• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Low voltage at injectors, ECU ground?

janky240

New member
Joined
May 21, 2021
Location
PDX
Hi there.

My 87 244DL is having trouble idling smoothly. I checked the injector voltage by backprobing them, and was only seeing 0.5-2.0 volts across the terminals. Unplugging them during idle seems to have no effect either.

I've even tried backprobing PIN 13 on the ECU, and I have similar readings. This is the 2nd ECU I've tried. I'm stumped! Any ideas?

**Forgot to mention, the positive pins at the injectors have 12 volts.
 
Try using a incandescent test light on the injector connections. You should see the light flashing very quickly as the injector is fired by the ecu. That low voltage on the negative terminal is normal as the transistor firing the injector switches to fire the injector. the injectors are grounded to fire and the transistor switches off when not firing. It might be too fast for your meter to react if it's not a fast sampling meter.
 
Try using a incandescent test light on the injector connections. You should see the light flashing very quickly as the injector is fired by the ecu. That low voltage on the negative terminal is normal as the transistor firing the injector switches to fire the injector. the injectors are grounded to fire and the transistor switches off when not firing. It might be too fast for your meter to react if it's not a fast sampling meter.

Ok I've gone and tried that, and the incandescent bulb lights up, but it's very weak
 
Incandescent will be weak because it's being PWMed with a low duty, use a resistor and LED suitable for 12V direct connection in parallel with the injector and you'll:

A. probably kill it with back EMF but before that
B. see it flashing because LEDs don't have any thermal lag like incandescents do
 
You want to know if the low side of the injectors are switching rapidly between 12V and ground, thus drawing current through the injector. A smooth idle is more about the correct pulse width and fuel pressure than are the injectors operating at all.
 
Incandescent will be weak because it's being PWMed with a low duty, use a resistor and LED suitable for 12V direct connection in parallel with the injector and you'll:

A. probably kill it with back EMF but before that
B. see it flashing because LEDs don't have any thermal lag like incandescents do

If the LED is getting killed you could put a diode across it. Reversed polarity of course.
 
Correct, I just could not be bothered explaining that :-D Check out the schematics of the LEDs on the FET outputs in the old Jaguar design for FreeEMS if you want a good reliable well thought through example of what johfraser said. But for a quick test I'd just wing it and if the LED didn't survive then I'd do it properly, but it probably will survive long enough.
 
The proper light to use is a noid light. A set of them can be had cheap. The connectors are specific to the injector brand. Bosch is the Volvo and GM brand . Connected across the harness, it will pulse when cranking and running. Harbor freight or any parts store will have a set.
 
Unplug the connector, peel back the boot, and backprobe a test lamp between the two terminals.
 
Last edited:
Before you listen to ZVOLV, be sure you know what back EMF is, how much of it is present off each injector when they turn *off*, and how much your particular oscilloscope can handle in the way you intend to connect it up, to avoid frying expensive test gear.

Also, a scope is the last thing I'd look to when trying to verify the basics like this. No need to over complicate things. My 2c.
 
Back
Top