Continuity between +12v and Ground/Sensor ground? Bad FP?

[sweet_baby_james]

I am running a Microsquirt and I was in the process of doing some additional wiring to accommodate my new MAF sensor when I stumbled upon something strange in my wiring: There was continuity between my +12v mini bus (which is powered by the fuel pump relay) and BOTH the sensor ground mini buss and main ground mini buss that I have in my setup. My battery is disconnected and no power or ignition switches are on.

This was an immediate red flag, there shouldnt be any continuity between +12v and ground, right? I measured the resistance to be 5ohms.

Upon further investigation, I found that if I pulled the fuse for the fuel pump, the problem goes away. But the fuel pump doesn't ground to the sensor ground so WTF is going on here?

This is what I have deduced so far:

If the fuel pump has failed, and there is no resistance across the coils (ie. there is no load) there will be continuity between the +12v supply to the pump and the ground side which is part of the system ground (where the FP is ground).
The ECU is also connected to the system ground and because the system ground and sensor ground are connected internally (im assuming thats the case), I now have continuity all the way to my sensor ground buss.

I tested my theory by disconnecting the ECU ground, and voila, no more continuity from the +12v buss to the sensor ground.

So, my theory is that I have a bad fuel pump that is essentially shorting out.

But heres the kicker, I have run this fuel pump is this same setup before, never blown any fuses or drained any batteries.

Any more thoughts or suggestions for things to test would be welcomed. I have followed the wiring in the manual as closely as possibly for my application

Thanks

James

[LAV1000]

How are the ignition coils wired ?
If you change or switch over the testing leads stil the same readings ?

[sweet_baby_james]

I am running LS2 coils wired per the manual.

Swapping test leads and meter gives the same results.

The fuel pump is a Walbro GSL-391. I check the resistance across the pump itself and it reads 5 ohms. Can't find any electrical specs for this pump anywhere.

James

[LAV1000]

James,

Did reread your post, its hard to me to visualize a schematic just by plane text.

If your fuelpump is connected between +12V through a fuse and ground, dan your fine with 5 Ohm.
I think that value is to high, expect somewhat lower for a fuel pump.
It could be deviation in your digital meter, most ones are not accurate at low Ohm values.

A small pump takes 5-6 amp, thats a theoratic value of 12V/6Amps= 2 Ohm.
Probably less in real live due to the electrical motor principle.

I think you don't have a problem, unless your pump doesn't run or flow enough.

[sweet_baby_james]

I'll snap a photo of my setup, might make it a little easier to understand.

From the runtime I've had on the pump it seems to be working fine, as does the electrical system in general.

It just surprised me that I would have continuity from the +12v to the grounds, seems like the ECU and other components wouldn't be happy with that?

Thanks for your help.

James

[noahr31]

Thats normal for a motor to read like that when tested with a meter.

[LAV1000]

James,

Check this.
http://www.msextra.com/doc/pdf/html/Mic ... e-3.4.html
Esspecialy page 15 take a look at the fuel pump.

[sweet_baby_james]

That's how I have it wired. I guess my question would be what is a "power ground"? Does that mean direct to the battery?

James

[LAV1000]

Yep, probably trough the chassis.

[sweet_baby_james]

Ok, attached is a photo of the main wiring. This is on a motorcycle, but that doesnt really matter:

The 'System Grounds' buss is connected to the frame. On that buss are the grounds for the O2 sensor, ECU (pins 22 & 23), the fuel pump and the O2 sensor display.

'Sensor Grounds' are pretty self-explanatory, MAP, TPS, Fuel Pressure Sensor, ignition trigger pickup, and the ECU (pin 20).

'Aux Power Fed by FP Relay' gets powered up when the fuel pump relay is pulled in (by the ECU grounding pin . As you see in the photo, this then gives power to the Fuel Pump, Left and Right Injectors, and coils through individual fuses.

'Main Power' buss is fed directly from the battery and feeds the Main Relay (through a switch), the Fuel Pump Relay, and a voltage reference to the R/R.

When the Main Relay is pulled in, this gives power the ECU and the 02 sensor.

The battery -ve terminal is connected to the frame.

Hope that clears it up a little!

James

[grom_e30]

looks and sounds fairly normal. even the signal grounds have to make there way back to the battery ground eventually but they go via the ecu, then through the ecu grounds before the get back to the battery.

is it just the 1 wire going to the frame from the bus bar? you may want to either run more of them or use a bigger wire for the main ground path of all that lot.

[sweet_baby_james]

Its a 10ga wire for from the buss bar to the frame, figured that would be enough.

So if its all pretty normal, do I assume that continuity from +12v to ground through the FP is OK?

James

[grom_e30]

Thats normal for a motor to read like that when tested with a meter.

[LAV1000]

So I gues fuel pump mystery is solved.

Thats a specious set up for a motorcycle !
I saw a kickstarter, what type of bike is this ?

[sweet_baby_james]

Well I guess I'll just keep running that way. Like I said, I've had it running this way before so it was a surprise to find this quirk.

It's a Yamaha RZ350. Converted to fuel injection and a turbo.

James