Intermittent low voltage to starter solenoid fixed

[Texasroadrunner]

For the last month or so my '72 would not turn over for two or three tries. Then it would. My wiring has all been replaced as well as the ignition switch. But sometimes there's not enough juice to fire the solenoid. Yesterday I put in a 40-amp relay and connected it directly to the battery to actuate it. Well what do you know, it starts up great and has not missed once. Nothing like a $5 part to fix a problem.

 

[Robotsan]

Nice one!
For those of us with poor electrical knowledge, could you please explain how that helped?

 

[Texasroadrunner]

Even though my wiring and the ignition switch have all been replaced, the entire circuit with connectors and switch can have some voltage drop. The relay gets full voltage from the battery and will send that straight to the stater solenoid making it reliably engage the starter motor. The relay is actuated by the original wire from the ignition switch. That way the hard job is done by the points and coil in the relay. It's a clean connection illustrated by the following diagram. I have the same type relays sending power to my high beam and low beam headlights, taking the load off of the headlight switch. If the relay ever wears out, which is doubtful, just plug in a new one.

 

[Robotsan]

Thank you for taking the time to explain that, much appreciated! So now does the power circumnavigate the original circuit, and instead takes a new, shorter route via the relay?

 

[Texasroadrunner]

Not really, the original circuit power to the ignition switch is still needed and used to turn on the low current draw coil in the relay. The relay just makes sure that maximum voltage from the battery is sent to the solenoid. Newer vehicles do this, but our old Datsuns didn't. It's up to us to modernize them.

 

[Rob Gaskin]

I understand what you have done but I can't help thinking that it is masking a fault - high resistance somewhere or faulty solenoid. I've not heard of another person having this problem.

 

[Texasroadrunner]

I understand what you have done but I can't help thinking that it is masking a fault - high resistance somewhere or faulty solenoid. I've not heard of another person having this problem.

I have read about this problem in other Z forums. I didn't have the intermittent until a few months ago. It really doesn't happen often but with a relay running the solenoid, I don't think it will happen again. Cheap and easy insurance.