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Thread: Engine dies while driving, tach drops to zero.

  1. #1

    Default Engine dies while driving, tach drops to zero.

    My car has just developed a problem. I can be driving, idling, anything, then all of a sudden, the tach drops to 0 and the engine stalls. If I'm moving, I can turn the key off and then on again and let the clutch out, or just restart it, and it's fine. I was thinking the ignition switch might be going bad, but I do not lose any other electrical at all. Plus, today I was driving and decided to turn the key off while in gear and the tach does not drop to zero. The problem is happening more frequently now.

    I've checked the spark with the paper clip trick and it's fine. I've checked the ohm reading on the magnetic pickup and it's within spec. All grounds appear to be OK. I'm at a loss as to what to look for.

    The car is a new-to-me Factory Five Cobra that only has 1,600 miles on it, but is 6 years old. It's a 347 cid with a Holley, mechanical fuel pump.

    MSD Digital 6AL box, MSD 8598 distributor. Coil Blaster 2. I actually replaced it last month because the coil died on me. Totally different symptoms: car stumbled when I first started it, then it dies and wouldn't restart. Coil did not have continuity. New coil fixed it right up.

    Cap, rotor and plugs are new. Car runs great when it's running.

    The tach is from Speedhut.

    I hate to point to the little red box, so please advise on how to troubleshoot. As I said, the entire car looks brand new, so no rusty ground bolts, and every ground wire I can see is really well secured.

    Thoughts and suggestions welcome! Adam

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  2. #2
    Join Date
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    Quote Originally Posted by AdamIsAdam View Post
    My car has just developed a problem. I can be driving, idling, anything, then all of a sudden, the tach drops to 0 and the engine stalls. If I'm moving, I can turn the key off and then on again and let the clutch out, or just restart it, and it's fine. I was thinking the ignition switch might be going bad, but I do not lose any other electrical at all. Plus, today I was driving and decided to turn the key off while in gear and the tach does not drop to zero. The problem is happening more frequently now.
    That's unusual. I'd want to know why that happens. The wiring for tachometer and ignition system must be incorrect at some point. Gary
    Regards, Gary

  3. #3

    Default

    Are you asking why doesn't the tach drop to zero when I turn the key off while in gear at cruising speed? I thought that was odd too. I figured the magnetic pickup is still reading and feeding the tach. Hmm, but come to think of it, why does the tach have power when I turn the key off? It never acted up before. And the car ran fine for the few hundred miles I've put on it since buying it this winter. I'm debating unplugging the tach and driving around to see if the stalling problem goes away. Could the tach be shorting out the ignition system?

  4. #4
    Join Date
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    Yes, that's correct Adam. The engine still turning over would have a trigger, but nothing should be powered. So perhaps your alternator is now feeding everything. Your tachometer is clearly powered, and the ignition module also must be. So you have two faults. Your original fault of instant cutout could be module or trigger related. I'd rig up a high brightness LED to the 12V positive of the 6AL module as the first test. Then you can confirm whether 12V loss is the momentary cause. I see you've asked the same on the Cobra website, so others may have help there as well. Gary
    Regards, Gary

  5. #5
    Join Date
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    Quote Originally Posted by AdamIsAdam View Post
    Hmm, but come to think of it, why does the tach have power when I turn the key off?
    Maybe this? https://www.msdperformance.com/suppo...s/#EngineRunOn (MSD Troubleshooting - Engine Run-On)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  6. #6

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    Quote Originally Posted by Gaz64 View Post
    ...I see you've asked the same on the Cobra website, so others may have help there as well. Gary
    You caught me ;-) Funny thing, owning a kit car, I wonder where to get the most brand-specific advice. My last few muscle cars were Pontiacs, so all questions are on the Pontiac forums, and the knowledge there runs VERY deep. But this issue, for example, is pretty generic to hot rodding. MSD electrical type stuff. So I wonder where the best place is for info, hence my double post. Anyway, thanks for the advice. Yes, I wonder if there are two things going on. I was floored when I turned the key off and the tach still registered RPM.

    Nothing like working under the dash of a Cobra. Plenty of room back there - NOT.

  7. #7

    Default

    It's not run-on, because when I cut the key while moving, the tach drops to 0 and the engine shuts off. If I turn the key back on, it resumes normal driving.

  8. #8

    Default

    I rewired two new lines directly from the battery and so far the problem has NOT resurfaced.

    I also added an in-line light on the switched power so I can see if that fails should the car stall again. So far, power from ignition switch has been constant and good.
    Last edited by AdamIsAdam; 05-26-2020 at 12:37 PM. Reason: corrected typo

  9. #9
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    Please proof read your post. Has the problem resurfaced or not?
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

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