Tach function drops out intermittently Options

[digit3]

Hey folks,

Trying to figure out what I suspect is an electrical gremlin that causes my VDO tachometer to suddenly lose function.

I'll be driving along and seemingly out of nowhere I see the tach drop out. The needle bumps a tiny bit and my throttle sensitivity "dulls". I can feed it a little gas to keep the car running to get over to the side of the road while my alt light goes on and off.

Once to the side of the road, when I stop feeding it fuel, the car stalls. I can start it right back up and tach function is restored and I may have the rest of my drive back to normal.

My mechanic suspected an ignition switch which we changed out yet the problem remains.

I suspect that it could possibly be a bad ground?

Running a MSD box and 123 distributor on a 2.5SS motor.

Any idea on how to chase this down? I'm not super tech savvy and trying to avoid bringing it into the shop for every small detail however this issue is a PITA!

Thanks

[SirAndy]

Do you still have the relay board in the engine bay?

I once had similar issues with a faulty fuel pump relay on the relay board.
(IMG:style_emoticons/default/idea.gif)

PS: If you still have the relay board, it could also be a broken solder joint going to the traces on the underside.

[digit3]

Do you still have the relay board in the engine bay?

I once had similar issues with a faulty fuel pump relay on the relay board.
(IMG:style_emoticons/default/idea.gif)

PS: If you still have the relay board, it could also be a broken solder joint going to the traces on the underside.

Thanks for your reply. The relay board was removed when we swapped out the motors.

[rfinegan]

a quick check at the battery harness connections for loose or corrosion..see this in the past

[JamesM]

If the tach goes from a running RPM immediately to zero than it isnt a fueling issue, its an ignition system issue, or a tach issue (or related to the wiring of either). Potentially the tach/wiring could be causing the ignition issue as well.

Having both a 123 and the MSD box complicates the troubleshooting a bit as their electronics are both "black boxes" and the failure of either could cause intermittent ignition issues.


BUT

the basics of what you are looking at:

Assuming you haven't changed things around with the MSD box, the tach signal itself is a direct connection from the negative terminal of the coil to the tach. If this wire shorts to ground or if for some reason there is an intermittent short in the tach it will cause your tach signal to immediately cut out and will also cause ignition issues. To rule this out you can disconnect your tach signal wire from the negative side of your coil. This will leave you without a tach but also without the possibility of that circuit shorting out and causing ignition issues. If you are still having ignition issues with this disconnected then the issue is not your tach/circuit. This wouldn't be my first guess as to the problem described but its quick and easy to rule out.

The + side of the coil gets 12v switched from the relay board. Can be impacted by the ignition switch or relay board failing intermittently. I haven't seen this circuit fail on relay boards to often but anything is possible. This one is also fairly simple to rule out as your can temporarily switch your coil to a 12v feed from another source (direct from battery), bypassing both the switch and the relay board and see if this resolves the issue.

The remaining culprits are the 123, the MSD box, or your coil itself. You should be able to eliminate the MSD box completely from the system to rule that out. You may just want to leave it out as i doubt its providing you any benefit.
I have rarely seen a non logic level coil fail but that is also an easy replacement to rule out if it is your problem.
I saved the 123 for last as I dont have a good way to troubleshoot an intermittent failure there so check everything else first and wind up at the 123 by process of elimination.

Not sure how you wired all the extras up but you could potentially be getting a voltage drop/intermitant connection on the 123 or MSD that could cause strange issues as well

[digit3]

If the tach goes from a running RPM immediately to zero than it isnt a fueling issue, its an ignition system issue, or a tach issue (or related to the wiring of either). Potentially the tach/wiring could be causing the ignition issue as well.

Having both a 123 and the MSD box complicates the troubleshooting a bit as their electronics are both "black boxes" and the failure of either could cause intermittent ignition issues.


BUT

the basics of what you are looking at:

Assuming you haven't changed things around with the MSD box, the tach signal itself is a direct connection from the negative terminal of the coil to the tach. If this wire shorts to ground or if for some reason there is an intermittent short in the tach it will cause your tach signal to immediately cut out and will also cause ignition issues. To rule this out you can disconnect your tach signal wire from the negative side of your coil. This will leave you without a tach but also without the possibility of that circuit shorting out and causing ignition issues. If you are still having ignition issues with this disconnected then the issue is not your tach/circuit. This wouldn't be my first guess as to the problem described but its quick and easy to rule out.

The + side of the coil gets 12v switched from the relay board. Can be impacted by the ignition switch or relay board failing intermittently. I haven't seen this circuit fail on relay boards to often but anything is possible. This one is also fairly simple to rule out as your can temporarily switch your coil to a 12v feed from another source (direct from battery), bypassing both the switch and the relay board and see if this resolves the issue.

The remaining culprits are the 123, the MSD box, or your coil itself. You should be able to eliminate the MSD box completely from the system to rule that out. You may just want to leave it out as i doubt its providing you any benefit.
I have rarely seen a non logic level coil fail but that is also an easy replacement to rule out if it is your problem.
I saved the 123 for last as I dont have a good way to troubleshoot an intermittent failure there so check everything else first and wind up at the 123 by process of elimination.

Not sure how you wired all the extras up but you could potentially be getting a voltage drop/intermitant connection on the 123 or MSD that could cause strange issues as well

Great stuff here! Thank you for the troubleshooting ideas. I will get on this next week and hopefully get it sorted out. Once I figure out the culprit I will post here again with my findings in case someone else experiences a similar issue. Thanks again! Dave

[Spoke]

the tach signal itself is a direct connection from the negative terminal of the coil to the tach. If this wire shorts to ground or if for some reason there is an intermittent short in the tach it will cause your tach signal to immediately cut out and will also cause ignition issues. To rule this out you can disconnect your tach signal wire from the negative side of your coil.

I had this failure several years ago with my 914. The engine would buck and tach drop to zero momentarily until it stopped once totally. Tach was a zero. I removed the tach wire from the coil and it ran like a champ. Turns out the wire in the engine harness was shorting to ground in the harness. I ran a separate wire to the relay board and all was well.

Moral of the story is disconnect the tach wire from the coil and test. Like James said you won't have a tach but all other systems should work.

[914rrr]

Will be following this. Mine is acting up as well.