Jump to content
LegacyGT.com

FPCM Wiring Upgrade - Relay Staying Switched w/ Key-Off


Recommended Posts

Problem: After modification, battery was draining to the point of no-start when car sat for 2+ weeks.

Troubleshooting: Follow this thread and included links from the original post quoted below through to resolution.

Resolution:

In the original modification per this thread and the diagram below, the new relay is getting signal by tapping off the B+ wire that goes to the FPCM. The new relay sends battery power direct to the pump via the FP+ wire, with ground-side switching still being controlled by the FPCM. When the ECU stops commanding the FPCM to drive the pump, the FP- side of the circuit comes up to the same voltage as FP+. Due to the internal circuitry design of the FPCM, the 12V coming direct from the battery through the FP+/pump/FP- can then jump over to the B+ terminal and keep the relay switched, even though power from the stock FP relay (at the other end of the red/black wire) has been removed.

924314651_FPCMOriginal.thumb.jpg.71f23e426acbb9b058bbe587ce1cc429.jpg

 

In my modification as outlined in this next diagram, rather than tap off the B+ wire I disconnect it from the FPCM entirely, and use it to switch the new relay. The new relay sends battery power direct to the FPCM. This eliminates the voltage drop from the main fuse box #11 fuse to the stock FP relay, and through the long 18ga red/black wire back to the FPCM. Relevant to the original problem, this also allows the new relay to be switched open/closed by the stock circuitry, eliminating the battery drain.

251924376_FPCMUnderdog.thumb.jpg.92f0ddc32e216e6548676df09c79ffc4.jpg

 

With the factory wiring I found ~.75V drop between the battery and B+ terminal at the FPCM. With the new wiring they are within .05V. Voltage at the FP+ terminal is within .05V of the B+ supply voltage, meaning the FPCM is not a significant restriction. I was not able to test the output of the STI FPCM as neither module procured from eBay worked for me.

The other key element of this modification is beefing up the FP+ and FP- wires to the pump. By running the 12ga wire in parallel we can be certain the pump is getting as much of the voltage coming out of the FPCM as possible. I did not measure the drop across the original or modified wiring so I don't have numbers for this.

Voltage gain from direct feed to FPCM B+ = .7V

Voltage gain from 12ga wire FPCM-to-pump = unknown

It is important to note that the numbers I observed were during warm idle when the pump is only driven 33% duty cycle. Under 100% duty cycle it is likely that the stock wiring dropped significantly more voltage due to the higher current passing through the wires. This is true for the modified wiring too, but the drop is proportional to the resistance (i.e. triple the current means 2.25V loss from battery voltage on original wiring and .15V on modified wiring).

You might be able to get a similar result by soldering in some 10-12ga wire in parallel with the red/black wire from the stock FP relay back to the FPCM, but the passenger seat and carpet would need to be removed to get access to the wire chase. Also, there could be significant voltage drop between the main fuse box #11 fuse and the FP relay, which would be more difficult to resolve cleanly since the wires are tightly loomed and run along the firewall.

 

Original Post said:

Hey Folks,

I have a bit of a head-scratcher here. Ever since I completed the last round of modifications on my '06 LGT I have had a small drain on the battery (~.1A). This causes a battery voltage drop of ~.1V/day. In the process of troubleshooting I discovered that pulling the FP relay's 30A fuse resolved the issue.

I can make this problem occur simply by cycling the key to IGN and switching back off. Once I pull the 30A fuse the relay switches off and does not switch back on when the fuse is reinstalled. Once the contact is broken the current draw drops to ~.01A.

I have verified that the wiring between the battery/relay/FPCM is correct. My battery is in my trunk and the wire runs are very short. 12V+ goes from the 30A fuse to Pin 30 on the relay. Pin 87 goes to FP+ at the pump. Pin 85 to chassis ground. Pin 86 to ("B+", red/black) at the FPCM. This has been verified against the instructions here, and in the DW FPHWK install instructions.

I have used two different standard 40A/12V SPST relays in the process of troubleshooting.

What appears to be happening is, once the connection is made, the current is feeding through the FP wiring and back into the FPCM via the "FP-" where it somehow keeps the "B+" pin hot. Since the FPCM "B+" is the trigger for the relay, it is keeping itself switched.

The FPCM is the original from my car. Entirely possible it has an internal fault, although it seems unlikely. I do have a spare STI FPCM I received as a replacement for a DOA unit I bought off eBay, and I can try it.

I have tried to duplicate the problem with the FPCM removed and it does not occur. The relay still switches with IGN since it is tapped off "B+" which gets power from the factory FP relay; but when the key is turned off the relay contacts open since there is no back-feed through the FPCM keeping B+ hot.

P.S. The car drives fine and if anything is running quite rich (still on break-in map; pulling fuel, but not pegged, in A/F learning A-C).

Link to comment
Share on other sites

  • Replies 98
  • Created
  • Last Reply

Top Posters In This Topic

Interesting. I have removed the Green/Yellow wire by removing the terminal from the FPCM connector housing and snipping off the last 3" of wire (and insulating the cut end). My relay switch signal is teed from the red/black ("B+") which is still connected to the FPCM. The only thing left to confirm is that my connections at the pump side are correct.

 

It also seems weird that neither STI FPCM will allow the FP to run but are still receiving power at the pins when key is in IGN.

Edited by Underdog
Link to comment
Share on other sites

As further proof that it is coming from the power side of the new relay and back through the FPCM I just cycled the key and pulled the M/B #11 (15A) fuse which is for the power side of the factory FP relay (and provides power to the red/black "B+" wire at the FPCM) - no change, relay is still switched.
Link to comment
Share on other sites

I believe I experienced/am experiencing the same issue, also having completed the FP/FPCM wiring upgrade.

 

Normal day to day driving I never noticed an issue, but when I would come back from offshore (the car being parked for 15 days) the battery would be dead.

 

I'd jump it and not experience any issues over the 12 days I would be home.

 

 

Sent from my iPhone using Tapatalk

Link to comment
Share on other sites

I am starting to think that everyone with the upgraded wiring may be having this problem, but it is only observable if the car is left undriven for an extended period of time. It is not unusual for me to go 2+ weeks in between drives, which is when I experienced the dead battery. Ever since I discovered that pulling the new fuse stopped the draw I have not had that problem (plus I've been keeping it on a trickle charger since I proved the cause and solution).

 

Here are the internals of my factory FPCM. When I probe between the red (FP-) and blue (FP+) locations there is definitely a connection - with my meter set in the 200kohm range it climbs from about ~35 and settles around ~44. Between the blue (FP+) and green (B+) locations there is a direct and obvious current path.

 

With the key in IGN and probing the inside of the FPCM the FP+ and FP- are hovering right around the same voltage, but with the key off there seems to be a .6-.7V drop between the red (FP-) and blue (FP+) locations, i.e. 12.2V at the FP- and 11.6V at the FP+ and B+.

IMG_5765.jpg.0f0ce723f67adb61bea1b3394655bce4.jpg

Edited by Underdog
Link to comment
Share on other sites

I'm gonna chew on it for a bit - maybe hack up a junk FPCM to better understand the circuitry. I checked the voltage at the FPCM B+ and it was only .05V lower than at the battery. With the uprated wire between the FPCM and FP, and the beefed up ground wire from the FPCM to chassis, I shouldn't be losing too much at the pump.
Link to comment
Share on other sites

I ended up with a slight variation of the wiring that resolved this issue for me. Instead of tapping off B+ to feed the FP+ wire directly, I cut the B+ wire and use it to trigger the relay and feed battery voltage direct to the B+ terminal on the FPCM. The FP+, FP-, and E wires are all left intact from the factory, with new 10-12GA wire soldered in parallel to the stock wires. Basically this repurposes the red/black B+ wire from power to signal for the new relay.

 

With the conventional wiring I observed a drop of ~.75V between the battery and B+ terminal at the FPCM. Granted, I have the battery in the trunk so the power had to run to the front of the car on the main 1/0GA wire before going through the main fuse box, to the relay, and back to the FPCM. However, I think it is safe to assume that the long run of 18GA red/black wire was the primary contributor to that voltage drop.

 

With the standard LGT FPCM I observed ~.05V drop between the B+ and FP+ terminals. Neither STI FPCM worked for me.

 

With the new wiring I see ~.05V drop between the battery and FPCM B+, which combined with the .05V drop inside the FPCM means my pump is seeing just .1V less than battery voltage at all times (well, less any losses along the 12GA wires I ran in parallel to the pump).

 

Furthermore, when the key is switched off, the power to B+ from the factory relay is removed, switching off the new relay. I have verified that the drain on the battery has been eliminated.

 

Attached schematic of my version of the mod. Big thanks to all who pioneered this before me.

1030855169_FPCMWiringUpgrade.thumb.jpg.869d4c073cdfb79e7875a59be1c8a515.jpg

Link to comment
Share on other sites

  • 2 months later...

Thanks for this underdog. I recently discovered my car was draining the battery.

 

I never thought about the fact the relay was not turning off.

 

Ill have to look into fixing mine with your new diagram.

 

You saved me some troubleshooting!!

Link to comment
Share on other sites

Yes but if you are switching negative side (FP-) with FPCM then the relay & module is staying powered with ignition off. I originally had it wired per your walkthrough and found FP- feeds back to B+ through the FPCM so the relay would essentially keep itself switched on once the key was cycled, until the in-line fuse to the relay was pulled. Edited by Underdog
Link to comment
Share on other sites

I don't believe I'm having that issue. The car sat for 9 days a couple weeks ago with no problems. I'll have to take it apart and probe the pins to be sure.

 

The only thing feeding power to my FPCM is the stock Red/Black, switched by the stock fuel pump relay. No additional +12 goes to my FPCM.

Link to comment
Share on other sites

9 days would be less than 1v drop for me at the battery. It was the 2-3+ week sits that I would start to have trouble.

 

Does your FP- get switched by the FPCM? If so then the 12v from the battery is going through the pump and back into the FPCM on the FP- wire. That is assuming you have it wired per your own walkthrough like I did.

Link to comment
Share on other sites

If so then the 12v from the battery is going through the pump and back into the FPCM on the FP- wire.

My +12 battery feed is cut by the relay, so there is no positive connection to the pump when the key is off.

 

Looking at your diagram, I'm guessing you didn't un-pin the FP+ from the FPCM plug.

Link to comment
Share on other sites

  • Unplug the FPCM plug and remove the Green/Yellow. Pry up the white retainer, then un-pin the terminal.

http://i.imgur.com/mSydYWPl.jpghttp://i.imgur.com/8AwVJ5zl.jpg

  • I loomed the harness back up and taped the terminal to it making sure it was well covered after this pic was taken.

http://i.imgur.com/jrONDuDl.jpg

Link to comment
Share on other sites

My relay opens and closes with the key, no latching problem.

 

I still need to A/B test the STi FPCM. When I do that I will confirm my wiring and power state of the relay and FPCM.

 

I didn't provide an overall schematic in my walk through. SBT did, but I don't think it's 100% correct.

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

Terms of Use