Coil Voltage Problem

[Rstorrie]

I am at a loss. Bought a 1987 Chaparral 20" with a GM Mercruiser 230 Alpha One Thunderbolt Ignition as a project boat. It now runs great, when it starts after working on it. The problem I am having is when it will not start, the + side of the coil reads about 8v and about 4v on the - side. When the + side of the coil reads around 12v, as does the - side, it will start. This comes and goes. I replaced the cap and rotor. new kill switch, new ignition switch, new coil, new starter, and new plug wires. Is this an issue with an ignition componant or does the ignition switch feed directly to the + side of the coil and that is where the issue is in that ignition wire? seems to me the key would feed straight to the coil and supply 12v not matter what. Any clues? Thanks.

 

[Scott Danforth]

the ignition switch feeds the + side of the coil on every motor with a powered ignition

the negative side is pulsed to ground by the ignition system

it sounds like you have bad connections somewhere.

start at the battery and clean every connection to the block and starter to the harness circuit breaker to the big plug to the helm harness to the ignition switch and back.

 

[Rstorrie]

I did clean all the connections I had access to on the engine, starter, battery, etc. Sounds like I might need to run a new 12v + wire from the battery to the ignition switch to the coil then?

 

[wellcraft-classic210]

Could be a faulty wire connection //Key switch is suspect also //

I would check the voltage at the key switch in and out // a corroded wire would not be likely to repair itself -- but a key switch contact that's worn can cause your scenario.

 

[Rstorrie]

Brand new key switch

 

[Rstorrie]

Ok, took the volt meter, 8v on + side of coil again. Checked the voltage under the dash at the kill switch, ignition dash mount breakers and key switch, all showng 12v, coil still showing 8v. Pulled the big harness plug off the engine, no corrosion what so ever.

 

[wellcraft-classic210]

Hard to say without being their

Have you tried resistance checks key switch to coil wire at coil + side etc ?

Is 12 v at the connector also ?

Does the 8 v reading hold steady if you jiggle wires and the harness plug ?

 

[Rstorrie]

Haven't tried juggling the plug yet. I will have to try that. Thinking since I have 12v at all the connections at the dash, maybe run a new wire from the key switch to the coil?

 

[wellcraft-classic210]

It seems s 2nd wire should be OK for debug purposes & getting the boat running - // I guess its your call on how to achieve a final fix.

I would lean toward trying to keep the boat original unless its a beater. A second wire could buy you some time and usage to do that if its of interest.

 

[Benny67]

and about 4v on the - side.

With respect to ground? Meaning, when you use your meter are you making contact with the ground on the engine in reference to that reading?

Take a peek at the - wires on the coil as well..might be a bad crimp or corroded wire

 

[Scott Danforth]

if the motor originally had points, then 8-9 volts at the coil would be right as the wire would be a resistor wire (ballast wire)

however I would suspect the 2-3 harness connections between the motor plug and the helm plug. the harness should have an 8 pin connector that looks like a trailer plug style connector.

 

[Rstorrie]

Was a corrosion problem. I had cleaned every connection I could find on the motor and battery. Ended up running a new 12v wire from the key to the + side of the coil. Still lower volts when it should be 12v with the new wire. Started tracing the power from the red wire from the battery post and found a twist electrical cap that was connecting two wires together and covered in electrical tape. Took it apart and the connection was really nasty and corroded. Cut off that mess, stripped the wire back and put a new ring connector on it at the battery. Seems to be solved now. Thanks for everyone's help and advice!!

 

[Rstorrie]

Well, scratch that. Went out this morning and it had dropped to 8v again. I have no clue as to what is going on at this point.

 

[Rstorrie]

I have checked the voltage everywhere and get 11.5 - 12v, except at the coil which is intermittent between 8v-12v.

 

[wellcraft-classic210]

OK
Some questions

1) Are you logging your results on these tests ? Taking notes on condition and findings often helps especially for intermittent issues )

When you say checked everywhere -- what exact points are you checking on the ignition ckt form key switch to the coil? What were the readings at that time ? ( as in was this with 8 or 12 volts present to the coil )



( 2) How are you verifying the gnd connection on your meter during these tests ?
( as in does the same gnd connection show 12 volts at some points and 8 at others without moving it ?) ( very important to ensure your not chasing your own tail )


3) Are you able to use an ohm meter ? ( as in this can help you find the issue with some power off resistance readings done while the 8 volt signal is present. ( Ask us if needed )

4) Pretty likely its dropping voltage across the key switch or a connection but anything in the ckt is possible.


5) How did the redundant spare wire work ( Was it present when you saw 8 volts ???? ) ( if so its likely an upstream problem ( such as key switch ) or a measuring issue with weak gnd ) // assuming 12 v into the key switch input // And key switches can definitely be intermittent when contacts corrode.


( Its all in the details )

 

[Rstorrie]

Update. Getting 8v at the coil, 12v everywhere else. Remove the ignition wire from the key to the coil, get 12v, Put the + wire back on the coil, drops to 8v. Remove the + wire from the coil, back to 12v. New coil shorting out?

 

[wellcraft-classic210]

Likely Voltage dropping as load the increases // Often Sign of a weak connection.

And you answer to question 5 regarding what happens with a jumper wire is ?????????????????

 

[wellcraft-classic210]

Not trying to give you a hard time -- but if you can answer the questions coming from people trying to help you its likely the problem will be pinpointed quicker less or little wasted $ and efforts..


Their are currently a number of unanswered or partially answered questions from prior posts.

 

[Rstorrie]

Ok, what are the questions?

Intermittent 8-12v off Purple + wire. New battery, key, kill, starter, coil, cap/rotor, plug wires. One second it will be 12v, then the next second it will be 8v, on and on. Only thing touched is the turning the key to run. Even died once the other day while idling, then started right back up. Cleaned and scrubbed every connection I can find. By passed the breaker to test, nothing changes, ran a new + wire from the key to coil, changed from 8v, to 10v, still won't start first time seeing 10v.

 

[Rstorrie]

Just looked up under the engine near the alternator and found only the orange wire connected and two other wires cut up underneath. One is red battery and the other is another Purple ignition wire controlled by the key. No spots on the alternator to connect them, there is just one Battery/R post on the back of the alternator and the Thunderbolt IV wiring diagram show the orange, red and purple connected to the alternator? WT?