Hey gang,

My friend picked up a 75 280 and its not running all that great.  We found that injectors 5 and 6 have 10.3 volts with the key in 'on' (but not running) but injectors 1-4 have something like 0.3v.  According to the FSM, there are two dropping resistors, one that covers injectors 1-4, and one that covers injectors 5-6 so I'm assuming those (or the connectors) would be the problem.  I have never heard of those going bad or causing problems though.

I thought the injectors were supposed to get 6v after the dropping resistors - anyone know how they could be getting 10.3?

The resistors won't drop the voltage until the injector is activated.  I think in this condition (ignition on, not running), you should see full voltage on all injectors.

There could be corrosion at the connector for the dropping resistors for 1-4. 

Also where are you measuring voltage? Don't measure across the injector. Measure from one side of the injector connector to ground. When the car is running, you should see the voltage fluctuate rapidly.

Also, the best way to test injector firing is with a noid light. If I don't miss my guess, I would say this one would work for you: https://www.amazon.com/OTC-6266-Noid-Lite-Bosch/dp/B0050SGHMW

22 minutes ago, SteveJ said:

There could be corrosion at the connector for the dropping resistors for 1-4. 

Also where are you measuring voltage? Don't measure across the injector. Measure from one side of the injector connector to ground. When the car is running, you should see the voltage fluctuate rapidly.

Also, the best way to test injector firing is with a noid light. If I don't miss my guess, I would say this one would work for you: https://www.amazon.com/OTC-6266-Noid-Lite-Bosch/dp/B0050SGHMW

We measured across the injector connector.  Wouldn't that be the best place to measure it since it would isolate the plug itself?

Nissan wrote a book.  It covers all years through 1980.  Many possible reasons for not running great.

 

25 minutes ago, chaseincats said:

We measured across the injector connector.  Wouldn't that be the best place to measure it since it would isolate the plug itself?

No, it is not. If you want to test if the injector is getting the pulse to fire, use a noid light.

34 minutes ago, SteveJ said:

No, it is not. If you want to test if the injector is getting the pulse to fire, use a noid light.

Gotcha - it's getting pulse to fire, its just very weak.

That merits looking at the connectors for corrosion. 

Have you calibrated your eyeballs?

7 minutes ago, SteveJ said:

That merits looking at the connectors for corrosion. 

The injector connectors have been replaced and look fine - I will let him know to check out the dropping resistor and ECU connectors.  What is the connector you circled on the bottom of the chart - I don't see it labeled

That is the electronic fuel injection relay.

11 minutes ago, SteveJ said:

That is the electronic fuel injection relay.

Is that one of the relays located in the under-hood relay bracket?