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78 Datsun 280Z Power Line Circuit

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Old 09-17-2016, 05:44 PM
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78 Datsun 280Z Power Line Circuit

Hello, I am working on a 78 280Z that will not start. We have used the troubleshooting steps in the FSM and have performed the continuity check at the 35-pin computer connector. In doing so we had very low voltage (0.4 volts) in the “Power Line Circuit” which was item 2-(2). We also had this same low voltage for each of the “Injector and Resistors”, item 2-(3). Where do I look to find the drop in voltage? I assume maybe a relay but have no idea. Can someone please tell me what to do to find the drop in voltage? Thanks in advance.
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Old 09-17-2016, 08:59 PM
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Well first, what is your battery voltage?
Second, check fusible links.
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Old 09-18-2016, 07:45 AM
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The battery has a right around 12-volts and reads 12-volts on pin number 1. Is there a easy way to check the fusible links. Sorry, I have more experience working with much older cars. Thanks for the help.
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Old 09-18-2016, 11:09 AM
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hate to burst your bubble but real old cars still used batteries and wires. You need a VOM that doesn't read AROUND 12 volts altho older cars used 6 volts. perhaps that is where your confusion arises. Electricity has not changed in the years between the model T and today. Georg Ohm figured that out around 1800. you check fusible links like any other fuse. put one lead on one end the other on the other end. Set VOM to resistance not volts and not amps.
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Old 09-19-2016, 05:34 PM
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Yes I am familiar with a voltmeter and its operation and I have used them on many old cars, but old cars have no computers, relays, fusible links, etc. I believe that the actual voltage on the battery was 12.1 volts but I truly did not record it since the voltage on the computer pins read 0.4 volts (and not anywhere close to full battery voltage). I truly did not think that it was that important. Thanks for your thoughtful assistance.
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Old 09-19-2016, 06:47 PM
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To properly check continuity for a fusible link or fuse you need to disconnect one side so it can't "read" through the rest of the loom. The low voltage could be due to the fact that with the key on and no start, the system senses a stall and turns off the fuel pump and injectors. When you first turn the ign. on you should hear the fuel pump run for about 5 seconds. Start with that and move on. The fsm is different for the '78-'83 cars so make sure you have the right one and take each step from the beginning, if you do this you will better understand how it works.
I did this with my '77 and damn near went bald.
Also make sure you are checking each pin to a good ground.

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Old 09-20-2016, 07:10 PM
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I have the FSM for the 78 280Z so I believe that I have done it correctly. I have also checked to make sure that my meter has a good ground by reading the voltage at the fuse box prior to the other readings and verified again afterwards. I will try to check the fusible link and see if that is it. Could it also be a relay? I am truly blind when it comes to newer cars. Thank you for the helpful reply
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Old 09-21-2016, 09:42 AM
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If the fuel pump is not coming on with key on then it might be the ign relay. I don't have the fsm for that car so I'm not sure. Check the ecu program for each sequence step from the beginning.
With your meter on the ecu see if you get a drop when you turn the key to start. Something could be drawing too many amps and not having enough power left to run the rest of the circuit. The '77 I had used a neg wire from the ecu to the battery not the chassis ground.
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