Baffling electrical issue please help

[Uncle Gump]

Regarding the perceived ground problem;

Since the PCM supplies ground to the fuel pump relay coil, if you measure across the coil contacts where the relay plugs in, it WILL look like there is no ground, unless you turn the key to the start position to make the PCM supply a ground. And, even then, it will only be for a few seconds.

The best you can do is measure from the coil contacts to ground and make sure the PCM power relay is sending voltage to the coil.

I don't typically question your electrical chops... but... ECM provides ground to the fuel pump relay in RUN... not START.

I agree with the perceived "ground" issue. It's unlikely because there is no solid ground source at the fuel pump relay... only a switched ground from the ECM to energize the fuel pump relay under predetermined states. If it was a ground issue... you would have bigger problems.

I'm still confused on how the relay was tested to see this low voltage condition the OP described.

 

[Uncle Gump]

Here are the schematics from my 86 EVTM. They may not be exact to 88 but I think the two pump systems were pretty much carbon copy.

 

[Curious Hound]

I don't typically question your electrical chops... but... ECM provides ground to the fuel pump relay in RUN... not START

Actually, in both. I mis-spoke. My mind was on the initial few seconds to prime for starting.

I'm still confused on how the relay was tested to see this low voltage condition the OP described.

Could be a function of test equipment quality. Or a tiny leak through due to the PCM probably using a transistor instead of a relay internally. That tiny voltage I don't generally worry about in this type if circuit. I don't ignore it. But, it's not significant.

Thanks for posting the diagrams.

 

[thatbotguy]

It should of started on 5 cylinders.

You sure its a pump issue and not injectors?

Pretty sure. The truck shut off at a stop light and it seems unlikely that all 6 injectors would fail at the same time. Also the pump bench tested bad.
Though I tried jumping the fuel pumps and the truck still wouldn't start so I'm thinking maybe the computer is involved.

Any ideas on what could cause both the injectors to not fire and the fuel pumps to not run?

 

[thatbotguy]

I don't typically question your electrical chops... but... ECM provides ground to the fuel pump relay in RUN... not START.

I agree with the perceived "ground" issue. It's unlikely because there is no solid ground source at the fuel pump relay... only a switched ground from the ECM to energize the fuel pump relay under predetermined states. If it was a ground issue... you would have bigger problems.

I'm still confused on how the relay was tested to see this low voltage condition the OP described.

The pin that I have determined to be signal positive on the relay is currently testing at 2 volts to ground on the battery. It reads the same when testing between signal positive and signal ground. This is with the key on run, when it's off there is nothing.

 

[Uncle Gump]

Hard for me to tell... but are you sure your meter leads are connected properly? Looks like the top lead port is for DC volts.

 

[Uncle Gump]

Your terminology is confusing things fir me a bit too.

Once you've verified your leads...

Test both the Yellow ( power from a fusible link) which is Hot at all times and Red (power from the EEC relay) which is Hot with Key in RUN. You should have 12 volts at the above described times.

 

[thatbotguy]

Hard for me to tell... but are you sure your meter leads are connected properly? Looks like the top lead port is for DC volts.

Top is for 10A dc, bottom is for everything else

 

[Uncle Gump]

And so we get terminology for the circuits correct. There are four circuits...

B+ yellow... fused link
Key ON power red... from EEC relay
Relay control tan/light green... switched ground from the ECM
Load green light blue... to the fuel pump

 

[Curious Hound]

Hard for me to tell... but are you sure your meter leads are connected properly? Looks like the top lead port is for DC volts.

He's got it right. That top port is labeled DC amps

 

[thatbotguy]

Your terminology is confusing things fir me a bit too.

Once you've verified your leads...

Test both the Yellow ( power from a fusible link) which is Hot at all times and Red (power from the EEC relay) which is Hot with Key in RUN. You should have 12 volts at the above described times.

Yellow is 12 volts as expected, red is ~2 with the key on

 

[Uncle Gump]

Ok...

So that red wire goes a bunch of places. One is your IAC solenoid. Check the red wire at the IAC connector... with it disconnected. Should be 12 volts...

 

[Uncle Gump]

It also powers all your injectors... should be 12 volts.

 

[Uncle Gump]

If those are the same as the fuel pump reading... refer back to the schematics... there is a fusible link between the EEC relay and the splice that feeds key ON power to all your engine components.

 

[thatbotguy]

Ok...

So that red wire goes a bunch of places. One is your IAC solenoid. Check the red wire at the IAC connector... with it disconnected. Should be 12 volts...

2 volts here as well

After looking at the schematics I noticed the fusable link as well, where is that roughly?