Help! Tach Signal Disappears - Holley HP EFI

[ROKN69]

Hi guys (and gals),

I have an interesting issue I've been chasing for over two months. I have a '69 Camaro with an LSx (RHS block and heads) that is ProCharged making over 1000 HP.

Issue: When the car reaches full operating temperature (not before) it will lose the CRANK signal. This happens at first under heavy acceleration (about 50% or more throttle), but as you drive the car it happens under slight acceleration, and finally after about another 5 minutes after the symptom first presents itself, it will happen even if you're just on the throttle cruising at a steady RPM. When under acceleration, if you keep your foot on the throttle (instead of lifting immediately) when the engine quits (due to loss of CRANK signal) the engine will stay dead. As soon as you lift your foot off of the throttle, the engine will spring back to life like nothing ever happened.

Over a year ago a similar event happened, and it was only under heavy acceleration when the engine quit running. As soon as I would lift the throttle it would spring back to life. It was repeatable and only when the car warmed up. I didn't keep running the car like I am now, though, so I don't know if the problem would have progressively gotten worse (as it is today). I took the car to my tuner, Dave from Auto Trend EFI, and Dave said that the car was missing some necessary grounds (head to head ground, direct ground to battery, some loose connections, etc.). After Dave worked on the car, it ran perfectly for a bit over a year.

Now the issue has presented itself (it feels like the same issue, anyway), again, and we've checked and all the grounds are intact. All connections are secure. We've changed the crank and O2 sensors (the O2 was due to another issue). We've pulled a system log and it shows that the CRANK signal is disappearing, but the CAM signal is still present. We tried a different Holley HP EFI ECU, with the same, identical behavior. We have changed the wiring harness, and verified no breaks in the lines. The throttle cable is nowhere near any wiring harness or cables (it is NOT a drive by wire setup).

I believe I've covered everything we've done, from a troubleshooting perspective. I'm hoping that someone will read this and either have experienced the same issue or point us in the right direction to resolve this nasty bug. Your help is very much appreciated!

Best regards,

Roger

[dhutton]

Find someone with an oscilloscope who knows how to use it. I am guessing the crank sensor signal might have slow ristetime possibly due to capacitive loading or incorrect pull-up resistor value. At low rpm the signal can still rise far enough for detection but the higher rpm signal can't get high enough to be detected. High temperature generally slows things down more. Is the pro charger heating up the area where the crank sensor is? Is the pro charger heating up the ECU?

Don

[ROKN69]

Hi Don,

Thank you for replying.

No, the ProCharger is not heading up the ECU - the ECU is in the passenger compartment, under the dash just under the ashtray. It also isn't heating up the crank signal sensor - they are about 4 feet apart in the engine bay.

Do you think a brand new crank signal sensor would exhibit the same issue as an older one? I'm asking because we've changed crank signal sensors and the issue is still present.

[dhutton]

Originally Posted by ROKN69

Hi Don,

Thank you for replying.

No, the ProCharger is not heading up the ECU - the ECU is in the passenger compartment, under the dash just under the ashtray. It also isn't heating up the crank signal sensor - they are about 4 feet apart in the engine bay.

Do you think a brand new crank signal sensor would exhibit the same issue as an older one? I'm asking because we've changed crank signal sensors and the issue is still present.

I think it is likely an issue with what is connected to the sensor. It is loading it or not pulling it up properly.

This is all a bit of navel gazing without putting a scope on the crank signal. It really is the best way to troubleshoot it.

One question. You said the tach signal in the title and then talked about the crank signal. The tach signal is generally an output from the ECU. So which signal is the problem?

Don

[dhutton]

Do you have the LS1/LS6 or LS2/LS3/LS7 harness and what crank sensor are you using? Reason I ask is the LS1/ harness supplies 12 volts to the crank sensor and the LS2/ harness supplies 5 volts. If you have mismatched the sensor with the wrong voltage harness you will likely have a problem.

Don

[Che70velle]

I'd put the car on a lift and inspect very carefully my harness at the crank sensor, and surrounding area for heat related issues, cracking, etc.
This means probably pulling the starter to make sure you see everything.
That area down there is an extreme heat area due to headers being close to it all. Pay close attention to your pins on the connector. Don't be afraid to remove whatever covering you have on the harness down there, to get a real good look at the wiring. Get a friend to help you do a pin to pin continuity check of each conductor, from the connector at the crank sensor, to the ECM. It will be a lot of work, but you should take the time here to cross your t's, and dot your i's.

If that checks out ok, I'd start thinking about engine thrust clearance, and the possibility of possible crank walk, or the reluctor itself being improperly attached.
Doubtful, as I'm sure your running top shelf stuff with that build, but anything is possible. You mention that when you "unload" the engine, it "springs back to life", so reluctor wheel is a remote possibility...

[ROKN69]

Originally Posted by dhutton

I think it is likely an issue with what is connected to the sensor. It is loading it or not pulling it up properly.

This is all a bit of navel gazing without putting a scope on the crank signal. It really is the best way to troubleshoot it.

One question. You said the tach signal in the title and then talked about the crank signal. The tach signal is generally an output from the ECU. So which signal is the problem?

Don

Thank you, Don. I changed the title of the post from Tach to Crank, thanks for catching that mistake. I changed all the text verbiage from tach to crank and forgot to change the title.

I don't have access to a scope, unfortunately. But, logically speaking, if we have changed ECU's, we've changed the harness wiring, and reverified the harness wiring is OK, then the next step is to change the crank sensor itself (the last piece of the puzzle). I think you meant to use the scope to test the crank sensor, and in my mind it's much easier to just swap in a new crank sensor than to try and test the old one. If I'm missing something here, please let me know.

I appreciate your input and in no way am attempting to shut you down. This is a dialog, and I hope you receive my reply in a positive manner. Thank you.

[ROKN69]

Originally Posted by dhutton

Do you have the LS1/LS6 or LS2/LS3/LS7 harness and what crank sensor are you using? Reason I ask is the LS1/ harness supplies 12 volts to the crank sensor and the LS2/ harness supplies 5 volts. If you have mismatched the sensor with the wrong voltage harness you will likely have a problem.

Don

Hi Don,

We have a 24 tooth reluctor wheel; it's a stock LS1 sensor (recommended by Holley). We are using the correct harness (supplied by Dave from Auto Trend EFI and verified by the current technician working on my car).

[ROKN69]

Originally Posted by Che70velle

I'd put the car on a lift and inspect very carefully my harness at the crank sensor, and surrounding area for heat related issues, cracking, etc.

Thank you. We have done this, and we (eventually) went so far as to completely replace the harness with a new one. The car has not had any modifications relating to exhaust or the passenger side of the engine compartment.

Originally Posted by Che70velle

This means probably pulling the starter to make sure you see everything.
That area down there is an extreme heat area due to headers being close to it all. Pay close attention to your pins on the connector. Don't be afraid to remove whatever covering you have on the harness down there, to get a real good look at the wiring. Get a friend to help you do a pin to pin continuity check of each conductor, from the connector at the crank sensor, to the ECM. It will be a lot of work, but you should take the time here to cross your t's, and dot your i's.

Good idea, every one of them. I wish I could say we hadn't done this, yet, so we'd have something more to do! It's all very frustrating. We have a lift, and have pulled the starter, including dropping the oil pan, to inspect the reluctor wheel. It's intact and within tolerance, according to Holley. We have checked each PIN and they are all fully intact. We have even wiggled all the wires with the car idling and it nothing acts up - it just runs. As I stated earlier, we replaced the entire harness, after first just replacing the crank sensor wire with a factory replacement from GM.

Originally Posted by Che70velle

If that checks out ok, I'd start thinking about engine thrust clearance, and the possibility of possible crank walk, or the reluctor itself being improperly attached.
Doubtful, as I'm sure your running top shelf stuff with that build, but anything is possible. You mention that when you "unload" the engine, it "springs back to life", so reluctor wheel is a remote possibility...

The thrust clearances are OK, the bearings are like new, and the reluctor wheel is properly attached. It's a Callies Dragonslayer crank with Callies rods and a stock LS1 24 tooth reluctor wheel attached.

The unload part is weird. When it's not quite fully warm, but almost fully warm, it only acts up when under moderate/hard(ish) acceleration. Let me explain what I mean by hard(ish) acceleration: I can't give it full throttle on the street because it makes too much power for the rear tires to handle. I can give it up to about 60% throttle depending on my current speed before it starts spinning.

As it warms up more, you don't have to depress the accelerator pedal to get it to act up. It starts cutting out while you're just cruising along at a steady speed and RPM. It "hiccups", for lack of a better word.

We spoke with a couple more GM technicians, today, and were told to swap out the crank sensor with yet another new one, and see if that helps. One tech told us he has had to go through "several" sensors before he found one that worked properly. I haven't heard yet from my shop, but I'm hoping they made some breakthrough, today. I have heard that my shop has followed all the troubleshooting avenues recommended by the pros and my hope, here, is that someone will come up with an idea that we haven't tried, yet.

Thank you for your time in replying! It's much appreciated.

[dhutton]

Originally Posted by ROKN69

Thank you, Don. I changed the title of the post from Tach to Crank, thanks for catching that mistake. I changed all the text verbiage from tach to crank and forgot to change the title.

I don't have access to a scope, unfortunately. But, logically speaking, if we have changed ECU's, we've changed the harness wiring, and reverified the harness wiring is OK, then the next step is to change the crank sensor itself (the last piece of the puzzle). I think you meant to use the scope to test the crank sensor, and in my mind it's much easier to just swap in a new crank sensor than to try and test the old one. If I'm missing something here, please let me know.

I appreciate your input and in no way am attempting to shut you down. This is a dialog, and I hope you receive my reply in a positive manner. Thank you.

I thought you already swapped out the sensor.

I suggested the scope because it is an ideal way to troubleshoot the problem. I am an electronics engineer and we troubleshoot problems like this by observing signal waveforms on an oscilloscope. I am pretty sure your crank sensor waveform has a lousy risetime for some yet unknown reason. One reason is because it may be too hot as suggested a couple of times.

Another idea I had is to get yourself a can of freeze spray and hit the sensor hard with it the next time it happens to see if it goes away briefly.

Don