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Need help troubleshooting...

The MicroSquirt ecu has one dedicated sensor ground for the sensors. I tied the sensor grounds for the map, air temp, coolant temp, wb, tps, and coils into that per their wiring diagram.
Assuming you mean the coil and wb "logic ground"s, that should be fine. Connecting anything that draws high current to the sensor ground wire(s) is bad. So is running the sensor ground to anything other than the MS sensor ground pin.

As far as power grounds...the coils go to the intake manifold, ecu and wb to negative battery terminal. My shield ground for the cas wiring is grounded to the intake mani.
I'd run the ecu ground wires to the intake manifold. The wb heater ground to the battery is OK.

Ideally, the cas shield should go to the sensor ground pin/wire close to the MicroSquirt, but it really shouldn't matter since the cas uses hall effect sensors, which are pretty robust. If you were using the factory VR-type CPS sensor, it would be worthwhile to move the shield ground. I'd leave it alone unless you're seeing lost sync problems.

B21 battery ground goes to engine and the chassis at the apron just behind the battery tray.
I'm not sure if it matters, but do you have the flat woven ground strap between the back valve cover acorn bolt and the bulkhead?

None of the above would explain your weird Vref and MAP voltages. A few tenths of a volt maybe, but many volts, no. I'm still thinking something is broken or shorted in the wiring.

On other stuff, where is your MAP sensor mounted? It should be good up to 125degC, but might be happier in the passenger compartment.
 
MAP is mounted on the intake manifold support where the IAC would go (no IAC on this car). Hind sight it would have been easier just to house it under the dash near my ECU.

Hoping to get out there to in the next couple days with the multimeter and get those measurements you (bob) had posted.

In the mean time I was considering trying a new MAP sensor as DIY suggested. The one I have on it now was a non OE from DIY which is supplied by a reportedly reputable US manufacturer. Should I be looking at a genuine GM part? Seems Summit has those for under $90. I had considered picking up a cheap knockoff just to try out, but those are still fairly pricey.
 
I just realized where some of the problem may be. In most cases, shield grounds should only be connected at one end. If you're using the pre-made MicroSquirt harness, then the CAS shield ground is already connected correctly to the sensor ground pin near the MS connector. If the other end of the shield wire (near the CAS connector) is connected to the intake manifold, it forms a bad ground loop between sensor ground and block ground. If this matches your wiring, just cut the shield wire off near the CAS.

For the MAP sensor, I'd try relocating it before buying a replacement.
 
Hmmm...I'll have to double check that. I should see that ground wire coming out of that end of the harness right? I don't recall seeing the shield ground at the ECU connector side tied to anything. I was under the impression I'd have to dig it out to ground near the ECU or at the other end. Chose the other end since I was cutting the wiring down anyways.
 
I have a microsquirt with the short harness on the shelf at home - I'll check the shield ground tonight. If I remember, the shielded tach/cas cable is just cut off flush on a brand new harness, and the customer needs to strip back the layers.

If you can easily unconnect the shield from the intake, you can then just measure resistance to ground. If it's connected at the MS connector end (under the heat shrink and harness wrap), it will be ~0 ohms.
 
Was able to get away from work for a bit to poke around. Disconnected the shield ground and started the car. Not sure if thinking clearly as its been a rough week, but I can't figure out how to measure resistance on that ground lead. Did a quick google and couldn't find anything that relates. I might be over or under thinking at this point. Honestly my electrical diag knowledge is amateur at best.

While car was running with the shield ground disconnected I decided to measure to see if there was an difference from what I've measured previously to see if maybe this is what is causing my issues. I'm still showing about 3-4v at my sensor/logic grounds. Interestingly the same voltage is showing when I check that now disconnected shield ground.

VREF still showing 6-7 at idle and increases with rpm.

I've rechecked my ground for the coils (not the logic) and no voltage there. My ECU ground at battery showing no voltage there.


Is it possible an internal failure in the MAP could cause this feeding voltage into the ground? Still stuck on this all happening after after blowing my charge pipe. Wouldn't think that sudden change from 14psi down to vacuum would cause a MAP to fail...read up on aftermarket 3bar MAP sensors being prone to failure however perhaps the timing is coincidental.
 
I checked my short MS harness at home and it does NOT have the CAS (VR1,VR2) shields connected to sensor ground within the connector/shrinkwrap. I must have been mistaken, or was thinking of a different harness. The resistance measurement is simple. If the shield is disconnected at both ends, there should be no continuity, aka infinite resistance, between the shield and sensor ground - just put a probe on shield and one on sensor ground to measure.

Change of topics.
Whenever a coil is disconnected, the collapsing magnetic field generates a brief high voltage spike across the coil. For MegaSquirt installs, the coils are the injectors, the ignition coils, the IAC (not present), and small relays. All of these _should_ be connected to +12v power away from the MS (mostly at the main relay), and _should_ be connected to power grounds. None of the parts normally connected to +5Vref and sensor ground can generate a voltage.

That being said, it would be good to verify that something isn't misconnected. Can you look up the pinout on the ignition coils and make sure that power and sensor grounds aren't swapped? Likewise for the WBO2 (it's not a coil, but it does switch heater power rapidly).

If you want to try a cheap single channel oscilloscope, you could get one of these (or ebay equivalent):
https://www.mpja.com/200KHz-Handheld-Mini-DSO-Oscilloscope/productinfo/35797+TE/
https://www.mpja.com/60MHz-Oscilloscope-Probe-for-Mini-DSOs/productinfo/35886+TE/
https://www.mpja.com/9V-Battery-Snap-to-55mm-21mm-Coaxial-Plug-Adapter-9/productinfo/35965+AD/
Read the instructions to understand the non-obvious features.
 
Bit of an update since I sidelined this for a while with other things going on. Decided to bite the bullet and order another 3bar MAP (AC Delco) per DIY's recommendation to try along with sorting the grounds. New MAP in and same story with 7v VREF at idle increasing with RPM. Didn't get a chance to drive it but I am certain I'm going to get the same scenario. So seems my original MAP is good and I shot the parts cannon at it for nothing...but hey at least I have a spare map sensor now.

Going to get a friend over here with a second multimeter to go through all the grounds, diag, etc to get a second pair of eyes on it. Have to start on my kitchen this weekend which will last who knows how long...which will sideline the car for a bit. Still motivated to get this sorted just a bit frustrating with the timeline to get it done. Appreciate the information everyone has provided so far.
 
This car is going to be the death of me. So here is where I am:

I had a friend come over to run some diag with me while it was still running. Conveniently he has an LS swapped car so we were able to compare some coil readings while running between our two cars just to get an idea of voltage fluctuation. I'm having some more issues than originally thought...

So we are seeing some fluctuations in voltage at the coils...spikes north of 40-50v per my multimeter. Perhaps it is picking up some AC interference from the coil firing, but we are seeing no such spikes on the LS2 coils in his car. Still getting about 2-4v in the sensor grounds which fluctuates with RPM. Fluctuation in the VREF over 5v also increases with RPM. We pull the belts off the alternator to confirm again if it may be an alternator issue. Same results. These issues are only present when the car is running. Key on not running I have 0v on sensor grounds and 4.8-4.9v on the VREF.

We pulled the CAS, backprobe, and it seems to be testing fine on the outputs. I can't seem to find the point of reference for the testing...it was here and a DSM thread elsewhere.

So at this point we are thinking grounding issue again potentially related to the coils when running. Continuity on all the power grounds are good. Pull the ECU and test the sensor grounds...continuity good. No continuity between power and sensor grounds. I've done this with or without any sensors connected. I'm getting continuity when I probe the 12v supply at the coils or injectors and the negative battery terminal (battery disconnected). This continuity will break when I pull my coil fuse. With coil fuse back in it will break when I pull my FP fuse. FP fuse goes back under the dash into my stock FP wiring.

So at this point I'm thinking I have a short somewhere that I missed in my harness. Pull off the intake mani and unwrap my harness. All my connections are good. No abrasions, miswired leads, etc. We test some of the general continuity again on the grounds with everything off and out...no issues with things grounding where they shouldn't under the hood.

So we dig in a little more using the continuity reference point from the 12v at the coil harness to the neg battery terminal. I pull the factory torpedo fuse that still goes to the in tank pump. No changes. I go under the dash to pull the hot wire lead from my MS harness FP fuse to the factory FP wiring that goes back to the two pumps. This breaks continuity. I work further back to the FP wiring under the seat and disconnect the chassis feed to the FP. This breaks continuity too.

I tried running a test jumper wire from my FP lead out of my MS fuse box directly to the main pump thinking maybe it was a wiring issue between the dash and the main pump. Same continuity as before.

When I measure the 12v and ground wires from the FP through the floor I'm getting 1.3ohms.

I'm at a complete loss on where to go next. I'm certainly an electrical diag amateur but we've nailed this down to an issue, I believe, with the FP circuit. My WB, Coils, and injectors share this same circuit (all individually fused but on same relay). I've tested continuity from the CAS to the neg terminal and I don't get continuity (its on a separate circuit with the MS itself).

Is the main FP suspect? Thoughts on next steps?

Sorry if this is a bit all over the place...so scatter brained right now with the time we've put into this.
 
Your high voltage readings on +5Vref and on +12V are still very strange.

For your Fuel Pump concerns, you may just be seeing the resistance of the main pump. I'd guess that the main pump is only 1 or 2 ohms resistance when cold, and only slightly higher when warm. With the pump connected between the MS FP relay and ground, all the other stuff powered by that relay will show a near short (1-2 ohms) to ground. OK? Does this make sense?

A labscope would really help, but you can try to isolate the source of the problem using the MicroSquirt Testmodes/Output Test - Inj/Ign screen. With this screen, you can turn on and off the fuel pump, and repetitively trigger the ignition or injectors. A 50ms interval is equivalent to 1200rpm. Since you have inj/ign/wb wired to the FP relay, you'll need to turn on the FP relay first to test inj/ign/wb. You probably want to pull the FP fuse(s) before testing the injectors.

I'd start by turning on the FP and see what the voltage readings are. If they're bad, try pulling the WB fuse and see if the readings change. Next, try running the IGN coils and measure voltages. You can finish up with quickly testing the injectors (there's a repetition count to prevent flooding the cyls too much). If the FP has been running, there will be residual pressure even after pulling the FP fuse, so you'll still get some raw gas in the cyls. After testing, I'd start the engine to flush the gas and re-oil the cyls. (If you have a spare set of injectors, you could plug them in outside the engine for testing.)
 
Thanks Bob...makes sense. And I'll work on the tests as suggested this weekend I think.

So this wild goose chase I took back to the fuel pump found completely rotted leads at the pump. A prior owner just applied electrical tape over the nonexistent insulation and called it good. He/she didn't go back far enough and looks like there was potential shorting from even more crumbling north of the tape fix. Cause of my problem or not it had to be fixed so I yanked that short harness section, built my own, and reinstalled on my lunch break today.

My Vref and voltage at sensor grounds still exist (kind of figured that) but its getting a nice steady 13ish volts at idle now. Wish I had measured it before to compare as those wires were really bad.
 
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One thing I've learned in all of this is wiring my pull up resistors in the loom under the manifold was a dumb idea. I was checking out the Yoshifab diagram for his harness and I think I might replicate that to easily test or replace if needed.

So I reached a dead end and ended up sending it out to diy for bench testing. Hope to hear something back in another week.
 
So DIY bench tested and couldn't replicate the issue (story of my life). ECU checks out good.

They think it's something to do with the MAP, MAP wiring, or MAP connector. I've replaced with a new gm unit and it made no change. Connector is new from DIY but I'll check that and the wiring yet again.

Truthfully I haven't tested the pull ups themselves as my CAS tested out fine when rotating separately.

The only components I haven't tested are the coils themselves. I'm in the process of finding testing data on those or I might just borrow a set of known good LS2 coils from a friend and see what happens before I go down the labscope rabbit hole.
 
1) With the repaired fuel pump wires, what bad voltages are you seeing? Measured from where-to-where? What brand/model is your volt meter? (Some fancy meters will capture much shorter noise spikes, while inexpensive meters usually average it out.)

2) Have you tried the MS testmodes to see if you can narrow down the noise source any further?

3) Good to know that the MS tests good and wasn't damaged.

4) Have you tried driving it? Or are you worried about being stranded, or another MAP failing?

Good luck, hope it turns out to be a simple mis-wiring issue.
 
The bad voltages were the VREF (at any point), sensor grounds, and coil 12v with meter grounded at the battery. I'm not sure of the model but it's an older Craftsman meter I bought about 15 years ago. Prob mid range model.

I was holding off on the test modes until verifying the ECU was good. Seemed logical in my mind not to go down that road until I figured out the ECU was good.

I haven't tried driving it since the last time. I'm still getting that wonky >5v reading on the VREF so I wanted to spare myself the exercise of pushing it back.


The ECU gets back this week. I'm going to give the known good coilpacks swap a shot to see what happens before digging into test modes.
 
Interesting findings today...

Known good coils made no difference. We added a better ground off the cylinder head to the strut tower go replace the vintage ground strap to the cowl. Made no difference. So the coils are good.

We pulled the FP fuse to crank. Vref over 5v at cranking. Remove coil fuse as well. Vref holds steady at 5v cranking.

For ****s and grins we tried grounding the sensor ground for the coils to the strut tower to see what would happen. Vref at idle reduced dramatically but still exceeded 5v. While it was running we were probing around and found if we probed the sensor ground to the battery neg terminal we saw about 1-2v there but my vref dropped and was rock solid at 5v at idle and above.

So seems like I've got some kind of grounding problem. I'm still getting great continuity from the ground at the ecu harness to the other end. It's grounded at the battery.
 
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^^^ That's great info. I'd check really carefully that the sensor ground and the power ground on the injectors isn't swapped somewhere. Check too that your original injector pinout info is from a good source, and that there's not way to accidentally mis-identify the pin numbers.
 
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