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sprinkles08
sprinkles08, ASE Certified Technician
Category: Pontiac
Satisfied Customers: 21600
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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96 Trans Am LT1 Tach goes to zero Engine sputters and then

Customer Question

96 Trans Am LT1 Tach goes to zero Engine sputters and then runs fine. Tach still erratic! Any clue out there??
Submitted: 13 days ago.
Category: Pontiac
Expert:  sprinkles08 replied 13 days ago.

Hello and welcome to JustAnswer!

If the engine runs poorly and the tach drops out then odds are you're losing crank sensor signal. The crank sensor signal is needed for spark and is where the engine speed signal comes from.

The first step in diagnosing the problem is to use a scan tool or code reader to see what fault codes are setting. Have you checked for codes?

Customer: replied 13 days ago.
No codes are present. Drove it today. Tach was very erratic. Ran fine. Stopped for 2 hours and was very hard to start. When it did finally crank it sounded like
popping through the intake several times. The tach came on and off then ran fine again. Stopped again for 15 min. Didn't think I would get home. The same thing happened.
Expert:  sprinkles08 replied 13 days ago.

Do you have an actual scan tool or a labscope?

Customer: replied 13 days ago.
I scanned for codes and came back PASS
Expert:  sprinkles08 replied 13 days ago.

If you have a scan tool then I would recommend monitoring the crank sensor signal when the problem is happening. It may take a labscope to see the signal dropping though because a scan tool may not update quickly enough.

Customer: replied 13 days ago.
My cheap scan tool don't give live data. It just pulls codes. This started yesterday. I got home went to back into the garage and it died but started right back up. Spit and Sputtered but got inside. It seems to be getting worse.
Expert:  sprinkles08 replied 13 days ago.

That;s a code reader then rather than a scan tool. A scan tool is a multi thousand dollar tool that allows for reading live data, performing actuator tests and reading fault codes.

It's likely that you have a bad crank sensor but it would take some testing to know for sure.

Customer: replied 13 days ago.
I've been calling around since you first said that. It's only about $75. It's worth a try. I will let you know.
Expert:  sprinkles08 replied 13 days ago.

Great!

Customer: replied 12 days ago.
Of course the parts store was out of stock on the $75 Brand so I opted for new AC Delco crank sensor. $155.00. Didn't do it. Same problems!
Expert:  sprinkles08 replied 12 days ago.

I assumed you meant $75 was the diagnostic charge wherever you called. Anything at $75 is going to be a code reader, an actual scan tool will start at around $2,000.

You could have a crank sensor wiring or connection issue or the tach may be dropping because the engine controller is actually going down due to an internal failure or a loss of power or ground to it. You can do a visual inspection of wiring and connections and you can try to monitor sensor reference voltages when the problem happens to see if you can catch something dropping, but the ability to monitor data is most likely going to be necessary to accurately diagnose it.

Customer: replied 7 days ago.
Come to find out the crank sensor has nothing to do with spark or tach signal. It only detects misfires. Thanks. Still having the issue.
Expert:  sprinkles08 replied 7 days ago.

The crank sensor is the main input needed for spark and is where the engine speed signal comes from, the tach shows engine speed. You either have a crank signal issue or the engine controller is going down.

Customer: replied 7 days ago.
Well I have replaced the crank sensor and still have the problem. What is the engine controller?
Expert:  sprinkles08 replied 7 days ago.

The module that controls engine functions.

Customer: replied 7 days ago.
Oh Good Grief! You're no help at All !!! Forget it!

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