1981 Datsun 720 Starting Issue

[Codywiley]

Hi guys how's it going? I have a standard 1981 Datsun 720, with a manual transmission and a mechanical fuel pump. I recently had to replace the alternator, as I had been having problems with it not charging/overcharging my battery. The problem was fixed relatively quickly, however, after I put the new one on, it refused to start. I had my starter tested as custom, and it was fine. I put that back in, and unsurprisingly, it started and continued to do so for about another week. I stopped for gas, and went to leave, annoyingly it refused to start. I was able to get a jump, and once again, it was fine for another week. In an unrelated incident, my positive battery cable had been slowly deteriorating (the plastic) leaving exposed copper rubbing on the starter, and melted off. This caused damage to my positive battery post. I replaced that, and it started, so to make sure, I shut it off, and tried again. It did not start. After 10 minutes, it fired right up, and I took it home. That night at work it refused to start again, this time not even jumping it worked. So I pushed it.

In case the long story threw you off, the issue here is that it does not start. I turn the key, and here a single click, but that's it. I replaced the starter (even with doubts that was the issue), the alternator, the battery, and positive battery cable. The negative cable was replaced recently (but is maybe worth a shot?). With a mechanical fuel pump, I can say that a fuel relay is likely not the problem as I've read in many forums. Any ideas?

 

 

I should also note that after I replaced the battery, it didn't start, but after sitting for 6 hours, it started once more, but that has been it. I highly doubt it's the alternator, but I won't rule that completely out

 

Thanks, Cody

[datzenmike]

Hi Cody, welcome to Ratsun.

 

 

First make sure your battery terminals and the cable ends are clean and tight. Now clean and tighten the ground cable where it is bolted to the intake by the thermostat housing.

 

 

NOW

Make a jumper wire and connect it to the battery positive side. With the transmission in neutral touch the other end to the solenoid terminal on the starter. This is the smaller terminal with the smaller Black/Red wire on it. You will likely have to pull this wire off for this, don't forget to put it back on.

 

What we are trying to eliminate is the start signal from the ignition switch with a direct and strong 12 volts from the battery.

 

If it starts every time this way, it might indicate that the start signal is weak. Perhaps 6 or 8 volts to the solenoid instead of a strong 12 volts and this is not enough to hold the starter contacts together. Now the cause, could just be old wiring or some intermittent ignition switch problem.

 

Give this a try next time the starter refuses to work. If this does work there is a simple enough fix for this.

[Codywiley]

Okay so by passing the ignition gets the starter to kick on and start to crank the engine. I'm guessing there's an issue in the wiring as you said, or an issue with the ignition switch. That has been replaced within a year or two though. Where should I start looking to get this thing running again?

[datzenmike]

Well if you are getting a click sound from the starter solenoid but no cranking, it sounds just like a weak signal. You need to make up a 'Hot Start Relay' that uses the weak signal to power a relay. Like this...

 

 

 

Get one of these nifty Nissan relays. They're in all 80s vehicles and they will easily fir your pocket, I know because I've tried it.. This is from a 720 truck but the S110/S12/S13 280zx Maxima all have them and others, but you want one like this. Preferably cut the wiring and get the plug that goes with it.

 

Top two 3 and 5 terminals. Connect one to a 12 volt source and the other to the solenoid terminal on the starter. Does not matter which one.

 

Bottom two, 1 and 2 terminals. Connect one to the Brown/Black wire that used to go to the solenoid and the other one to ground. Does not matter which one.

 

 

Now when the key is turned to start, the weak signal travels to the relay and through it to ground, energizing the coil. (terminal 1 and 2) The coil pulls the relay closed and connects terminal 3 and 5 sending 12 volts to the starter solenoid.

[DanielC]

The "hot start" bypass relay does get the engine to crank on most cases. 

But the starter not kicking in with the key is a symptom, not the root cause of the problem.  The root cause is excessive resistance somewhere between the positive battery connection, to the key, and back out to the starter solenoid.  If the excessive resistance is between the battery, and the key, that means the excessive resistance is also in the ignition circuit, and anything else the key switch powers.

It is possible there is excessive resistance from the ignition switch to the wire going to the starter solenoid, and the relay will bypass that.

[banzai510(hainz)]

pin 3 or 5 goes to the starter on the relay

 find out which goes to the battery. then the other goes to the starter. then take a wire put in the relay where it goes to the starter and put it on the +side battery if it cranks then you have a weak signal  orginally

[Charlie69]

I had a similar problem on my86 720, even though it has an electric fuel pump it turned out to be my ignition switch. 

 

Even though you say your ignition switch was replaced a year or two ago it still could be defective.  Might have been replaced with a cheep Chinese switch.  I try to buy quality when it comes to electrical parts.

 

Rock Auto both descent quality switches.  depends on which style of switch you have.

 

http://www.rockauto.com/en/catalog/nissan,1981,720+pickup,2.2l+l4,1210217,electrical-switch+&+relay,ignition+starter+switch,4700

 

I am betting on the second one.

[Codywiley]

Thanks for the help so far guys... after replacing the ignition switch, I still have yet to get my truck to start. My guess is that there is something wrong with that black and red wire on the starter? Any thoughts? Any tips on replacing it?

[banzai510(hainz)]

if you take a wire from the + side battery and go right to the seliniod tab( or the connector to it if it uses a wire) it should turn over!!!!!!!!!! this will by pass the switch and relay. then youll know. If still no crank then its the starter

[Codywiley]

It was actually the ignition switch. I'm not sure why it didn't initially start.