Fuel pump rleay troubles (ECM related)

[Suprzuk]

Ok .... I have been the search route all day and havne't found anything that looks like what I am dealing with.

Truck is an '87 1 ton, TBI 350

It will start after considerable cranking to get the oil pressure switch to activate the pump. If I jumper the relay in the harness the truck fires right up.

I have narrowed it down to the green/white wire coming to the relay harness. If I am following the diagrams correctly this is the feed from A1 on the ECM. I know that it is seeing the ECM because that pin has a ground on it when the ign is in the off position. When I turn the ign to the run position the ground goes away but I don't get 12V on it.

Any ideas on a fix or can I just bypass it and wire that off the ign switch and eliminate the ECM controlling it?

TIA

Kevin

 

[HarryH3]

The relay should have a constant 12 volts to one side of the energizing coil. The other side will go to the ECM and gets pulled to ground when the ECM requests fuel pump enable.

The ECM should request fuel pump enable when you first switch the key "on". The fuel pump will run for a short time and then shut off, unless the ECM gets distributor reference pulses from the ignition module. However, if the pulses aren't getting to the ECM, then it should be setting a trouble code and turning on the check engine light.

Just thought of another thing that I recall reading in a service manual. The 1-tons have something called a "fuel module" that keeps the fuel pump running for an extra 20 seconds or so when the engine is shut off. Perhaps that has failed and is causing the problem that you're seeing?

 

[Suprzuk]

According to the attached diagram the relay has a constant ground and should pull the 12V from the ECM when the relay is to be energized.

I am not seeing any trouble codes on the check engine light....that is probably because the highway dept pulled the light when they got rid of the O2 sensor .... I gotta go back through and get it all back up to snuff.

I am thinking that I will just bypass the ECM controlling the relay and let the ign switch do it for me.

The grn/wht wire that is giving me trouble is marked with the red arrow.

there is a larger version of this diagram on my server HERE

The "fuel module" looks to me like it is just wired in parallel with the relay and oil switch so that shouldn't be causing the trouble.

Thanks

Kevin

 

[dyeager535]

I agree with the fuel module issue, first I've seen that particular component in a diagram. Same as the oil pressure switch in that it can control the fuel pump independent of the ECM.

Only thing about running the relay with ignition is that the fuel pump would continue to work even after the engine died...accident, etc. Not that the engine will always shut off, but that's about the only reason I can think to run it based on the distributor signal.

 

[Suprzuk]

I'll have to dig into it at the ECM tomorow night and see what is going on there. I am pretty sure that the ground being removed from the grn/wht wire though means that I am reading into the ECM.

kevin

 

[Suprzuk]

Anybody have a schematic of the ECM that I could get a copy of ? ? ?

Thanks

Kevin

 

[dyeager535]

You want the pinout?

I've posted it, but the FTP section over at diy-efi.org (gmecm) has the ECM pinout for the '747.

 

[Suprzuk]

I have the pinout...I was looking for the schematic.....I found it last night after running around on google for a while....This is just a crop out of the whole schematic.....just the portion that deals with the fuel pump pin A1

From what I am seeing either one or more of these components are bad or the "VIGN" signal is't getting to this part of the ECM circuit......onto the test bench it goes.

I have been collecting all the diagrams that I find as I am troubleshooting this and adding them to my server......... CLICKY>>>>CLICKY

Kevin

 

[Suprzuk]

Well after looking and hashing out the schematic this morning I am thinking that transistor "Q6" is the culprit. The ground is being read through diode "CR12" back through the ign switch.

I'll update with how I make out once I pop the cover on the ECM tonight.

Kevin