New to this forum. Hey all!!!

I have an 1989 S-10 LWB Pick-up with a 4.3 TBI in it. I've owned this truck since new and it recently has set for approximately 3 years (btw I'm proud to say this truck only has 126,042 miles on it! and has only needed a starter, heater core, and water pump over the past 24 years ! ) Anyway, back to the problem at hand. After some fluid changes and such I tried to crank the truck and realized "right off the bat" that the fuel pump was not kicking in. I checked the relay and it was good, checked the fuse and it read 7.04 volts... I then checked the harness going to the relay and it check out ok. 12 volts on orange wire, black was grounded good, 12 volts for 2 seconds when key turned on, etc. No codes stored. I then put a jumper wire on the wiring harness of fuel pump to force it on and nothing happened. So I think "dead fuel pump." But... why the 7.0 volts on fuse. Its that way on all the fuses except Instrument Lamps and Crank, which are -4.9 (yes, negative or reversed polarity!?!). If I pull the ECM B fuses I get 12 volts where I'm suppose to. Anyone have any ideas on this? Is it a partially shorted wire that separates under load causing a voltage drop, bad ECM, ground fault???

Thanks in advance.

P.S. No fuses were blown and the ECM B fuse port has a 0.3 volt on the left side and 12 volts on the right. It should be 0V and 12V, respectively. Also my oil gauge jumps erratically after turning engine over and the overhead light stays on with door shut and off with door open (backwards). Ironically, all these problems have one wire in common. The orange wire going to and from ECM B and is the 12 battery port (port B1) of ECM.

Anything will help. I'm grasping at straws at this point.

 

I finally found the problem. Although, I hate to admit my ignorance, in my defense, I did catch the problem lol. In my absence of mind I attached my volt meter to the ASDL (OBDI Link) port B which I thought was ground because my view was obscured. Port B is the ECM Test Port. Port A (just beside Port B) is the ground link. DUH!!!!.

Why did I miss this? Read on...
I plugged each fuse in 1 at a time and tested everything on the fuse block and saved the ECM B and ECM I for last, respectively. Everything 12 volts until BOTH (not just 1) ECM related fuses were plugged in. I noticed the expected sudden voltage drop over the entire fuse block. It the hit me that even though my meter was reading a voltage drop the courtesy floor light didn't dim as expected (especially when the voltage was nearly halved!!!) I thought to my self (still thinking Port B was ground BTW) maybe the ECM Ground on the ASDL Port is faulty. So I attached my meter to the chassis (as I should have to begin with...Lesson 1!!!) Bam! 12 Volts everywhere. I begin to look at the ASDL Link closer and notice that I wasn't plugged into a ground this entire time! So why the false voltages and why did the meter read a voltage while attached to 2 positives and no ground? Physics 2 friends lol!

The simple answer... backfeed. This is why you should also use a test light to find shorts (the light adds a draws a load from the circuit... sort of like testing an alternator. The meter does not).

The theoretical answer... while the ECM fuses were out of the fuse box the PORT B of the ASDL link acted as a ground (due to backfeed). When re attached the voltage was nearly halved because the meter perceived it as that. It couldn't do anything else, but what I was forcing it to do on accident of course. I also noticed this when I had all the fuses out the 0.3 volts that should have been 0V on the ECM B Port for the fuse (e.g. left prong read 0.3 and right prong read 12.2) was actually discharging (slowly headed to 0V). I assumed this was resistance induced by my meter which happens quite often on electronics. It wasn't. I hit me that this was backfeed voltage from the ECM Unit and it was actually a capacitor discharging from the unit!!! Always use a known, good chassis ground and don't forget to use a test light along with a volt meter to backup your findings!