Hi guys! Like the title says, I'm getting no voltage at my injectors. Vehicle is a '90 DX hatch with mpfi conversion. Conversion was done in February, no hack job on wiring, I bought the monotech harness so everything went smooth, car ran great for almost 9 months. Last week my fuel pump took a dive (could hear main relay clicking, had voltage under seat, but pump wasn't priming) so I replaced pump (walbro 190), pickup strainer (Airtex fs121), and filter (Purolator f54638) and now pump primes with key in "on" position. Yay, success! Except it won't start. :/ I don't have a fuel pressure gauge but I ran the fuel return line to a bucket and fuel is definitely pumping out everytime I turn the key. So I don't really know if the pressure is in spec, but I know pump polarity is right, main relay does at least half it's job (primes pump), and there's no blockage in filter or fuel lines. I have 3 main relays and all of them prime the pump so I kinda assumed they were all good, what are the chances they are all half-broken? I've checked all my fuses and they are all good. Injectors all have 2.2ohms resistance (spec is 1.5-2.5ohm). The monotech harness checks out from injectors to resistor box, resistor box has 6.1 resistance at each pin (spec is 5-7ohm). I have three main relays and two PM6 ECU's, doesn't matter what I plug in, results always the same. Been running through the flowcharts in the shop manual and I end up here: "Repair open in the YEL/BLK wire between the injector resistor and the main relay". Now there's 3 YEL/BLK wires at the relay, one at pin1, two at pin5. I tested for continuity from those pins to all the contacts in the 12p connector near driver shocktower. Pin1 has continuity at 3 points, and pin5 at another. Is this correct? If that's wrong it looks terribly difficult to fix. Should I just replace the main relay? Can you think of anything else I should check to get voltage to the injectors?

 

You did not describe any direct tests for voltage at the injector clips. How was it done?

Also do this:
Unplug ECU connector A.
Turn key to ON(II).
What voltage to body ground do you read at ECU connector A pins A13 and A15?

 

key in "on" position, Negative probe to body ground, Positive to pins in injector clips. I should get 12v in 4 of the 8 contacts right? I get nothing at all 8.

 

The Yel/Blk wire passes through connector C175/C313 (left shock tower). Do you measure voltage on the Yel/Blk wire (pin 8) there?

 

 

zero Volts at A13 & A15, out to check C175/C313 now.

 

Having trouble finding that 8pin connector pictured, muellersfan. At driver-side shocktower I have a connector that has 12 pins and looks like it could hold 14. I tested all the pins and none came back at 12v, a few had 0.4?

 

The Yel/Blk wires in the main relay connector should be at pin 3 and pin 7. Pin 3 likely has two wires.

 

Ah yes, you are correct. two at pin3 and one at pin7. in OP I called them pins 1 and 5 but I was mistaken.

 

***But ignore this connector for now because you can do tests for lack of voltage at A13/A15.

 

Disconnect the main relay connector.

Use a jumper wire to short pin 5 (Blk/Yel wire) to pin 3 (Yel/Blk wires) in the connector.

Turn key to ON(II).

Do you now find voltage at A13/A15?

 

with jumper i get voltage at A13, nothing at A15

EDIT: i can get Voltage at 15 too, just very finicky. sometimes there, sometimes not.

 

Your original diagnosis of a broken Yel/Blk wire(s) was correct.

Gently push a needle through the insulation to touch copper for each Yel/Blk wire an inch or two from the main relay connector. Do you measure voltage at the pierce point for both wires at that location?

 

I think I see where this is going... All my main relays are half-broken, huh?

Edit: ok, out to check YEL/BLK

 

No. The main relays are fine. You have one or more breaks in the Yel/Blk wires.

 

Kept jumper in place, "A" plug at ECU still unplugged: got voltage at both wires going to relay pin3, nothing at pin7.

 

Temporary test fix:

Cut both Yel/Blk wires at the pierce point and splice to long new wires of the same gauge.

Cut Yel/Blk wires that run to A13 and A15 (~2 inches from ECU connector).

Cut Yel/Blk wire that runs to injector resistors, also leaving ~2 inches of wire.

Run one of the two new main relay wires to the ECU and splice to the cut A13 and A15 wires and the other to injector resistors and splice to cut wire.

Install main relay and ECU connectors.

Does engine now start and run?

 

Will do! couple questions before I start snipping:

just the two that showed power with jumper in place? both going to relay pin3? ignore pin7 for now?

looks like I could go just a couple inches upstream and cut the red wire at the resistor box instead? thus leaving my monotech harness uncut?



Like, I can cut red wire inside orange circle instead of yellow/black one in red circle, right?

 

Yes

Don't touch the pin 7 wire at all.

Test for continuity between your desired cut point and the injector clips. If you detect continuity, then your desired cut point is fine.

 

Mkay, snippy-snip time! TY so much, muellersfan, I can feel the progress already! Wishing I made this thread a week ago, lol.

 

 

Ran out of sunlight. I was able to check continuity on that red wire so that's where I'll be cuttin tomorrow. Also depinned pin3 on the main relay plug and will do the same at ECU plugs A13 and A15. Why depin a wire that's probably damaged? Idk, if one of them is good I'd like to keep it in place I guess. Thanks again to Muellersfan, I'll let you all know how things turn out in the morning.

 

Best of luck

 

Well, I now have 12v at the injectors! I think that confirms the break in YEL/BLK wire? Car still won't start, so there are other problems. Strange that so many things went wrong all at once. FML

 

The next test is to determine whether the engine starts when you spray starting fluid into the TB.