Hey Everyone,

I was referred to this site by Soloman from LT1 PCM Tuning and said Injuneer was the man to talk to.

If I can't fix the car soon, I could use some referrals to get someone that can because I just want it to run again and I feel like I have wasted a lot of time and maybe don't have the skills to get it done. I live in the OC area of Southern California.

I've been trying to get my car running for 4 months now. I was doing some test runs and when I hit 5000rpm in 1st gear, the engine lost rpms, went down slowly. It hasn't run since. There were a few times where it started and ran for under a minute in the last four months but that is is. It immediately shows a code 41 when I crank it and turns off the injectors,etc. The voltage from the ICM to the PCM goes above 4.0 and I have seen it go to 5.0 AC volts sometimes. Also the Cardone manual for DTC 41 says it has to be below 4.5 volts? Is it 4.0 or 4.5? What is the main cause of the voltage being too high? It's not a bad PCM wire as I have checked it and ever replaced it to be sure.

When I wiggle the opti harness where it connects in the middle of the intake it primes my fuel pump several times and there is a beeping noise every once in a while on the driver side by the fender. I completely bypassed the harness connector and it still does it. I've also bough new male and female connectors for the opti harness. Why does it prime the pump when I wiggle it in that area (new connectors or no connectors). I checked all the wires to the PCM from the opti connector and the end where it plugs into the opti and they all have continuity and very low resistance. The voltage, ground is correct as well for all four wires.

I will list everything I have tried so far below. I haven't been able to get solid results testing the opti with an oscilloscope - I posted the graphs on another board and they were not square enough. I have another Hantek scope (the 6 plug one for automotive that I have yet to try). What I have done is bought two new optis and hooked them up with the key on, then used a drill to rotate the opti drive gear. It will not produce spark. Not sure if the spark plug has to be hooked to the number one plug on the opti or any cylinder will do?

Also, I do not have a ground wire for the PCM next to the knock sensor on the engine block. I think it was rerouted to the passenger shock tower. I jacked up my car last weekend and found a ground cable with eyelet that ran with the wire that bolts to the starter (the smaller one, not the big cable). It doesn't ever looked like it was hooked to anything as the starter has a heat shield jacket around it, it was stuck in there, and there is no place to bolt it. I tired grounding it with a wire hooked to the grounding wire tree by the battery but it did nothing. How do I make sure my PCM is grounded?

Does the coil have to be grounded? Because I don't have it bolted to the engine. When I did have it, it still wouldn't start.
Here is what I have done. Please post any ideas, and I will try them!

1. Went through the shbox test procedure several times. Both the ICM connector and opti harness tested fine for voltage, ground, etc. I also tested the opti harness, unplugging it from the opti and is shows the same numbers as the opti connector.

2. Replaced the ICM with a new AC Delco 1905E. Bought another ICM at Oreillys and did not bolt it to the cooling plate (AC Delco still on it), just unplugged the ICM harness and plugged it into the Oreilly and put it on a flat metal surface by the thottle body links. Will it work like this? Car still did not start.

3. Tried two new coils. Tested both coils I have and they both are within spec. One is MSD. I tested the 3 wires that run to the coil and they show 12 volts with the key on. I also disconnected the connectors at the coil and tested them with the key on. With this test, only the 1 wire primary connector showed 12 volts. What I did do was crank the engine with my positive multi meter electric probe taped to the coil spark plug connection, and the other probe to ground. The volts jumped from 12 to 18 when I cranked it. Not sure how accurate this was or if this is the number it should have.

4. Checked my fuel pressure and it holds at 41-42. The fuel pump primes with key in and the pressure drops a bit when I crank it.

5. Printed the GM Service manual troubleshooting for DTC Code 41. Went through all the tests on page 2 and it says its a bad PCM or PCM connector.

6. Tested all the wires in the ignition circuit for continuity and resistance, and all seem fine. Checked all the grounds

7. Cut my white ICM wire near the computer and routed a new wire to the ICM.

8. Replaced my PCM with one a reman off of Ebay.

9. Had Solomon from LT1 PCM Tuning bypass my Vats in the tune program I have installed. SaveShare
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So long, and a first post, it got placed in the “approval queue”.

 

Just in case….. what year is the car? There can be subtle wiring differences, and/or subtle differences in the factory service manual diagnostic measurements.

How long did you own the car before it died while doing test runs?

 

Hi Injuneer. I put too much in my signature, but have since fixed it.

I've had the car for almost two years. It didn't have any problems (hesitation, stalling, not being able to start) before it died four months ago. With the exception of a bad water pump gear that caused the car to overheat (and too me a long time to figure out and fix). I have since fixed it with a electric water pump. I haven't driven the car that many miles. It was supposed to have a new MSD opti, but I have read they can still go bad.

 

I can't believe this.... I just typed a response, and I lost it when I tried to post it because it said I was not signed in, and didn't enter the correct password. I have to change my password every 180 days, and I just successfully did it before writing the response. Sorry, but I need to find more time to recreate the response.

 

that happens all the time with me, so I get in the habit of a quick copy before I hit the post button.

 

Hi Injuneer,

Sorry to hear you lost your post. Thanks for taking the time to put something together. I will keep working on the car, post updates, and edit down my first post.

 

The PCM has two grounds. There is a single black wire, in the bundle with the starter solenoid wire, knock sensor wire, and passenger side O2 sensor wires. That sounds like the wire with the eyelet you found, and should be attached to the block above the starter. Shoebox has a "clickable" photo of a 95 engine harness, virtually identical to 1994. Click on the wires I mentioned for a detailed enlargement of the area:

1995 Z28 A4 Engine Harness

The second ground is the stud on the driver side head that holds the ICM/coil bracket. There are two tan/white wires from the PCM, joined to a single eyelet. That goes under the ICM bracket nut, along with the black ICM wire, along with a ground strap from the chassis. They all have to be there and have clean connections. The black wire from the ICM is spliced to the grounds for the A/C compressor clutch and both O2 sensors.

As far as the voltage on the white wire from the PCM, agree the factory service manual is unclear, referencing both 4.0 volts and 4.5 volts. But the exact voltage is not critical. The code sets when the voltage is 4.6 or above. You are seeing 5 volts intermittently, if I read the above correctly. You only get 5 volts if there is an open circuit. The ICM also gets its 12 volt power through the coil connectors, so those wires are critical.

 

When testing the Opti while not connected to the engine, I believe you need to ground the body of the Opti to the engine.

You reference a Cardone document..... stick with the factory service manual. I have never seen the Cardone document, but their Opti is not very good.

In your step #7, you spliced the white wire near the PCM. Have you verified the condition of pin B5 at the PCM?

Step #3 the 18 volts is a mystery. Did you trace the coill circuit from fuse #11 (under hood box) to coil connector 2/pin B? Then from coil connector 1/pin B to the ICM connector/pin A? The only way I could see 18 volts pop up might be internal leakage between the primary and secondary winding of the coil. But you have used two replacements. Shoebox has a check for the coil: (Be aware when considering my comments.... I am a mechanical engineer, not an electrical engineer..... )

http://shbox.com/1/coil_testing.jpg

 

Thanks Injuneer for the recommendations!

I finally got the car started a few times last night and today!

Last night, I decided to wire back my original PCM to ICM and heat shrink it (I had cut it and spliced in another wire next to the computer to completely rule out the wire being a problem). I had tested the ohms and resistance in both the original wire and temporary wire and both were okay, so I decided I may as well start cleaning things up.

I was about to stop for the night, but then decided to look at one other thing. While I was by the PCM, I started looking at an aftermarket terminal connector that was placed in-between the large stock terminal. I've looked at it before and wiggled it, but just figured it was working okay because all the wires in the ignition circuit tested okay for ohms and resistance.

The terminal is on the right side of the pic, beige in the picture. It also has a white label with a aftermarket part number 63011 on it (black shrink rap under) which I will research to see if I can find the orgin. I'm pretty sure it is part of the line lock setup as I followed the wires into the firewall where there is a switch to activate it (bottom left of the pic, there is a thick grey wire with two small wires going into the connector). I also have my electric water pump key on part of the relay spliced into it (blue wire with black tape at bottom of stock connector) - which was probably not the best idea.

So, what I did was disconnect the two terminals and take the aftermarket connector completely out, then connect the stock terminals. My car fired up immediately two times last night and two times today, but then did not start a third time and started to show the DTC Code 41. So, the extra terminal is not the only culprit. One thing I noticed in the stock terminal is that one of the pins is missing (picture attached). I still need to ground the PCM wire by the starter. Also, the fuel pump still primes when I wiggle the Opti wires near the connector with the key on. So, hopefully the car will continue to start and I will be able to isolate the problem. I'm not out of the woods yet but it was nice to get it started and listen to it rumble!!.

For the optispark test (not with oscillscope) it doesn't look like the optispark is grounded: It didn't work for me because there of the DTC 41 shutoff .

I was actually referencing the Carline manual, not Cardone parts, lol. page or section: 6E3-A-84

Stock Connector missing pin?



The guilty party!



Extra terminal connector part label



Extra connector (beige on right)

 

I did some searches on 63011 and some came up with VATS or Fan Switch. it looks like its plugged inline with the wire harness, it must be the diodes for a fan switch.

 

Looks like someone may have installed a manual fan switch. The circuit can require diodes to be added.

Info from Shoebox:

http://shbox.com/1/fan_switch_wires.jpg

http://shbox.com/1/fan_sw_diagram1.jpg

A line-lock would require wiring on the driver side of the car, and wouldn’t necessitate modifications of the harness near the PCM.

 

Shoebox also has pinout diagrams for the major connectors. The 10-pin connector in that area appears to be C100 or C105:

http://shbox.com/1/harness_connector_faces.htm

 

Thanks Injuneer and chevykid. I finally had some time to work on the car tonight since last posting on Wednesday.

1. The additional terminal connector is routed to a manual fan switch in the car. A mechanic that worked on the car told me the fan will go on at temp, even when the switch is off inside the car - which is good, I have yet to test this myself.
2. The missing pin in the pic I posted last time of the stock connector is for traction control which I don't have. The connector is the C100.
3. Yes, the line lock wiring is all on the driver side where the ABS is at. The switch is in the small ash tray in the car. So, it doesn't seem to be a factor.

4. Previous post answers:

In your step #7, you spliced the white wire near the PCM. Have you verified the condition of pin B5 at the PCM?. -- I looked at all the pins in the connector that has the ICM wire, and they all look good. I can post a pic. Also checked resisance/ohms all the way to the pins in the computer connector. Looked good.

Step #3 the 18 volts is a mystery. Did you trace the coill circuit from fuse #11 (under hood box) to coil connector 2/pin B? Then from coil connector 1/pin B to the ICM connector/pin A? The only way I could see 18 volts pop up might be internal leakage between the primary and secondary winding of the coil. But you have used two replacements. Shoebox has a check for the coil: (Be aware when considering my comments.... I am a mechanical engineer, not an electrical engineer..... ) - Yes, I checked the circuit a while back for volts, and more recently for ohms/resistance and everything seemed good. I will recheck. Also, I checked the MSD and two new parts store coils using the shoebox test and they both checked out okay.

Today, I attached the ground wire to the block. There was no place to attach it by the knock sensor in the area of the pic on shoebox, so I wired it to the where the battery cable/starter cable is grounded to the block to the right of the knock sensor and by the engine mount(removed the nut and put an eyelet connector on there, then tightened it back up).

The car wouldn't start and was showing the DTC 41 again. I feel like I am back at square one again and am sad.

I have a few thoughts and questions:

Does my coil need to bolted to it's holder or grounded? I have read it doesn't need to be? I started leaving it off because the supercharger intake tube makes it difficult to access while working/testing it, and trying to start it

My coil clips don't stay on and clipped. They are aftermarket from Michigan Motorsports. They will stay on pretty tight and are pushed all the way in, but i have taped around them in attempt to make sure I have a tight connection. I think part of the problem is the MSD coil and clips don't quite line up, so they don't clip on and snap.

Wiggling the opti harness wires at the connector by the injectors still causes the fuel pump to prime multiple times with the key on? Both the male and female connector are new and the opti plug in to the opti harness part is new. I've checked all for voltage using the shoebox test and ohm/resistance and everything seems okay? Is it possible that one of the opti wires grounded on the intake and that helped start the car? (they are taped up right now and I need to shrink wrap/clean up everything, but all the connections are tight).

Is it possible that the MSD opti is just being cruel and only starting the car every 100-200 attempts? How do I blow out the opti, as it is vented)? My opti oscilloscope attempts didn't help enough, but I am thinking I will give it another shot. I had the Hantek 6022BE as that was recommended a lot. It produced close to square graphs with the key on, but when I tried to start the car, the lines lines got thicker on one side of the vertical line of the square wave and that is it. It also would not work when I grounded it, so I sent it back. The results and pics I posted were inconclusive. I feel like I ruled it out with the two new optis I tested remotely with a drill. I now have the Hantek 1008B. Anyway, it seems difficult to use them and get conclusive results, and to find the right info on how to use them. I know how to set the time, volts and which opti wires to connect them to.

Any way I can find a good LT1 local mechanic in the LA/OC area? I am on a large LT1 Facebook group and the LS1LT1 website. I called a few local Corvette shops that work on LT1s, but they are not interested. My biggest fear is the car sitting at some shop for a long time or having to pay for a bunch of work and still not have the car fixed. Anyway, really looking forward to getting it running and moving on to my other plans. I would love to post some videos of some pulls with the supercharger. It's fun to drive! I'm hoping at least to help someone on the boards so they can find something helpful from my journey.

 

By "coil clips" do you mean harness connectors? Definitely a potential source of the problem with the ICM. Might want to check for an AC Delco pigtail (Shoebox only shows a 96/97 single connector pigtail). Or maybe a salvage yard. Hawks list the gray and black connectors:

https://www.hawksmotorsports.com/199...nnector-black/

https://www.hawksmotorsports.com/87-...onnector-gray/

As far as the fuel pump priming when the Opti harness is jiggled, that would seem to be indicating there is a break in the low resolution circuit, and it is intermittently connecting when the harness is moved. After the fuel pump primes for 2 seconds with the key in "RUN" position, and shuts down, the pump will not restart until the PCM sees the low resolution pulses from the Opti. It should set DTC 16, but it can only set DTC 16 for the low res pulses if it can see the high res pulses.

Stab in the dark.... are you sure you have the correct Opti harness connector? There are different connectors at the Opti for unvented (93-94) and vented (95-97). One is long and one is short and the short, so a mismatch might present intermittent connection.

No knowledge of anyone who works on LT1's. I converted my LT1 to an aftermarket engine management system 20+ years ago. And there are so few people left on this forum that it's a ling shot you will find someone from your area.