Jump to content

IGNORED

Injector Voltage


Recommended Posts

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?

Link to comment
Share on other sites


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

Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

7 minutes ago, SteveJ said:

That merits looking at the connectors for corrosion. 

image.png

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

Link to comment
Share on other sites

Just now, Zed Head said:

How long has it been sitting?  Here is what my six injectors produced at full flow on my engine after I bought a long-sitting car.  That's gasoline.

image.png

Yikes, it's very possible that is what is going into the engine - he really should empty the tank.

Link to comment
Share on other sites

The gasoline was fine.  The injectors just didn't flow evenly.  I drove it that way for quite a while.  

If you told use what "not great" means we could have more fun.   

Link to comment
Share on other sites

Dozens of possibilities why the car doesn’t run that great just on the EFI alone. Get that EFI book and a multimeter and go step by step and record your findings on each. If you don’t your headed down a rabbit hole. Bouncing around like I did wasted tons of time. Just my two cents 

Link to comment
Share on other sites

Posted (edited)
22 minutes ago, Zed Head said:

The gasoline was fine.  The injectors just didn't flow evenly.  I drove it that way for quite a while.  

If you told use what "not great" means we could have more fun.   

Fair point - basically it won't run unless the coolant temp plug is pulled so the car runs in 'max rich' mode.

You need to be VERY gentle with the throttle or it will die.  After it's warm, you can plug the coolant temp ECU plug in and it will idle but will die with pretty much any throttle input.  We smoke tested it and there are no vacuum leaks.  It has new air and fuel filters, we adjusted the valves to factory spec, and cleaned all the contacts here except the dropping resistor.

Edited by chaseincats
Link to comment
Share on other sites

8 hours 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?

When you measured across the injector connectors... Was this with the connectors attached to the injectors, or with the connectors not plugged onto the injectors?

Can you take a pic of how you took this measurement?

Link to comment
Share on other sites

2 hours ago, Captain Obvious said:

When you measured across the injector connectors... Was this with the connectors attached to the injectors, or with the connectors not plugged onto the injectors?

Can you take a pic of how you took this measurement?

This is with the connector disconnected from the injector - literally touching the female connector pins with a multimeter that the injector's would plug into.

We did confirm the the injectors work by using wire leads directly to a 9v battery with the connector disconnected.  Injectors 1-4 that were now accustomed to the low voltage had a considerably fainter click than injectors 5-6 but after clicking them on and off 9v a few times, they began to come back to life and the audible volume of their 'click' gradually began to match injectors 5 & 6.

Link to comment
Share on other sites

1 hour ago, Captain Obvious said:

Were all the injector connectors pulled off at the same time when you were taking those measurements?

Also, does the car run better now that you have cycled all the injectors a bunch of times manually?

Yeah, they were all off when we took measurements - we just went from one to the next.  It did run WAY better after cycling them all but that was only temporary.  My guess is cycling them in the cylinders so many times eventually got enough gas in each one for it to run properly.  After that was burned off it did go back to running how it was prior to cycling them.

Link to comment
Share on other sites

OK, got it. So getting into the details about the voltage number measurements you got... I'm pretty sure you already know this, but they don't really make sense.

If you pull the all the injector connectors off, turn the key to ON, and measure across the two injector connections inside any of the connectors, I would expect to see a low voltage on all of them. But it's a little unpredictable because of the way the circuit works. One side of each connector should be connected to the battery (through the dropping resistor) and the other side of each connector should be floating pretty much as a no-connect.

However, there is a capacitive load on each of those floating sides that could provide some weird numbers if you don't know what you're looking at. You could charge or discharge that capacitor through your meter and the numbers could change as you're watching them.

So with all that in mind (and as mentioned above), it would probably be easiest to change the way you are taking your measurements and see what you get when you just measure from the injector contacts to a known good ground instead of measuring between the two contacts in the same shell.

I mean, there's info to be gleaned from what you already did, but without being there and seeing what type of meter you're using and watching the numbers for stability, it would be simpler just to re-do the measurements and reference everything to ground.

  • Like 3
  • Agree 1
Link to comment
Share on other sites

43 minutes ago, Captain Obvious said:

OK, got it. So getting into the details about the voltage number measurements you got... I'm pretty sure you already know this, but they don't really make sense.

If you pull the all the injector connectors off, turn the key to ON, and measure across the two injector connections inside any of the connectors, I would expect to see a low voltage on all of them. But it's a little unpredictable because of the way the circuit works. One side of each connector should be connected to the battery (through the dropping resistor) and the other side of each connector should be floating pretty much as a no-connect.

However, there is a capacitive load on each of those floating sides that could provide some weird numbers if you don't know what you're looking at. You could charge or discharge that capacitor through your meter and the numbers could change as you're watching them.

So with all that in mind (and as mentioned above), it would probably be easiest to change the way you are taking your measurements and see what you get when you just measure from the injector contacts to a known good ground instead of measuring between the two contacts in the same shell.

I mean, there's info to be gleaned from what you already did, but without being there and seeing what type of meter you're using and watching the numbers for stability, it would be simpler just to re-do the measurements and reference everything to ground.

Sounds good.  He is getting the car back today and will clean the terminals on the dropping resistor just in case.  I'll let you know the results and take measurements next time I'm over.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

By using this site, you agree to our Privacy Policy and Guidelines. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.