Fuel Pump Relay ECM Signal (Page 1/1)
MrLonely AUG 26, 07:31 PM
Hello, this is my first post but I've spent quite a bit of time looking through old posts and comments. I have an 85 GT Auto V6. From what I have gathered it was originally a manual car but was converted. I've been trying to get this car running and I was wondering about the Green/White wire coming from the ECM to the fuel pump relay. From what I've read, when you turn your ignition key to ON, this wire should give a 12V signal to prime the fuel system. I was probing the relay connector and I found that I always have 12V when I turn the key to the ON position. It doesn't turn off after 2-3 second or at all. I was wondering if I am misunderstanding how the signal works or if something is wrong with the ECM. I know that my ECM is not the original as it has an 86+ computer. I should note that if I jump my fuel pump through the relay connector I can get it started and even drive it. Just wanted to make sure the signal circuit was working correctly, I am planning to do the "hot wire" for the fuel pump once I know the circuit will work correctly.

One other thing I wanted to add is that the orange/black wire going to the relay connector does not show 12V until turned to ON position, I was under the assumption that it should have 12V even with the ignition OFF. But I could be mistaken on how that works too.

Thanks. Any help much appreciated.
Patrick AUG 26, 07:55 PM

quote
Originally posted by MrLonely:

I should note that if I jump my fuel pump through the relay connector I can get it started and even drive it.



There should be at least two relays on the firewall in the engine bay behind the driver. They're all the same. Try swapping the relays to see if your fuel pump will prime properly.


quote
Originally posted by MrLonely:

I should note that if I jump my fuel pump through the relay connector I can get it started and even drive it. Just wanted to make sure the signal circuit was working correctly, I am planning to do the "hot wire" for the fuel pump once I know the circuit will work correctly.



I suspect that's a typo as you do not want to be driving around with the fuel pump "hot wired". Very dangerous. That could lead to big problems in the case of an accident and fire where the fuel pump will continue to run and supply gasoline to the fire despite the engine not running.
MrLonely AUG 26, 08:24 PM

quote
Originally posted by Patrick:

I suspect that's a typo as you do not want to be driving around with the fuel pump "hot wired". Very dangerous. That could lead to big problems in the case of an accident and fire where the fuel pump will continue to run and supply gasoline to the fire despite the engine not running.



I do not mean a true hotwire. I thought it was common knowledge about the fuel pump relay "hotwire" people will do to eliminate unnecessary wiring and to just run it straight from the battery through a new relay. Its done to get higher constant voltage to the fuel pump, especially when running a more powerful pump.

Now as for the relay I only have one good relay that I have tested with a 12V battery source and a multimeter. I can get my relay to click and it closes the secondary circuit to allow voltage to cross. However, that wasn't really the issue I was concerned about. I am more concerned with the fact that the fuel pump seems to continuously run as if the ECM is not giving it a 2-3 second signal for priming the system. Assuming that is what the green/white wire is supposed to be doing.
Patrick AUG 26, 08:41 PM

Are you troubleshooting a non-starting situation, or are you wanting to rig up non-factory fuel pump wiring?

I was offering fuel-pump troubleshooting advice, as most Fieros have at least two relays onboard to try with the fuel pump.
MrLonely AUG 26, 08:45 PM

quote
Originally posted by Patrick:


Are you troubleshooting a non-starting situation, or are you wanting to rig up non-factory fuel pump wiring?

I was offering fuel-pump troubleshooting advice, as most Fieros have at least two relays onboard to try with the fuel pump.



It's technically a non-start situation but I want to understand how the ecm signal works when you turn the key, more specifically, what should be the behavior of the green/white signal wire that goes the to the fuel pump relay. Because when I probe it with the key in the ON position, it has 12 volts all the time which I do not think is right. Hence my questions mainly having to do with out the ecm works rather than the wiring itself.

Sorry if that's confusing.
Patrick AUG 26, 09:05 PM

quote
Originally posted by MrLonely:

I thought it was common knowledge about the fuel pump relay "hotwire" people will do to eliminate unnecessary wiring and to just run it straight from the battery through a new relay. Its done to get higher constant voltage to the fuel pump, especially when running a more powerful pump.



It's not unheard of, but it's far from "common knowledge". Most people who've owned any vehicle have had no interest or desire to customize the wiring to their fuel pumps. It's just not something that's done all that often. I was considering doing it to my Subie WRX STi when I replaced its fuel pump, but I had no hard evidence that it was actually required.

In regards to the workings of the ECU and fuel pump... electronics is not my strong point.
MrLonely AUG 26, 09:33 PM

quote
Originally posted by Patrick:

It's not unheard of, but it's far from "common knowledge". Most people who've owned any vehicle have had no interest or desire to customize the wiring to their fuel pumps. It's just not something that's done all that often. I was considering doing it to my Subie WRX STi when I replaced its fuel pump, but I had no hard evidence that it was actually required.

In regards to the workings of the ECU and fuel pump... electronics is not my strong point.



Well I appreciate the insight, I am having trouble getting the pump to power through the relay but I wanted to ask about how the ecm works when turning the fuel pump on because everything else I've tested has shown to be good and working. I was going to wire the fuel pump directly as an attempt to give it a better ground and improve the voltage signal in hopes that it would fix my issue but I suspect something is up with the ECM signal.
fierosound AUG 27, 10:19 AM

quote
Originally posted by MrLonely:

Well I appreciate the insight, I am having trouble getting the pump to power through the relay but I wanted to ask about how the ecm works when turning the fuel pump on because everything else I've tested has shown to be good and working. I was going to wire the fuel pump directly as an attempt to give it a better ground and improve the voltage signal in hopes that it would fix my issue but I suspect something is up with the ECM signal.



Download the 1986 Factory Service Manual from here:
https://fieroinfo.com/

Follow the fuel system diagnostic charts on pages 6E3-A-24 to 6E3-A-31
A systematic approach should get things sorted out.

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MrLonely AUG 28, 12:17 AM

quote
Originally posted by fierosound:


Download the 1986 Factory Service Manual from here:
https://fieroinfo.com/

Follow the fuel system diagnostic charts on pages 6E3-A-24 to 6E3-A-31
A systematic approach should get things sorted out.



I'll take a closer look at those sections then. I should be able to figure it out but thought I'd ask just in case I was missing something.
MrLonely SEP 15, 06:58 PM
I've been pretty busy but I finally got around to checking through those diagnostic flowcharts and I've come to the conclusion that my ECM is in fact bad. Which would explain why I was not getting the ecm signal to activate my relay correctly.

So I can find a replacement ECM and get it working but I am concerned with why the ECM is bad. Is there something I should be checking to prevent the failure or was it just a matter of time before it stopped working?

I think I also saw somewhere that you can wire a newer Grand Prix ecm to work in the Fiero and I happen to have one as I was planning a 3800 swap.

Appreciate any thoughts, thanks!