How to check resistance in grounding?

[tapiol]

Hello! I've been running my ms3+msx 3.57 for few years now and always had an issue with my ve-values, they are waaaaaaay too high, like 100 at idle. This winter I re-did everything fueling related, new filters, pumps, regulator, injectors. And still have the same issue! Tunes have been made from scratch for few times to outrule issues on that side.

It has to be mechanical and something with my wiring.

If I enable the inj/spark disabling test mode and enable all injectors and measure resistance from the injector harness to the ecu ground on the cylinder head I get 7k ohms, that cant be right?

Using injector output test mode I enable the mode, turn on the fuel pump and measure voltage at the injector harness: 8.6 volts on harness injectors not firing, only the fuel pump on. 12.2 volts when measuring the positive lead on the harness and negative on the cylinder head.

Why am I seeing the 8.6 volts when the injector is not firing, shouldnt it be completely dead?

[Marek]

For "how does electricity work?", see my post http://www.msextra.com/forums/viewtopic ... 01&t=66380

In your case, the thing doing the work is the injector and the thing which switches to make a circuit is the transistor in the MS.

If the transistor isn't commanded to turn on, the circuit will not be complete, no current will flow and you ought to see 12v on both sides of the injector.

If MS tells the transistor to turn on, the transistor completes the circuit and opens up the path to ground for the injector one the injector side of the injector circuit.

(If you are using resistors with your injectors then consider them to be a single unit.)

You should have the power turned off when you are measuring resitances.

kind regards
Marek

[tapiol]

Thank you for your answer marek! I have my injectors umplugged now, measuring directly on the harness itself. With the ecu unplugged from the harness and 12 volts applied on the injector + lead (fp relay bypassed) I have 0 volts between the + and - leads on the injector harness as I should.

Now if I plug in the ecu, key turned off, megasquirt not powered and 12 volts applied to the injector + lead and measure the voltage on the injector harness I have 9 volts.

Why do I have 9 volts even the megasquirt not powered? The injector ground wire is getting grounded even when the megasquirt is not powered.

What to look for now?

[Marek]

If you think you have a problem with the wiring, unplug the injectors, unplug the connection to the MS and simply use the ohmmeter to see whether the harness is somehow somewhere connected to ground.

kind regards
Marek

[billr]

"Now if I plug in the ecu, key turned off, megasquirt not powered and 12 volts applied to the injector + lead and measure the voltage on the injector harness I have 9 volts."

Is that with the injector still disconnected, the meter is connected to the two harness wires in place of the injector? If so, the test is meaningless. The meter is a very high resistance as compared to an injector. The transistor drivers in the MS will always have a small bit of leakage to ground, even if the MS is not powered.

PS: post an MSQ so we can see what you are talking about without having to ask a bunch of questions.

[racingmini_mtl]

To add to what billr is mentioning, you never want to be in a situation where the MS is not powered and the injectors are. That can lead to issues and unwanted injector activation and is not a recommended wiring setup.

Jean