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Old 01-27-2021, 05:56 PM   #26
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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.
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Old 01-28-2021, 10:18 AM   #27
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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...info/35797+TE/
https://www.mpja.com/60MHz-Oscillosc...info/35886+TE/
https://www.mpja.com/9V-Battery-Snap...info/35965+AD/
Read the instructions to understand the non-obvious features.
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Old 03-03-2021, 04:23 PM   #28
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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.
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Old 03-03-2021, 04:53 PM   #29
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Old 03-05-2021, 03:03 PM   #30
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Old 07-27-2021, 02:44 PM   #31
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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.
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Old 07-29-2021, 10:03 PM   #32
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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.)
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Old 07-30-2021, 01:36 PM   #33
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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.

Last edited by Alan29; 07-30-2021 at 01:51 PM..
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Old 08-07-2021, 04:30 AM   #34
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Usually when analog signals start to behave strange, I usually check grounding and pull-up resistors.

Also if there are unconnected free analog inputs, it is a good practice to ground those.
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Old 08-09-2021, 09:45 AM   #35
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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.
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Old 08-13-2021, 09:06 PM   #36
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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.
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Old 08-14-2021, 12:26 PM   #37
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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.
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Old 08-17-2021, 09:04 PM   #38
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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.
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Old 08-22-2021, 01:27 PM   #39
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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.

Last edited by Alan29; 08-22-2021 at 01:34 PM..
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Old 08-22-2021, 05:33 PM   #40
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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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Old 09-12-2021, 03:47 PM   #41
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Finally getting around to working on the car again. Funny enough, watching Roadkill Garage has given me some motivation to get it going.

So today I removed the pull ups from the loom and did some significant cleaning up of the wiring (mostly rerouting/reorganizing) as it was a logistical mess. I still need to rewire my coils and injectors. Before I had all that bundled with everything and it passed around under the manifold, up between the mani and thermostat, then back to the coils and injectors. To simplify and make future diag easier these components will be separate. I'm either just going to branch off at the firewall and head straight to the injectors and coils, or I was considering maybe going under the manifold and popping up between the 2nd and 3rd runner.

I'm also working on separating the wb controller wiring from the main loom to facilitate ease of replacement if it goes bad or I upgrade to a different controller down the road.

This week I'm soldering jumpers with pull ups directly to the board. This is the way I wanted to do it from day 1 but I didn't want to risk voiding a warranty.

I guess this is the long way to fix my ground issues but figured why not just do all this to make life easier for the long run. Hope to have this thing fixed in the next couple weeks if my back will allow.

Last edited by Alan29; 09-12-2021 at 04:10 PM..
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Old 09-14-2021, 07:54 PM   #42
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Well things are getting better. Here is what is different from before:

1. Removed 5v pull ups from loom and soldered to board
2. Cleaned up and checked all grounds (sensor and power) in the MS harness work
3. Moved power ground for coils from intake mani to the cylinder head where the kjet hard line bracket was
4. Removed crusty ground strap from back of cylinder head to cowl. Used thick gauge ground from cylinder head at kjet line bracket to the strut tower.
5. Corrected Spartan 2 grounds per manufacturer power ground to cylinder head (where old crusty strap was) and sensor ground to battery neg terminal.

Key on not running I've got 4.92v at vref. Cold start and cold idle it was holding steady as well at same 4.92v. I turned off my meter at that point as I wanted to let it warm up. After warm I checked vref again and it's slightly higher than 5v. Some fluctuating at idle but it actually drops down towards normal instead of the jump up to 6v from before. I'm getting some voltage from the negative battery terminal to the block that fluctuates with rpm.

Drive link has vids of the two above items if anyone is curious:
https://drive.google.com/folderview?...QidJhoM2f8WUve

I ran out of time and didn't get a chance to get my laptop out to look for any abnormalities. I thought it odd the vref held initially but then fluctuated after some time. Tomorrow I'm going to cold start it again so I can keep a close eye on the vref and see how long it takes until it starts fluctuating again. See if maybe that correlates to something else.

So long story short some positive movement. The ground issue is odd.
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Old 09-15-2021, 10:06 AM   #43
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I got a few minutes free this morning to spend with the car. Probed the vref, completed the cold start, and watched the meter. It fluctuates slightly from 4.91 to 5.05 or thereabouts. There is no point when it just jumps to that higher reading I saw yesterday...the range of fluctuation increases as the car warms up. I'm going to guess the voltage at ground might do the same thing but I'll need to wait until the car is cold again to test that theory.

Since it was easy to do and because of my improper grounding of the WB controller on the initial install I pulled the fuse and both ground points thinking maybe there was something there screwing with my grounds. No change.

Since it seems to be temperature related I gave the air and coolant temp sensor connectors good shaking then disconnected to see if any impact. No change.

As soon as the car is cold again I'll test the ground as it warms up. I'll run a log at the same time too (didn't have time to mess with the laptop this morning).

I've ruled out the coils, MAP, TPS, WB, and temp sensors. All my wiring is rock solid. The only thing I can think is perhaps the CAS is going on it already? I'll need to see if the log can give me any indication there. My google searches aren't coming up with a lot of good testing info on the CAS but I'm going to hit that harder this week.
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Old 09-15-2021, 09:58 PM   #44
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Uhhh, I looked at your video and you want to set your meter to the 20V DC position. I don't know what the 9V BAT position does, but it might be screwing up all your measurements, especially if it's trying to load down the circuit until the voltage drops.
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Old 09-16-2021, 08:53 AM   #45
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Uhhh, I looked at your video and you want to set your meter to the 20V DC position. I don't know what the 9V BAT position does, but it might be screwing up all your measurements, especially if it's trying to load down the circuit until the voltage drops.
Oh wow...I have no explanation for why it was in this position yesterday morning as I've been using the 20v DC position all along this adventure. Verified I get the same readings with the 20v DC position, though.
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Old 09-17-2021, 07:44 PM   #46
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I found a manual for your craftsman meter, and the 9v BAT setting only loads the signal with a 1.5K resistor (6mA), which won't affect your measurements. The 1.5v BAT setting adds a 15ohm load (100mA), which will drag down most Vref and sensor measurements.

The variation shown in your video, ~4.9v to 5.6v, is still a little bit too much. Overall, it's doing _much_ better, but something is still weird.
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Old 09-18-2021, 08:55 PM   #47
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Bit of an update. The car had very little gas after all this work since January so I took it out today knowing it would likely stall again but thinking maybe it would last longer.

It died as expected. Got some logs at the time and it's the same **** really. Interesting thing is while it was hot and in a no start condition the vref held a solid 4.92v whilst cranking. After enough cool down time I was able to get it to start and idle. Measured again and vref was fairly solid at 4.92-5.00v. I haven't seen vref that solid at any point when running prior to this last batch of work.

I'm still getting .4-.9v on the power ground (positive probe to block and negative to better neg) when at idle.

I decided to bite the bullet and just order a replacement CAS to see what happens. I found some chatter on the DSM forums about this sort of failure (dying when hot). I can return it necessary.
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Old 09-21-2021, 01:18 PM   #48
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I suppose it should come as no surprise that firing the parts cannon in an act of desperation based on some Google searching didn't work. New CAS and same story as my last updates.
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Old 09-22-2021, 08:17 PM   #49
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IIRC, you had a log of your original failure where the MAP sensor just got stuck at a high value, which I assume flooded the engine. For the CAS, you can look at "Lost Sync Count", and "Lost Sync Reason" in MegaLogViewer to see if the CAS is mis-behaving.

Your latest measurement of 4.92-5.00 Vref is OK. The 0.4 to 0.9 on power ground is a bit much -- have you walked your meter probes along the power ground wire&connections to see if the loss is distributed or if you can pinpoint it?

Any recent logs that happened to capture misbehavior?
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Old 09-22-2021, 08:18 PM   #50
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Well good news and bad news. Good news is I figured out I, for some reason, bought non-resistor spark plugs for this when I first started the build. Nice new set of resistor NGKs and I've got no voltage at my ground now. My VREF is pretty steady now but periodically it will drop a couple tenths. Idle is kind of weird on the car as well...I do need to verify base timing again on it since I've had the CAS in and out.

So I let it warm up. Everything looks good. Idle gets much better, VREF is hardly ever fluctuating, and nothing at the ground so I take it for a spin. Car is seemingly running great. I'm not going too crazy since I'm in my neighborhood but its pulling pretty nice. Nothing weird until it cuts out for maybe a second...like a quick hiccup. I figured it was the end but the car was fine. A minute or so later the 244 once again asserts dominance and leaves me on the side of the road to reassess the meaning of life.

I've got a log of it idling warm then driving and dying. Another of a restart attempt that was cut short because my laptop is ****. I'll look at them tomorrow or maybe the next day. Mentally drained right now.
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