More E40 Mysteries - Fuel Pump Relay Control C1 Pin 45 Always Hot

[rao]

2005 Corvette E40 SPS flashed for a manual transmission in another non-GM chassis - no BCM and no CAN Bus contact with the PCM.

The engine starts and runs but I have the fuel pump wired to always be on with the key.

The fuel pump relay wire C1 Pin 45 is always hot +12v from the moment that the PCM sees key on power. It is supposed to only pulse on to prime the pump and then go on once cranking begins.

Any ideas what is causing this?

[blindsquirrel]

Disconnect ECM, ohm from C1 45 to C1 18 & 19 (or any of the 12v ckts). Trying to see if the pin is internally shorted/stuck - if it is, there's no need to check further.

If that checks open circuit as it should, then: How do you have the starter circuit wired? Straight from an ign switch to the starter or is the ECM controlling it?

[rao]

No short between C1 45 and C1 18, 19 &20.

The starter is independent of the GM PCM: I could use C1 51 to supply the starter relay coil and C1 13 to ground the relay but I have no way to tell the PCM that it is time to start the engine.

[blindsquirrel]

Ah... right. Using the Corvette stuff (well, using only half of it and hoping it works) strikes again. With no BCM the ECM has no way to receive a crank request signal and so doesn't know what state it is in and so doesn't know what it should do with the FP relay. I would really suggest rethinking this in favor of something more swap-friendly. Both GTO and Chevy SSR are available as '05 E40 manual trans options.

[rao]

So when the PCM doesn't get a CAN bus signal from the BCM it defaults to keeping the fuel pump on all of the time?

Interesting.

[blindsquirrel]

I assume so, only because that's how it's behaving. The pin isn't shorted to voltage internally and the 'Description and Operation' section of the service manuals don't really go into detail about how the system works when the ECM is used in a swap with no BCM present.

[rao]

It's too bad they don't

Using an 05 GTO PCM seems like the easiest way to sort this out.

Thank you.

[blindsquirrel]

Have you checked the harness side, with ECM unplugged? Make sure it's not external +12v applied to the wire from somewhere else. The pin assignments for this OS are very different from other E40s with conventional starter/ign switch setups. This is all due to the weird C6 pushbutton start bullshit.

[blindsquirrel]

And it was a one-year-only design before they went E38, which doesn't help any.

[rao]

I have checked the harness side several times.

I already did the SPS reflash to a GTO and am building a spreadsheet to compare the PCM pins. At first glance there are only a handful of differences.

I'll pull the engine and rebuild the harness after I finish the comparison.

The GTO should make several things easier; we'll see.

Next time I start with an E38.

[blindsquirrel]

I don't think any of this is E40-specific, it's '05 Vette-specific. E38s have their own BS issues to deal with. Although, you can get replacement E38s for like, what, $30?

[rao]

I'll take your word for it on the changes being Corvette specific.

Pointing out how cheap e38s are is just mean

[rao]

Here is another update for future searchers.

Before I tore everything apart I decided to test the C1 Pin 45 output with the GTO SPS flash. Even though several wires needed to be relocated they all had to do with the engine running would not hinder checking key on parameters.

Low and behold it acted exactly the same as before, once the key was on and the ECU was powered up C1 Pin 45 had +12v constant. This was very concerning because I was about to undertake a lot of work and if it didn't fix the problem that would be very bad.

I took a step back and thought about and after reading the description some more in the service manual I wondered if because this was a +12v trigger that the service mentions is monitored (including several codes for circuit failure) and not a trigger to ground that it might need a load to work correctly. I put in place a dummy relay which didn't control anything but which C1 Pin 45 triggered and guess what? It worked just as it should - went to +12v for 2 seconds after key on and I could trigger it to +12v with the scanner as well.

From there I did a SPS Corvette flash; flashed in my working tune and it works as it should.

The funny thing is that if I would have just wired it up and not tested it it would have worked fine and I wouldn't have been sent on this tangent.

The moral of the story is that sometimes +12v triggers need a load if the circuit is being monitored by the PCM.

TLDR: Everything was fine, the circuit needed a load to work properly.