Club Cobra

Club Cobra (
-   Ignition (
-   -   Engine dies while driving - tach drops to zero (MSD) (

AdamIsAdam 05-16-2020 06:00 PM

Engine dies while driving - tach drops to zero (MSD)
2 Attachment(s)
My FFR Cobra just developed a problem. I can be driving, idling, anything, then all of a sudden, the tach drops to 0 and the engine stalls as if someone turned the key off. 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 (gauges, lights turn signals all remain on). Plus, today I was driving and decided to turn the key off while in gear and the tach does NOT drop to zero when I do that; then I turned the key back on and it continued running of course. The problem is happening more frequently now too.

I've checked the spark with the paper clip trick and it's fine, which MSD says that means the box is fine. I've checked the ohm reading on the magnetic pickup and it's within their specs. 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 347cid with a holley DP and 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 and died when I first started it 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!

S1965C 05-16-2020 09:06 PM

Check your ground wire off the MSD box. Loose or bad connection can cause the intermittent issue as you described.

AdamIsAdam 05-16-2020 10:43 PM

1 Attachment(s)
That wire comes into the car behind the dash and right to the frame. See pic. Seems solid.

Moto One 05-17-2020 08:36 AM

We found that the MSD box, FI computer was on the same circuit as the fuel pump, and radiator fan. When the fan would come on the motor would stumble at the voltage drop.

Just a thought.


AdamIsAdam 05-17-2020 09:13 AM

I don't have fuel injection and I have a mechanical fuel pump. This is definitely electrical, not fuel related.

That ground wire I posted a pic of looks VERY secure, it's certainly very tight. I can't see how much paint was scraped off around it, but the bolt goes into the frame. And as I said, it's not moving or loose, that's for sure.

spdbrake 05-17-2020 09:51 AM

I'd suspect its the Ignition switch or the MSD box. My trust level in the 6AL boxes is pretty low. I switched to a MSD Street Fire 5520 after the last failure. It has the same options as the 6AL but not the same MJ amperage output. $100 cheaper. Meets my needs fine.

You don't have a heavy key ring per chance?

I've had pushed back pins (not locked) on the Green/Purple Dizzy connector at the MSD dizzy giving intermit connections. A wiggle check with the engine running should work.

Below is the FFR wiring diagrams.

The Gray wire on the MSD boxes is your Tach feed. Since the tach dies at the same time as the engine its either the Ignition switch or the MSD box or a loose connection. You can use a piercing probe to monitor the power wire to the the MSD box.

Tom Wells 05-17-2020 10:20 AM


This may not apply to your immediate problem, but may bite you again in the near future: I notice you mount the coil to the engine.

If you want the coil to be reliable, get this one: MSD Blaster High-Vibration. It is epoxy-filled and will last much longer when bolted to the engine. The heat combined with the vibration of the engine will kill a normal oil-filled coil fairly quickly.

Alternatively, the coil can be moved to a structural location such as an inner fender or frame brace to isolate it from the engine a bit.

Just a heads-up,


PS: Way back I had a similar problem. It turned out to be a bad butt splice connector in a wire going to the coil. At first it was intermittent, later had to trailer the car home. Took some real detective work to find it as it was located under a fender in an enclosed space. The clue was no voltage at the coil. Maybe you could connect a voltmeter to the hot side of the coil and wiggle wires here and there while watching (or your assistant watches) the voltage.

patrickt 05-17-2020 12:49 PM

About ten years ago I had a similar intermittent electrical problem. I was able to pinpoint it by having temporary 12v lights wired to the critical circuit points so that when the problem occurred I could just look to see what light was out and that tipped me off to where the problem was. Try wiring a light to the On/Off 12v+ feed to the MSD box right at the box itself. Take her for a run and, when she cuts out, tell us if the light is still on. Likewise, have a second light wired to the always-present 12v+ and 12v- feeds to the box. Since that light will always be on, what I did was just hook it up with alligator clips before a run. If it is indeed an electrical issue, this is a pretty good way of identifying it. It won't spot everything (like a flaky coil), but it will eliminate a lot of possibilities. I do like the MSD Hi-Vibration coil, and that's what I have. But don't start replacing things until you've exhausted the light bulbs trick. The problem with an intermittent fault is that you can accidentally "jiggle" a bad connection somewhere in to working right and you think you've fixed it, but you really haven't.

AdamIsAdam 05-17-2020 01:09 PM

Yea, good idea. What do you suggest, Scotchlok wire taps?

patrickt 05-17-2020 01:30 PM


Originally Posted by AdamIsAdam (Post 1476747)
Yea, good idea. What do you suggest, Scotchlok wire taps?

I used Posi-Taps, but anything similar like that would probably be just fine.

FredG 05-17-2020 02:30 PM

MSD ughh
I too have suffered the ills of the 6al but mine have been limited to the coil. I would not trust any of them. I have gone through 2 MSD 8207 Baster coils in 2 years and the third isn't looking so good. First is the intermittent firing then it will shut off. Will start up again after a bit. They don't have a good reputation. I came across the attached document showing MSD recommended coils. I am going to research and see which of these are good. For now I am just going to go with one that is compatible from Summit. Going to look at the high Vibration coil a few people talked about but it will be fender mounted like the current coil.

studebaker53 05-17-2020 03:04 PM

temporarily disconnect tachometer and see if problem persists. I had same problem years ago and the tach was getting hot and shorting out ignition system thus the killing engine.

AdamIsAdam 05-17-2020 04:11 PM


Originally Posted by studebaker53 (Post 1476752)
temporarily disconnect tachometer and see if problem persists. I had same problem years ago and the tach was getting hot and shorting out ignition system thus the killing engine.

I was thinking the same thing.

I found the factory wiring schematics onto which the builder notated where he connected the MSD gray to the harness' purple which then goes to the tach. Of course, it's not a simple plug that I can unplug, it's hard connected. This would all be a lot easier if the darned thing wasn't so darned small behind the darned dash and the interior in general! At least the darned thing doesn't take up much space in the garage! lol Just venting.

(Before I start cutting wires, I want to figure out how I can access the wires to re-crimp them later.)

I'll play w/this all this week and report back. Thanks all for your suggestions.

patrickt 05-17-2020 04:33 PM

Just pull the fuse that feeds the dash gauges and take her for a spin. IIRC, that tach wire is just a 12v square wave.

jhv48 05-17-2020 05:02 PM

Ignition switch is my bet. Start simple then go complex.

AdamIsAdam 05-17-2020 06:17 PM


Originally Posted by patrickt (Post 1476758)
Just pull the fuse that feeds the dash gauges and take her for a spin. IIRC, that tach wire is just a 12v square wave.

Doh! I hate when I don't think of the obvious. That fuse controls all gauges, but also apparently the radiator fan, but I should be ok for a while anyway without over heating, not that I'll know without a temp gauge lol. I'll go tomorrow in the daylight. tnx


Originally Posted by jhv48 (Post 1476760)
Ignition switch is my bet. Start simple then go complex.

Yea, I ordered one (a different brand) already that someone (a professional builder) on the FFR forums said is more reliable. But with only 1,600 miles and it feeling so good/solid, I wonder how likely that is.

I verified that the ground being used has the same volts as when I put my meter to the battery directly. I'll relocate it anyway, but I don't think that's the problem.

My money is on either the ignition switch or the tach. I feel that if it's worked fine for the first 300 miles and a few months I've owned it, and assume it worked for the prior owner, then why would wiring suddenly go bad?

I don't think it's the MSD box itself and I am pretty confident it's not the coil, although I have a spare new one I can swap in if need be.

patrickt 05-17-2020 06:26 PM

Here, try just attaching a 12v light, via alligator clips, to the IGN post of your ignition switch and a good ground. Then turn your switch ON. The light should be on. Now, try gently tapping the ignition switch with the handle of a screwdriver and lightly wiggle the key without turning it to crank or ACC. The object is to get the light to wink when it shouldn't. If you can get the light to wink with a wiggle or a tap, you've found the problem.:cool:

Gaz64 05-17-2020 08:27 PM

The tachometer will not be the cause since it would be connected to the "tach out" terminal on the 6AL. This is an output signal, and cannot be an "input signal" to stop the ignition system from operating.

AdamIsAdam 05-17-2020 09:02 PM

It's not easy, but I'm able to get a test light on the ING wire. It's illuminated with the key ON and tapping the switch or key didn't effect the light. It also functioned properly, illuminating only with key ON, but not on ACC or OFF.

Thanks for the info about the tach output wire.

Ozzie Goat 05-18-2020 06:39 AM

MSD instructions state the box must be grounded to the battery.

I've had this twice. Both times, foe me, was connectivity to ground
Your mileage may vary.

All times are GMT -7. The time now is 05:51 PM.

Powered by vBulletin® Version 3.8.0
Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO 3.6.0
The representations expressed are the representations and opinions of the forum members and do not necessarily reflect the opinions and viewpoints of the site owners, moderators, Shelby American, any other replica manufacturer, Ford Motor Company. This website has been planned and developed by and its forum members and should not be construed as being endorsed by Ford Motor Company, or Shelby American or any other manufacturer unless expressly noted by that entity. "Cobra" and the Cobra logo are registered trademarks for Ford Motor Co., Inc. forum members agree not to post any copyrighted material unless the copyrighted material is owned by you. Although we do not and cannot review the messages posted and are not responsible for the content of any of these messages, we reserve the right to delete any message for any reason whatsoever. You remain solely responsible for the content of your messages, and you agree to indemnify and hold us harmless with respect to any claim based upon transmission of your message(s). Thank you for visiting For full policy documentation refer to the following link: