Engine Fails to Turn Over Sometimes Con't

[Steve Carpman22174]

A couple of weeks ago I posted a thread regarding the intermittant failure of the Yanmar on our Legend 37.5 to turn over. I received great advice and have concluded the problem is electrical - the dreaded increased impedance in the wiring harness between the momentary switch and the solenoid. The solution seems to be to check and clean all connections (including the grounds) in the ignition circuit, add a momentary switch at the low current side of the solenoid, add a starter relay per all the discussion on the archives, or change out the ignition wiring in the harness. In the spring we will do some of these and I fully expect the problem to be eliminated.

The only aspect of the discussion that does not seem logical to me is why was the problem intermittant? If the impedance in the wiring harness increased to the point where the solenoid was not receiving proper voltage/current, why did this not prevent the engine from starting all of the time? Our Yanmar started with no hesitation probably 95% of the time. Why did this electircal problem emerge as infrequently as it did?

 

[Schatzi]

Steve:
On the 1994 40.5 we changed all the wiring from the switch to the starter (#10). It has worked 100% since and yes it was intermittent (towards the end 80 - 20 would not start, took 6-12 tries). We paralleled the existing switch leg to the starter and supply leg to the switch and left original wiring in place. Between the connections and small old wires, it just could not get it started. Loose connections and corrosion would not deliver the amperage consistently. The starter would draw it down and there was not enough amperage to start. An electrical engineer refused to add a relay saying that it would be just hiding the problem. The wires worked fine for years, replacing them with new wire and hydraulic crimped connections did the trick. The job didn't take all that long (3-4 hrs) and we put an emergency start switch at the starter. Good Luck

 

[shemandr]

Different temperatures? Heat increases the resistance of a wire (I'm not an EE but I did pass Physics 101 and I have toaster). The wiring harness of my Yanmar runs over the engine from rear to forward. It gets pretty warm there. You don't have to increase resistance of the wires that much to reduce voltage enough to not run the starter. The voltage drop may already be on the edge just from the connections and undersized wire. Just an hypothesis for consideration.

 

[Bill Roosa]

Variables in DC circuits

The correct term is not impedance it is resistance. Impedance is the resistance to the flow of power in AC circuits due to the combination of inductance and capacitance. AC circuits also have resistance. DC circuits don't have inductance or capacitance typically. At least not that you have to account for from a power flow aspect.

Things that change resistance in DC circuits;
temperature
corrosion
humidity
vibration (effects the physical corrosion)
current flow (produces heat)

Note that you can measure 12ish volts at the solenoid control terminal wire with no current flowing but once you reconnect it and let current flow the voltage can drop (even in a well maintained circuit) to a level that will not produce enough magnetic field to overcome the solenoid return spring.

 

[Bill Roosa]

Also the battery voltage

The battery voltage also changes with state of charge and any other current is has to supply when the starter is engaged.

 

[Benny]

Intermitent electrical problems just seem to be random. Luckily the problems are easier to diagnoze and fix than to understand their causes. Problem solved? Forget about the reasons. There are to many variables which influence the process.