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Automatic choke


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It's not the relay. Never is.


 For the relay to remain ON there must be an equal voltage across the energizing coil on the relay. One side goes to the battery. The other to the alternator/charge lamp. When the alternator is stopped or not charging the White/Blue wire to the alternator/charge light goes to ground. Power flows from the battery to the alternator energizing the relay and switching it from ON to OFF and turning off the choke heater.


A poorly charging alternator can also do this. Does your charge light flicker? Should be charging at over 14 volts. Check with meter. Now check that the Yellow/Red and the White/Blue wires on the relay have the same voltage when engine running.

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The relay can only be on or off. The choke circuit is wired into the ON side of the relay. When running, there is equal voltage across the coil in the relay and nothing happens, power flows through the ON relay to the choke. If the engine stalls, the alternator terminal goes to ground. Now power does flow through the coil tripping it from the ON position shutting off the power to the coil.


This relay buzzz is a very common problem for the 720.

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I don't think the choke but unplug it and see. It's how the coil in the relay is powered. One side goes to the battery positive / alternator output and the other to the alternator plug that turns the charge light on in the dash. As long as running that charge light circuit should be +14.  With equal voltage on either side of the relay coil nothing happens. If the engine stalls the alternator charge light terminal goes to ground and current flows through the relay coiol and turns it OFF. Seems like it's osculating onoffonoffonoff.


I'm curious. VERY carefully pull the plastic plug out of the back of the alternator while running and the relay is buzzing. The relay should stop, does it?


If yes, then I would have to say the alternator is at fault.

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I'd like to see what this really is. It's a common problem with the 720 and could be those reconditioned alternators.


It doesn't take much to power those relays, maybe a few volts. A few volts isn't enough to make the red charge bright enough to see.

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I've seen alternators with a red charge light on start up. Give it a rev and it begins charging and goes away.


Check belt tension.

Check battery posts and cables are clean tight and check your three fusible links for good solid plug in connections. The Fusible links are connected to the positive battery cable at or very near the post Two Greens and a Black.

Check the plastic plug fits securely in the back of the alternator.

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Here are my thoughts on a lifetime guarantee.


You buy a new alternator/starter/distributor (whatever) because the price is right and you can't go wrong with a lifetime guarantee. So you're out and about and the car quits because the starter won't turn, alternator let the battery die or there is suddenly no spark. It's dark after 6 now and your were on the way home at 8, so dark as shit, you're on the side of the highway,10 miles from home and it's raining or snowing and you aren't dressed for it. But you do have a phone and the tow is only $70 to get you home. Tomorrow is a work day so you make other arrangements to get to work and home. The next night you remove the offending part and get a ride to where you got it before they close and they order you a new one. It's Fri night and it will be in Monday. At least two of these conditions will happen in this scenario likely more. Thank god it has a lifetime guarantee though, I'm really going to stick it to them.   


What you really need is a guarantee of it working flawlessly for 10 years. I would take a $30 original Nissan starter that's been sitting on an engine in the junk yard over a reconditioned one any day.

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