Electric Choke Sucking Away Ignition Voltage?!

[TBB]

Having recently replaced the breaker points and coil on my Mercruiser 170 with a Pertronix Ignitor and Flamethrower coil, I would like to get a solid 12 volts to the ignition coil. I have circumvented the ballast resistor but I still get only 9 volts at the ignition coil and at the ignition key switch. However, if I disconnect the electric carburetor choke, the voltage at the coil and ignition switch jumps up to 12 volts.

Is this normal? What is the likely culprit and fix? I've got a 1986 170 (3.7L/470) Mercruiser

 

[Bondo]

Re: Electric Choke Sucking Away Ignition Voltage?!

Ayuh,.... A Weak Battery...

 

[bruceb58]

Re: Electric Choke Sucking Away Ignition Voltage?!

You have a resistive connection somewhere. Check the voltage at the ignition switch first and then move back or forward towards the engine. Reference the ground at the battery so you don't get fooled by a ground drop at the helm fuse panel if there is a drop in the ground too.

Your drop could easily be in the main engine harness connector in either direction. Basically, the circuit starts at the battery, goes to the starter, goes to a circuit breaker back at your engine, goes through the main engine harness, to the ignition switch, back to the engine harness and then to your coil. Many places for the drop. Ignition switch is a good place to start since it is in the middle.

 

[TBB]

Re: Electric Choke Sucking Away Ignition Voltage?!

Hmmm. The key on / engine off voltage (as measured by the dash gauge) has always shown 8-9 volts in the 8 years I've owned the boat. I previously thought it was just the gauge until I discovered that disconnecting the choke caused it to jump up to 12 volts. Could the choke itself be the resistive problem?

More background -- the low voltage registers both at the coil and at the ignition switch. I replaced both batterly lines last fall when I replaced the starter (heavy 0 guage, clean connections). I also replaced the ignition switch at the same time. Finally, I cleaned the wiring harness connections.

I will trace backwards and forwards of the ignition switch, but I am not sure exactly what to look for? Presumably, I have 12 volts at the battery side of the ignition switch, but I will check. I currenty have 9 volts at the switched (+) ignition terminal when it is on (unless I disconnect the choke as previously mentioned). Any suggestions on how to diagnose the resistive problem would be appreciated.

Thanks.

 

[bruceb58]

Re: Electric Choke Sucking Away Ignition Voltage?!

No...the choke itself is the load on the circuit. It is way at the end of the circuit. I am predicting that you are going to measure 8-9 volts at the ignition switch. You are going to have a resistive connection between the battery and the ignition switch somewhere.

More than likely it is going to be in the engine connector. Probably want to pull it apart and inspect for corrosion in there and clean it up.

Just for fun measure the battery voltage when the key is on. Might as well start from there.

 

[TBB]

Re: Electric Choke Sucking Away Ignition Voltage?!

Okay, wanted to share my 90% success.

I had a good connection from Battery to Ignition switch. Had a poor ground connection at helm bus, replaced wire, now good. Got 12 V at the key. Checked continuity between ignition switch and harness connection in engine compartment and between harness connection and coil (with coil ballast resistor wire removed). Both show almost zero resistance (0.2-0.3 ohms). Cleaned all connections at ignition switch, harness connector, coil, etc.

I now have 11 to 11.5 volts at coil (key on, engine off), up from 8 volts! Still not quite the 12 volts I get straight off the battery, but significantly better. Also, connecting the choke no longer has a material effect on the voltage delivered to the coil. With the engine running, the coil is getting 12.5 to 13 volts, which should keep my Pertronix Flame Thrower coil happy. Engine runs great and starts much easier.

Thanks for the suggestions.

 

[bruceb58]

Re: Electric Choke Sucking Away Ignition Voltage?!

Good news. Just for future reference, it is WAY easier measuring voltage drops than measuring resistance when you are looking for tenths of an ohm. For example, a 1 volt drop that you are seing coupled with a wire that is carrying 10A is only 0.1 Ohm. That would be very difficult to measure with a meter.

 

[JustJason]

Re: Electric Choke Sucking Away Ignition Voltage?!

what bruce says is true, and ill add to that.

it doesn't take much resistance to drop a volt or so.
when you measure a circuit cold, your going to see the least amount of resistance that circuit could possibly have.
But when you start applying power through circuits, your applying heat via electricity to the circuit. The hotter a circuit gets the more resistive its going to be.
so keep that in mind when your checking for drops. what you measure as .1ohm cold could easily increase to anything possible when hot.

when the engine is running you should be getting 14-14.6 volts out of the back of that alternater.
In a perfect system. if you took your meter and put the negative on the battery negative, and took the positive lead and touch every positive wire right before the load, you should see the exact same voltage. if you don't, it's because your losing voltage via a drop someplace along the line.

 

[TBB]

Re: Electric Choke Sucking Away Ignition Voltage?!

Bruce, Jason -- Thanks for the further tips. Yes, I remember V=IR. I'll keep working to chase down that last one volt drop.

Brent