Cliff Harris

I just read on another forum that VATS enables the fuel pump relay. The car in question was a '91. Is that true?

I thought it was only the starter enable relay and the ECM/PCM would not pulse the injectors if there was a VATS problem.

crowz

I went thru my diagrams and it doesn't go thru the vats but that isn't to say the computer cant be set to decide to not energize it because of the vats. So I cant tell for sure.

Cliff Harris

The part that I don't know about is that the VATS function was moved into the computer, I think in 1990, so it's possible that it could control the fuel pump.

In the 1227165 ECM used from '86 through '89 there is a FMD (Fuel Management Device) integrated circuit that controls the fuel pump. It's all done inside the chip. It is NOT under program control. During those years VATS is a separate module that is not connected to the fuel pump relay.

Researching this could turn into a big project and I really don't want to spend the time for something that doesn't have much return on investment for me. I thought it would be easier to ask and see if anybody knows.

l98tpi

I think that VATS enables the starter and the DERM (air bag control module) enables the fuel pump relay.

When I had VATS issues on one of my 91's the starter would not rotate.

When I just removed the air bag system from my 91 racr, the fuel pump relay nor the ecm would get enabled. So I had to re-route the circuit.

strotter13

Hey Cliff, when I pulled my steering column apart to replace the blinker switch, I disconnected the VATS, (long story on why). I found that the starter would not turn over. When I turned over the key it was like the car was completely dead. I have no idea if it cuts the fuel pump as well. But to me it seems like it wouldn't cut the fuel pump because it wouldn't need to. Unless it was some sort of double redundancy sort of thing...

However if the ECM fuse is pulled (and the vats is connected) which I did also mistaking it for the Air Bag fuse, the car will just turn over and turn over and will not start (of course) which could seem like a fuel pump issue.

Cliff Harris

The ECM fuse is between the ignition switch and the ECM. It's only function it to tell the ECM that the ignition is turned on (or off). The ECM gets its power from a fusible link.

The injector disable feature is to prevent "jump starting" at the starter solenoid. Yes you can crank but the engine will not run.

Related to that: It just occurred to me that the shield under the knock sensor might not be there to protect the knock sensor but to prevent access to the starter and solenoid terminals.

HlhnEast

I dunno about the C4 but the fuel pump on later GM vehicles goes thru the PCM. When the fuel pump goes bad it will pop the PCM fuse.

trackbird

When my '89 Formula Firebird broke the VATS wires on the ignition switch the fuel pump didn't prime or run. (just as a data point).

I was just going through my 1991 FSM in preparation for installing a FAST XFI computer and the CCM has a fuel pump output signal that appears to drive the fuel pump relay assuming the VATS doesn't fail and tell it not to operate the pump. So, it appears (from my research so far) that the VATS talks to the CCM (doesn't everything?) and the CCM sends out the fuel pump enable signal. I'll have to verify the wiring and make sure the CCM goes to the relay instead of the CCM feeding back to the ECM and the ECM handling the fuel pump enable. I'm not 100% sure on which one ultimately handles the output to the pump...yet.

aDigitalPhantom

I think it works the same as later years where it disables the injectors, but I cant verify that. There is a "VATS SIGNAL" at B6 on the PCM.

Tom400CFI

Fuel pump runs on a '92 no matter what. VATS doesn't influence the fuel pump.

trackbird

Per page 8D-8 of the 1991 FSM, under diagnostic tests "display ccm input status, diagnostic test 1.3" input ID #1 is "pass-key fuel" and enabled or disabled. So the ccm can see it.

Digging into the book a bit more, ccm fault code list for module 1.1 shows code 54 as "pass-key fuel enable failure".

Page 8D-19 shows pin F4 as starter enable and it grounds the start relay to enable cranking. Pin D3 is marked as "pass-key fuel enable" and shows as "229 Dark blue" and to ecm (vin 8 is pin B6 and vin J is pin D7 of the ECM)

So, based on this it appears that the pass-key does control the fuel pump by sending the fuel pump enable command to the ecm to turn on the pump.

As a side note pass-key "input" to the ccm shows as pin E12 and pass-key "return" as pin F5.

EDIT:

Page 6E3-A-58 (vin 8) describes "code 46" and shows a block diagram with a module marked "pass-key fuel enable" and shows a wire from "GD3" to a "10 way IP connector" and into pin B6 on the ECM. The ECM shows "pass-key enable signal", a symbol indicating a resistor and "5 volts".

They note that CKT 229 being open or shorted to ground will set code 46 in the ECM and code 54 in the ccm.

And after typing all of that I found the most important part.

"When a complete pass key circuit is detected by the ECM the fuel injector circuits will be enabled and allow the vehicle to start and run".

They don't mention the fuel pump in that case. So, despite what I thought I was reading...it appears to disable the injectors only.

I hope that helps.

Tom400CFI

No. "Fuel enable" is referring to the pulsing of the injectors. Not the fuel pump.



Bingo.

trackbird

Sorry about the tortured route to get there. I was reading the book and typing this on my phone.