WIll36

iTrader: (12)

I'm working on the external wirring. I have a couple questions

how did you guys wire up the distributor and coil to each other and the ECM?

Did anybody have to wire the F idle up?

Where did you guys run the main grounds?

Car is an 88 IROC with a 350 TPI and 6 speed. The original harness was damaged in an engine bay fire so I'm using a DIY autotune MS harness and a donor engine bay harness from an 88 car but I could only find a TBI harness so I'm modifying it as I go.

ZZ3Astro

I just finished this install on my 87 TPI 5.7 auto. You won't need to use F-Idle, just wire up the four wire IAC per the instructions. The manual gives a lot less information on how to properly set up the IAC than I wish, but after a few days of crazy IAC behaviour I figured out the problem. Here is my IAC setup:

IAC Stepper "moving only"
Step Size 3.5 msec
Acceleration step size 0.0 msec
Minimum steps to move 3
PWM freq 80
Start Value (retract) 255 <---- this was defaulted to 0 and was the cause of my problems
Cranking Position 20
Crank to Run taper time 5
Hysteresis 5.0
cold temp 0F
Cold position 40 steps
Cold taper time 40 sec

This is not perfect yet but works pretty well.

As for grounds, I split them up. I ran two of them to a common ground next to the ECM (where I also ground my wide band O2 and PLX map sensor), several to the stock ground location on the back of the passenger side head and used the rest for signal grounds (such as the TPS).

For the distributor, there is some variation from one car to the next from what I have been reading. I have the small cap external coil HEI so yours may be different. I reused the 5v bypass wire from the stock harness. In case you aren't familiar, it's the wire you have to disconnect to properly set the base timing. This wire carries 5 volts from the ECM to the distributor module. When 5 volts is present, the distributor timing obeys the ECM's timing signal, otherwise it falls back to whatever mechanical base timing you have set. I set mine to 8 degrees base (stock is 5 or 6 i think).

This 5v signal is the first place where things get kinda grey. You can get it from the TPS +5v signal (thats what I did) or you can feed it from another source in the Megasquirt. Some people have to set up a relay to shut off the 5 volt signal (during cranking only) to get the car to start up. Others, such as myself, have had no problem just leaving the 5v signal on during crank. I tapped into the TPS +5 line with the original bypass wire from the old harness. The bypass quick disconnect is in the stock location above the heater/ac box on the firewall.

The second place of grey area is the actual setup of the ignition in Megatune. I followed instructions to perfection and had a misfire and jumping tach. Looking at various posts and forums, I found where someone changed a setting from what the instructions said to the opposite setting and voila! I did the same thing and it fixed all of my ignition problems. The setting is either the ignition input capture or the cranking trigger.. can't remember at the moment.. here is how mine is set up:

Trigger offset 8.00
Skip Pulses: 3
Predictor: Alpha-Beta-Gamma
Alpha % 90
Beta % 80
Gamma % 10
Time Mask: 0
Percent Mask: 50
Cranking % 50
After-Start % 70
Normal Running % 25

check tach sync: Always
Ignition Input Capture: Falling Edge
Cranking Trigger: Trigger Rise
Coil Charging: Standard Coil Charge
Spark Output: Going High (inverted)

As for wiring.. I purchased the HEI mod kit but you can wire it just the same without the kit. Just follow the manual for the wiring of the HEI. It's very straight forward.. you basically just connect one wire to sends the signal from the distributor to the megasquirt.. then the second wire sends the timing control back to the distributor module. The third wire is the +5v bypass I mentioned above.

Hopefully this will help you out a bit..

Steve

WIll36

iTrader: (12)

Thanks for the reply I did manage to figure that out the other day I wirred mine up the same way you did. I just got confused from reading the 7 and 8 pins one. The guys at DIY auto tune straigthened me out though.

So my only other wirring issue left is the fuel pump wirring and relay. From what I can tell based on the GM diagrams the fuel pump is fed through the auxilery oil pressure sensor just over the filter. So that as long as the motor is making oil pressure the fuel pump is running. Other than that the ECM just triggers the relay at start up to give it that initially fuel pressure for start up. So do I just wire the fuel pump wire on my megasquirt to trigger that relay? Thanks again for all the help I'm good at diagnosing electrical issues but I get a little nervous when it comes to building stuff from scratch like this.

ZZ3Astro

That fuel pump relay got me good. First I reused some of the ground and +12v ign wiring from the original harness. GM used the same wire color (tan w/black) in several areas. One is the fuel pump +12 and the other is a ground circuit. I made the mistake of tieing those together and blew some fused before I realized what I had done. Anyway the deal is the Megasquirt's fuel pump relay circuit is opposite from the factory ECM. Megasquirt provides the ground signal while the factory provides +12v. I repinned the relay and fed +12v ignition to the relay input where the factory ecm turned it on. I then fed the Megasquirt's fuel pump output into the ground pin of the relay in place of the original chassis ground input. A friend of mine does it a little different. He puts a relay on the circuit that is controlled by the aftermarket ecm. This relay provides +12v to the original fuel pump relay. I prefer less relays myself... less things to fail!

As for the oil pressure switch, there is no risk in keeping it hooked up since it is on the other side of the relay from the MS. I just haven't plugged mine back in yet.

WIll36

iTrader: (12)

Ok I'm a little confused from what I read in the factory electrical manual the fuel ecm provides power to the pump but it also sends some of it to ground to vary the voltage and thus the volume of fuel sent to the motor.

So with your setup you just provided the relay a constant 12v feed on it's hot side and wired the MS harness fuel pump wire to the ground side of the relay? Wouldn't that just make the pump run at full volume all the time? I wanted to keep the auxilery oil pressure switch cause I like the idea of the car losing fuel if the motor isn't making oil pressure.

ZZ3Astro

Well the 7730 ECM pin A11 (pin A1 on the 165 ecm) outputs a +12v signal to the relay, so it can't control how much is actually going to the fuel pump. What you might be thinking of is the fuel pump signal pin E13 (pin B2 on the 165) to the factory ECM, which is an input of the voltage at the fuel pump. I believe it's only used to adjust the base pulse width of the injectors slightly based on the voltage seen at the pump.

WASyL

ZZ3Astro why have You set ignition input capture to Falling Edge rather than Rising Edge ??

best regards

ZZ3Astro

All I know is I had a misfire problem, jumping tach and the spark offset setting didnt match the actual degrees of base timing. After seeing a post from someone with a similar problem, I followed his lead and changed the setting to Falling and everything cleared up and the base timing matched the offset in megatune.

I'm guessing the pickup coil in my distributor is wired backwards. The plastic part of the plug fell apart so there is no way to know if someone put the wires on backwards. That's the only explaination but the only way I can verify is to try another distributor. I had one on hand with a good plug, but the wire colors on the pickup were different. My dist is stuck in the block (140k miles without being touched) so I'm not gonna try to sort it out until I tear the engine down later.

Toehead

That's interesting. On my car the ECM provides a ground for the fuel pump.