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I've spent quite a bit of time trouble-shooting an intermittent no-start problem, that only occured after the engine had been heat-soaked (ie, re-start an hour after shutdown). The solenoid went click, but the starter did not spin up.

There are many things that can go wrong.

1) You need a healthy voltage to activate the solenoid. This is down to the ignition switch and wiring.

2) You need a healthy voltage at the starter main terminals. This depends on the cabling, battery switch, and starter battery.

3) You need a healthy starter motor.

Firstly, you need to systematically measure the voltage drops in your system. The starter motor needs at least 9V, ideally 10. You need to measure the voltage at the starter terminals WHILE CRANKING.

For voltage drops, connect the meter between battery + and starter +. Then repeat for battery - and engine ground. Again, measure while cranking. Please note that the losses on + and - sides add up.

Good cabling and battery switch should give you a 1V drop or less. A good battery should hold up to 11V while cranking, leaving you a healthy 10V at the starter.

In my case pretty much everything was wrong. Poor battery cables, an ageing 1-2-both switch, weak battery, ageing ignition switch, poor quality wiring to the solenoid.

So I now have new 2/0 battery cables from Genuinedealz.com, a new ignition switch, new wiring to the solenoid, a new 1-2-both switch from Blue Sea Systems, and a new group 31 battery from US Battery. Very healthy voltage at the starter now.

But guess what? The intermittent start problem was still present. I replaced the starter motor, all is good so far. Will need 6 months of use before I declare it fixed!
 

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I disagree. The starter solenoid doesn't use much current. That is why there is a solenoid there to start with. If the voltage at the solenoid isn't as high as it should be there is a wiring problem to or from the panel, either gauge, connections corroded, or age of wire. Adding a relay (really another solenoid) will not solve the problem only mask it and add more connections at the same time.

Now if there are glow plugs and they are powered off the panel a relay does make sense as they do use a lot of current.
I agree. The starter solenoid IS a relay. So you're adding a relay for a relay. Will that relay need a relay too?
 

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Now if there are glow plugs and they are powered off the panel a relay does make sense as they do use a lot of current.
My Universal was wired so that the start button is only energised via the glow plug switch. The problem with that is, you have the glow plug current dropping the starter solenoid voltage!

I have rewired it so the two are separate.
 

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Found a new, geared starter on Ebay for my Kubota, for $80. It's half the size and weight of the old one. The reduced size helps because the bolts are now kind of accessible. (they weren't at all, before)
 
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