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ignition problem

G

gmcinnes

Member
Ok,Ok, so I changed over to Pertronix ignition from points system, now I have this problem. This is a 1978 Mercruiser MCM470 4 banger. I also in stalled new 3.0 ohm Pertronix coil and I have 12.4 volts on the Pos. terminal of the coil, wire connected at the terminal, but immediately when I connect the Red Ignition wire from the Petronix module, the voltage drops off to 7 volts. But get this the engine will start but I have had to disconnect the gray wire from the Neg. side of the coil for the tachometer.???? I have disconnected the wire on the electric choke. The wire with the 12.4 volts is brown, and I disconnected the purple/yellow as I thought this was the resistance wire. What should the voltage be at the coil (pos.) with the wires connected. I sure don't get why the gray wire should have anything to do with it starting or not starting, is it grounding out the ign. module. Wierd? I would appreciate all comments. Thank you.
 
in the original configuration you have a special wire (material not copper) in the harness that creates ~ 2 ohms of resistance. This typically drops the votage to the coil + when running to ~ 9.xx volts DC.
The coil did NOT have a internal resistor.....................

If you installed a coil with a internal resistor then it is possible to see 7.xx volts when running.

Electronic thunderbolt IV and V have a coil with internal resistance........NOT point ignitions coils......
 
Yes battery voltage at pos. on coil is 7.5 all connected ,key on. Tach wire connected or not, no difference. How all this started (points system) was I was running it in the yard with water hooked up, put it in reverse and it quit, could not get it started again. I never even thought it might be the tachometer, just could not get any spark so I ordered new coil and ign. module. got it running this am, leaving the tach wire off. Seems to be running ok. We'll see if I have any problems once I get it in the ocean.
Thanks
 
Yes, original Merc Manual I have shows resistance wire in line, runs to electric choke. Have that disonnected as well. get about extra 1 volt.
 
Yes original diagrams showed resistance wire in line. Fired it up this morning, with Tach wire disconnected. 7.5 Volts all wires connected not running. Running it was about 11 volts (positive terminal on coil and ground on engine) on the digital meter. This was Pertronix 40611 coil and does show it has internal resistance. Strange thing is I can attach the gray tach wire once the engines is running and the Tach works but if I shut the engine down and leave it for a few minutes it will not start again unless I disconnect the tach wire. The longer I live the more I know I don't know anything.
Thanks
I'll probably be back.
 
Master mechanic said:
Electric choke on a 78' ? The 470 was not made in 78'.
Resistor wire ? On omc yes. Merc. ?
You changed out the the complete dist. ?
Click to expand...


YES mercruiser has a resistor wire in the harness. I have actually unwrapped the wire harness in many of an older boat (point ignition) tracing other issues and have identified the resistor wire!! It is approx, 3 ft long and is in the harness tied to two or three other wires in a junction heavily wrapped in tape. The wire goes towards the coil and is folded back on itself. The length determines the resistance value.
OMC when they were using early ford/GM motors had the visable Ballast resistor.


Yes the 470 was available in 1978............

I believe he only removed the point plate and installed the kit, BUT I think he purchased a internal resistor coil, Not needed with a resistor wired harness/ignition. He has doubled the resistance from ~ 2 ohms to ~ 5 ohms.......

Most likley will see top end miss. Not enough umph.........
 
kghost, Yes, now thats what i'm talking about, you are correct in all those issues, Yes, I need to rectify this, hopefully without unwrapping a lot of this wiring harness. Will changing over to a not internally resisted coil correct this issue? Can you recommend a coil that would do this? Or should I start unwrapping the wiring and can you tell me what to look for in this resistance wire. I want to do this right and concerned about not enough uumph, and miss.
Can you comment of the Tachometer issue?
Thanks millions
 
If you have a bad tachometer it will short out the coil..........only way to tell is to get another one and see if it does the same thing, maybe borrow one from any source, A after market auto tach will work or if you have a friend with a older boat (pre efi) the borrow it.

It also could be that the high resistance of the wire and internal resistor coil may be the cause, add the tack and the voltage/current drop may be too great.....

You will need to unwrap the wire harness and find the multi wire junction where the resitor wire is connected. The core material looks similar to aluminum not typical copper. Remove that wire from the system and add a PURPLE 14 ga wire back at the junction and rewrap the harness.

Dont use cheap vynal elec tape get some quality 3m for the rewrap.....
Attached is a schematic, The ignition curcit should be correct even if the rest may not be for your application. there are several schematics for several variations. the ignition is basically the same for all.

One more thing, you mentioned when you shifted it it stalled.

You are aware there is a SHIFT INTERUPTOR SWITCH ????

This switch sits in a V notch plastic piece where the short shift cable and control cable meet (shift bracket) If this switch is made when shifting INTO gear it will short the ignition to ground and stall/not allow the motor to start. (either bad cable adjustment or bad short cable)

This interuptor switch is there for only one purpose, WHEN YOU SHIFT OUT OF GEAR (FORWARD OR REVERS) THE RESISTANCE ON THE SHORT SHIFT CABLE CAUSES THE V NOTCH TO MOVE AND ACTIVATE THE SWITCH. This kills the ignition for a split second allowing the gear and clutch dog in the lower unit to dissengage.
 
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Ok, yes, I got the resistance wire removed, quite a job unwrapping all that tape, especially in behind the engine ,yep about 3 feet long or more. Rewired purple wire direct to the coil and back to the electric choke. Now I have about 10 volts at the + pos. on coil, and it started instantly and measured 13.5 volts at pos. coil, when it was running. Did not try and start it with the Tach connected but I tried connecting it with jumper when it was running and now I have no signal at all. Now I believe this was my problem from day 1 when it stalled and would not start again. Anyways now It made me change over to electronic ignition, rotor, cap, and coil. Seems to run nice, better response, smoother. Guess I'll be looking around for a new Tach. Of course I was looking in the manual after and did notice that a shorted tach will prevent it from starting. duh.
Yes, I was aware of the Shift interrupter switch, and did some inspection on that and it seemed ok when I had it running and shifting gears a few times.
Thank you for your time, interest and help.
G. McInnes
Pender Island, British Columbia
 
In the last few years the areas between the mainland and Vancouver Island have become fairly slow fishing unless you happen to hit the salmon runs at the right time. The west side of Vancouver Island is where the good fishing is now.
 
kimcrwbr1 said:
My concern now is the 10 volt key on engine off it should be battery voltage or close. According to the schematic the purple /yellow wire from the start solenoid to the coil looks to be the resistor bypass. when you start the motor it gives the coil battery voltage in the start position for quick starts and then when you let off the key the coil gets ballast resistor voltage. Since you removed the resistor wire the purple yellow wire is not necessary and can be terminated, just disconnect both ends and tape or shrink wrap the connectors. Anyway fully charge the battery (slow charge) measure the voltage at the battery and the coil should be close to that key on engine off, then start the engine and after a few minutes running high idle measure battery voltage again and volts at the coil. Do a complete diagnostic on the charging system and that it is working to spec. Have you sanded all the grounds metal to metal. clean all the connectors with elect cleaner and use dielectric grease to put them together. If your not using the tach disconnect it at the tach and tape the connectors until you install the new one. Like I say the 10 volts has me concerned and you dont need any surprises out there be as thorough as possible now.
Click to expand...

No you can NOT disconnect the crank wire from the solenoid,

You can only do this if the ignition key is designed to supply the cranking voltage to the coil.

The old point ignition key switches did not have this mechanical design.

If he disconnects the solenoid wire the boat will only start when he releases the key from the crank to the run position.
 
Yes, that purple/yellow stripe wire (solonoid) I reconnected to the purple ignition wire the new one that I installed. It was on the resistor wire that was wired right to the coil, that is the one I removed, and if I would have looked at it close enough I could have seen that it was the silver or aluminum color, instead of copper. If I disconnect the electric choke I get about another 1 or 1.5 volts. I think all is good.
 
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