it and want to test the trigger. It has 3 leads one white, one brown, and one black. It appears that I have a Merc 1350 (1971) powerhead serial # 3116535 on a Merc 1000 labeled leg with a Merc 1250 Cowl wrap. The ignition appears to be from an earlier Merc (950, 1000, 1100.1250) as it is the merc distributor # 393-2799A9 . It has the merc trigger # 393-2918A6 The rotor appears to be from a merc 950 or 1100 part # 393-2921 Any help as to how to test this old trigger would be greatly appreciated!!! Thanks in advance.

Doug

Can you please direct me to where I can find the switchbox and coil testing procedure you refer to? I have the same ignition system in my 1970 mercruiser 427 325hp inboards. Thanks!

Burt
 

emckelvy

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Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Hi, Burt, here's the testing procedure for this type of Merc CDI ignition:

"MERCURY BATTERY-POWERED CDI TEST FOR DISTRIBUTOR MODELS

This test is for the 332-2986 switchbox used from 1967-1978 on all the inlines.

This test assumes your coil is good (problems with CDI coils are rare).

DISCONNECT BATTERY

1. Turn off ignition;

2. Disconnect all 3 distributor wires on the Port side of the switchbox (and the ?mercury switch? if present);

3. Remove the HV lead from the ign coil to the center of the dist. cap (remember it unplugs from the coil and unscrews from the cap);

4. Reconnect the HV lead to the COIL only;

5. Position the free end of the HV lead approx. 3/8" from ground (block, shrouds etc), and find a way to hold it there;

6. Jumper the brown and white terminals on the dist. side of the switchbox to each other.

RECONNECT BATTERY

7. Check that you have +12V at the red terminal (even with the ign off);

8. Turn on ignition and verify +12V at the white terminal (same side as the red terminal);

9. Ground the black terminal on the distributor side of the switchbox - this should cause a spark each time you touch ground.

If you get spark with the distributor bypassed, and it won't fire with the distributor connected, the trigger is bad and the entire distributor housing assy must be replaced.

If you get no spark using the test, the switchbox is probably bad. In that case, be sure to check for correct power on the switchbox, check all connections, and check the coil's resistance to make sure it's OK."

Note that these instructions are specific to a Merc outboard, and locations such as "port" ain't necessarily gonna apply to your Mercruiser. And I don't expect you have a "mercury" kickup-kill switch on your I/O's.

But function-wise, the ign works just the same, and if I recall correctly, a V8 Mercruiser and a 4-cyl outboard use the same switchbox. The power pulses per revolution would be identical for a 2-stroke 4-cyl and a 4-stroke 8-cyl.

HTH...........ed
 

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wheelerdealer

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Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Thanks Ed. This is very helpful. I think I can adapt it to an 8 cylinder inboard.
 

wheelerdealer

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Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Re: Testing ignition trigger on 1969 merc 1250 " FrankenMerc " trigger part # 393-29

Do you know what the resistance on my coil should be? My analog meter shows almost no resistance. Looks like a dead short. I will try it with a good fluke and see what I get. Thanks again
 
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