need wiring gurus

[DustinsDuster]

so i got the harness for my truck buttoned up, and i've started testing it out before i loom it all together. and then i ran into a strange issue...

key is in the "off" position, everything tests dead like you would expect. turn the key to "on", and i get power to the coil, and hot bundle that feeds all my sensors and what not, but no power going to the fuel pump. now here's where it gets weird: turn the key back to the "off" position, and the coil stays hot. doesnt slowly bleed down, stays at the same voltage. seemed to stay that way until i unplug the main relay, then it dies again. plug it back in, stays dead until i turn the key again.

i have a spare MPI relay, tried it and it seemed to act exactly the same way. i''ve gone back over things and everything seems to be hooked up as it should and i can't figure out whats wrong.

basically, i was wondering if theirs anyone on here who might be willing to take a look at it and see what they think. my grandpa is going to come by this weekend(he's a MUCH better electrician than me), but if we cant get it figured out, i'd still be interested in finding someone to come take a look. any takers?

[Drifte]

Does your truck ignition have accessory between off and on? The Fuel pump doesnt prime on the 1g. I swear I had one that did, but my last one didnt.

It's been a while since I did any dsm wiring, so the other problem cant really help yea. I'll talk to Brent tomorrow about it though.

[sparkles]

Weird. Can you pull up a schematic?

[sparkles]

Wondering if there's some sort of a deal like on the newer cars to where you still have power until you open the door OR possibly something shorted out inside the ignition switch.

[AutoMods]

the cam angle sensor needs to be spinning (motor running or starter engaged) to activate the pump .

not sure on the coil but maybe a capacitor is keeping the voltage up after the key is off

[DustinsDuster]

i got a really helpful list of things to check over at hotrodcoffeeshop.com. he thinks i have a couple wires crossed and the MPI is latching on. i've got a list of stuff to check today, and i'll get back to you guys.

just in case it ever comes up for someone else(unlikely, but always good to spread info):


I'm gonna ramble out loud for a bit.

I'm starting to think you've got your wires crossed somewhere and you are latching the MPI relay on. To test this, set it up so it does what you're seeing, key off but coil still hot, then pull the relay slowly and see if it clicks open. Even more simply, you should hear/feel the MPI relay click off about 8 seconds after key off. If it doesn't shut off on it's own, and it shuts off when you unplug it, it's either latched on, or the ECU isn't turning it off perhaps.

Quote:
the black wire with yellow stripe off the MPI shows ~10v in the start position, and nothing in the run, so i think i got that right,


If that wire goes to pin 108 of the ECU, that is correct, since it provides the starter cranking signal to the ECU. In stock form it ties into the BR wire between the starter relay and pin 9 on the MPI relay. It could easily come from the wire that goes right to the starter solenoid, same thing.

Quote:
so, i tested the coil with the MPI unplugged, and it functions as it should. now i'm wondering if i have pin 3 on the MPI correctly. pin 3 runs to pin 110 on the ECU, tied in with the fuel pump wire, the coil pack/transistor, and has another wire running over to the column and tied into the black wire with white stripe off the ignition switch that is hot with the key in the "on" position.


Pin 3 on the MPI relay is simply fuel pump power in.The ECU pin 56 grounds MPI pin 7 to close the contacts and MPI pin 3 is connected to MPI pin 2 out to the pump, pump voltage check connector, and pump voltage monitor input at the ECU (pin 13). That's all pin 3 does. Pin 3 power comes from the main 12 volt key on output (BW wire) that connects to the coil, ignitor, some dash stuff, and the input at the ECU to tell it the key is on (pin 110).

The coil gets power directly from the ignition switch and should be 0 volts as soon as the key is turned off. We know the ignition switch is turning off properly if it works without the MPI relay plugged in.

If the MPI relay is latching on, you might have crossed something between the BW wire (to ECU pin 110 specifically) and the A output on the MPI relay, or pins 4 and 5, which power all other 12 volt things in the engine harness. If I'm thinking correctly at 6 in the morning, that would maintain the power input to ECU pin 110 and the MPI relay would never turn off, and pins 4/5 would then power the coil as well if they are tied together. The easiest way to isolate it would be to disconnect pins 4/5 if possible. Otherwise just trace all of the wiring from that output and make sure it doesn't connect to anything the BW wire is connected to (ECU pin 110, MPI pin 3, coil, ignitor, CEL, and pressure guage).

I would maintain the factory strategy as far as what 12v items are powered up by the ignition switch, and which ones are powered up through the MPI relay (pins 4/5). Moving things around can overload one output or the other. Also, anything on the ignition switch output simply shuts off with the key, which is certainly desireable in the case of the coil! The things powered up by the MPI relay will have that 8 second delay, which is speicifically to park the ISC motor. Moving things around can cause strange side effects. Double check everything and make sure they're properly segregated.

Hopefully something in there helps...

[sparkles]

Lol, that's where I started to look after I read your post.

[DustinsDuster]

got it all figured out. back when, i bought the harness and ECU from Derek's turbo truck. when i picked it up from him, one of the ECU plugs had a bent up paperclip sticking out of it. he said the ECU had a bad ground and you needed to ground the pin externally to get it to work. after sitting on the shelf for a few years i forgot which one was which, and forgot about the paperclip all together. as soon as a guy over on HRCS mentioned the ECU controls the MPI's ground, i remembered the whole deal, swapped ECU's, and everything worked fine.

i'm pretty happy about the whole thing, now i can get on to finishing up more details. i think my truck thread should get a much needed update soon.

[sparkles]

Does this mean I can't come drink beers?

[DustinsDuster]

sure you can- i'll just be working on something else by then. i need an electric fan yet, gotta hook up the turbo's oil drain, bolt the turbo up with gaskets finally, bolt the TB up with gaskets, run vacuum lines, put the dash back in, bunch of other little things, etc.