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Negative voltage spikes on 12volt power to car coil and injectors, help pls.

apples

Jul 1, 2012
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So, If I probe with my scope between the battery negative post and then probe my coil fuse, injector 1+4 fuse, or injector 2+3 fuse I get a negative voltage spike.

I think it shows in the photo it dropping to around 6 volts.

Coming from the coil that power feed wire shares with the injector feed power wire too. I am guessing that it's a ringing effect coming from the coil. But it probably could also come from the injectors too?

Green circles in the circuit diagram show the fuses that I probed.

How can I fix this.....Can I make put in a didoe or something? and how would I wire that in?

https://i.imgur.com/OefHuet.jpg wiring diagram
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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What's the duration of that pulse? I expect it's pretty short. I also suspect it's harmless, and possibly caused by inductive or capacitative effects.
 

apples

Jul 1, 2012
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My car has a missfire at idle. And I know that I get a no spark firing . So I am just looking at what it could be as I have all new parts everywhere else.

Harmless??? mmm okay if you say so.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Note my first question. Set the scope for one shot trigger, and set the trigger voltage to about 8 volts. Then set the horizontal speed to maybe 100us/div and see what you capture. Play around with the horizontal timebase until the pulse is between 1 and 5 divisions wide. Then post a picture of the screen along with the horizontal timebase setting.
 

Bluegrass

Dec 20, 2019
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Yes your seeing an inductive spike.
Most engines will have suppressor caps grounded near the coils and injectors. Same as the old points/ condenser of yester year.
This is to bypass noise to keep the AM radio quiet and spikes from affecting the rest of the system.
I know of one instance not commonly known, where a faulty coil pack with shorted turns actually hashes up the line going back to the computer to such a degree is stops the computer from normal processing for short moments.
This causes the vehicle to jerk violently because the whole drive train is connected while driving.
The only way to find the offending cylinder coil in this case is to use a Scanner looking at misfire data for all cylinder, looking for one cylinder with high counts.
In your case, the computer may or may not have this feature to look at. But a Scanner is the only way you will get a good lead to be sure what's going on.
Not to only read codes but to look at operating data and understand what the data tells.
Some times misfire only show up in certain drivability modes. This becomes a clue.
A coil can be open or shorted and set a code for that cylinder and easy to see.
But a coil with shorted turns is not a hard fault and may not set a code because it is still firing it's cylinder.
Coils' current signatures are monitored by the computer and codes assigned when fault is detected but not for shorted turns that may only reduce the high voltage level some amount and cause shuttering in certain drive modes.
Your vehicle year, make, and model will depend on how much diagnostics power the computer has built into it's program.
Good luck.
 

apples

Jul 1, 2012
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Well I have fitted it with a megasquirt thinking that my original ECU was the culprit, but it still does it.

New plugs, wires, sensors etc.

Should I add a capacitor across each injector? Well it's batch fire so 2 cylinders fire in 2 pairs. Maybe one capacitor per each pair of injectors.
 

Bluegrass

Dec 20, 2019
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First let see why there is a spike.
Each time a coil or injector has a ground applied from the computer, current flows through the device winding.
This current causes a magnetic field around the winding wire and the core it is wound on.
At the moment the current stops (ground is opened) the magnetic field developed collapses and "re-induces" a voltage in the same wire winding that created the field from current flow, but in opposite polarity.
The number of turns and the current flow largely determines the voltage developed as the spike you see and can be many times the 12 volts original feed.
The only way to clear this is to place a diode across each device in the "correct polarity" that conducts when the spike is present.
The conduction begins at about .7 volt +/- so you can see this acts as a short across the device winding to kill the spike but looks like an open to normal forward current because the Diode only conducts in one direction.

.
You can see the PCM (computer) is not cause of the of the spike >>>by being faulty<<< in any way. All device with a coil such as injectors, relays, solenoids etc. produce a spike that often needs to be controlled.
The PCM supplies a ground out to the device to complete the circuit, as the devisees already have 12 volts supplied when the ignition switch is turned on. The computer ' fires' each under fire order control sequence
I am familiar with the after market Megasquirt control system as we used it in a kit car with a 5L Ford engine.
Good luck.
 
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