1973 Merc 850 no spark when cranking ???

[tjerzak]

The motor ran at the end of last season. All summer I have been trouble shooting no spark. I replaced the distributor and had all the motor wiring and modual checked. The casings on the Stator wires were cracked and I taped them. I put a timing light on it and when I crank the motor I get no spark, but when I let off the key, every once in a while I get a quick flash. I tried to turn it over by hand with the key on, but only get spark once in a while. Is it possible the key switch is malfunctioning or could it be in the wire harness? I have paid 3 mechanics to tell me they can't figure it out. Any suggestions?
Thanks, Tom

 

[tjerzak]

Re: 1973 Merc 850 could it still be the Trigger ????

Re: 1973 Merc 850 could it still be the Trigger ????

The motor ran at the end of last season. All summer I have been trouble shooting no spark. I replaced the distributor and had all the motor wiring and modual checked. The casings on the Stator wires were cracked and I taped them. I put a timing light on it and when I crank the motor I get no spark, but when I let off the key, every once in a while I get a quick flash. I tried to turn it over by hand with the key on, but only get spark once in a while. Is it possible the key switch is malfunctioning or could it be in the wire harness? I have paid 3 mechanics to tell me they can't figure it out. Any suggestions?
Thanks, Tom

10:47 pm est
I took the merc control off and checked the key switch.
Checked all wires., ok
When I crank there is no spark.
When I let off the key I get spark once in a while. I turned on the key and turned the flywheel. When I did get spark after about 4 or 5 turns, if I rocked it back and forth I would get a spark. Could it still be the trigger?

 

[Chris1956]

Re: 1973 Merc 850 no spark when cranking ???

Tom, I have attached the troubleshooting procedure Clams Canino has provided. It should help you isolate the problem.

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 (most all are).

DISCONNCT BATTERY

1. Turn off ignion
2. Disconnect all 3 wires on the distributor side of the switchbox.
3. Remove the HV lead from the coil to the center cap (remember it unscrews
from cap)
4. Reconnect that HV center lead to the COIL side only.
5. Position that HV lead about about 3/8" from ground (block, shrouds etc)
and find a way to hold it there.
6. Jumber the brown and white terminals on the dist. side of the switchbox
together.

RECONNECT BATTERRY

7. Check that you have +12 V at red terminal.
8. Turn on ignition and verify +12V at white terminal (on red terminal side)
9. Touch the black terminal to ground - unit should cause spark each time
you touch ground.

If that passes and it won't fire at all, suspect the trigger

 

[Chris1956]

Re: 1973 Merc 850 no spark when cranking ???

Tom, The first simple check is to assure that you have +12V on the red and white wires of the switchbox, when the ignition key is on. No voltage on both = no spark at all.

 

[tjerzak]

Re: 1973 Merc 850 no spark when cranking ???

Thanks, I will try it over the weekend.
I did test voltage on the white wire going to the distributor while cranking and I was getting +12volts and I checked the red for power also.
The distributor I put in was off a parts motor so I don't know for sure that it was good. I hate to spend $400 for a rebuilt trigger if that isn't it but I don't know were else to go. One mechanic said the modual and coil were both good.
Thanks for the help.
Tom