Tach reading high

[4spdragtop]

While trying to work out other gremlins, all of a sudden the fn tach is bouncing/reading high. Both factory one and other one I was using underhood for carb and timing setup.
Could it be bad coil?
Thanks
Steve

 

[Super-Cuda]

While trying to work out other gremlins, all of a sudden the fn take is bouncing/reading high. Both factory one and other one I was using underhood for carb and timing setup.
Could it be bad coil?
Thanks
Steve

Hey Steve,

Two different tach's hooked up at the same time doing the same thing?

I'd be looking for a vacuum leak first. Next, check ignition wires for arcing. I have found that checking for them at night or in a darkened garage works best . (keep the garage door open a bit LOL !! ).
Not sure what parts you have replaced but, these days, just because they are new doesn't guarantee anything.
Cheers!

 

[4spdragtop]

Hi Al, thanks for chiming in, yes both tachs tested together and individually with same results. I pulled distributor and reset reluctor gap with VA on(mityvac).
No issues with arcing etc, I checked that a little while back.
I'll inspect the wires leading to coil, i may have pooched them when trying to install coil?
Google said that it may be a poor ground? Poor ground where??
Thanks!

Hey Steve,

Two different tach's hooked up at the same time doing the same thing?

I'd be looking for a vacuum leak first. Next, check ignition wires for arcing. I have found that checking for them at night or in a darkened garage works best . (keep the garage door open a bit LOL !! ).
Not sure what parts you have replaced but, these days, just because they are new doesn't guarantee anything.
Cheers!

 

[72bluNblu]

Did you install a new coil?

 

[4spdragtop]

Thanks, I swapped in a used one a couple days prior to the tach issues. No issues (other than carb tuning). While wrestling with carb/timing I noticed that the tach went wonky, both factory one(electronic board) and Equus Pro Tach.

Did you install a new coil?

 

[4spdragtop]

So I read that coil signal wire(grey) should be kept away from any other wiring. I had ziptied/tidied up underhood wiring in prep for wrapping. So I ran it away from other wiring. Altho I had ziptied these well over a month ago with zero issues.
I have 3 coils, 1 new SMP # UC12 and 2 used, MP stickered and another black one with "015" on it. The "015" was in the car, I replaced it with the MP stickered one. The MP one was in it while the tachs were bouncing and reading high(approx 1500 at idle).
I measured resistance on all 3 for comparison.
Black "015"....-ve to +ve = 1.5 ohms
Center to +ve = 8.83K
Center to -ve = 8.83K

Black MP ....-ve to +ve = 0.9 ohms
Center to +ve = 4.58
Center to -ve = 4.58

SMP UC12...-ve to +ve = 1.6 ohms
Center to +ve = 9.19
Center to -ve = 9.19

Black MP one has 1/2 the resistance??
The new SMP will be going back in, we will see.
Oh also, altho 67 cuda, I have 70+ square back installed.
Thanks all.

 

[RustyRatRod]

What ignition are you running Steve? With certain HEI ignitions, you must use a "tach filter" to help calm the tack down so to speak from spikes in the signal. I had to use one on my Ford truck.

 

[4spdragtop]

Mopar elec ign Rob, the "normal" stuff. Running it for quite a while now.

What ignition are you running Steve? With certain HEI ignitions, you must use a "tach filter" to help calm the tack down so to speak from spikes in the signal. I had to use one on my Ford truck.

 

[RustyRatRod]

Mopar elec ign Rob, the "normal" stuff. Running it for quite a while now.

I don't think anything is required there. I have that in Vixen and her tach works fine. I would check the ground real good and all the connections and make sure they're clean and tight.

 

[Super-Cuda]

So I read that coil signal wire(grey) should be kept away from any other wiring. I had ziptied/tidied up underhood wiring in prep for wrapping. So I ran it away from other wiring. Altho I had ziptied these well over a month ago with zero issues.
I have 3 coils, 1 new SMP # UC12 and 2 used, MP stickered and another black one with "015" on it. The "015" was in the car, I replaced it with the MP stickered one. The MP one was in it while the tachs were bouncing and reading high(approx 1500 at idle).
I measured resistance on all 3 for comparison.
Black "015"....-ve to +ve = 1.5 ohms
Center to +ve = 8.83K
Center to -ve = 8.83K

Black MP ....-ve to +ve = 0.9 ohms
Center to +ve = 4.58
Center to -ve = 4.58

SMP UC12...-ve to +ve = 1.6 ohms
Center to +ve = 9.19
Center to -ve = 9.19

Black MP one has 1/2 the resistance??
The new SMP will be going back in, we will see.
Oh also, altho 67 cuda, I have 70+ square back installed.
Thanks all.

View attachment 1716135286

Steve,

I've got the one in the middle with the Mopar logo. I'm using Hi-Rev Blue Ignition Module and have the Mr Heaterbox electronic board in the Factory tach. Factory electronic ignition harness.,
No issues.
BTW , Love the "Wall Art " on the right LOL !!

 

[4spdragtop]

Thanks Al, I can safely say the more exposure the "wall art" gets, the better the mood lol.
I have same board in our tach, as well as NOS MP orange ecu, with 70 up VR.
The MP "stickered" one, I have no idea if it is an MP one, but i doubt it?? I posted the resistance readings for info.
I'll put new SMP one in after work and see.
Anyone got coil specs for 70+ with elec ignition?
Sounds stupid, but which grounds are you referring to Rob?

I don't think anything is required there. I have that in Vixen and her tach works fine. I would check the ground real good and all the connections and make sure they're clean and tight.

Steve,

I've got the one in the middle with the Mopar logo. I'm using Hi-Rev Blue Ignition Module and have the Mr Heaterbox electronic board in the Factory tach. Factory electronic ignition harness.,
No issues.
BTW , Love the "Wall Art " on the right LOL !!

 

[4spdragtop]

I thought all was good, swapped in new SMP UC12 coil. Steady rpm reading, started to dial in carb and timing with tach and vac gauge. Grabbed vac gauge and tach, jumped in car to chk readings in gear and I noticed the tach started to jump again...ffs. I "thought " I heard a "tick" (electrical??) Under the hood, but I was in drivers seat. I shut car off, tried to restart, no spark. Pulled new coil and specs are good/same as outta the box.
Could alternator be causing this? Dist gapped at .08. Double checked va didnt interfere with reluctor under vacuum.
With dist out, spun shaft while hooked to continuity tester and it beeped.
SMP readings 1.9 ohms and 10.7 ohms. A little higher than cold, which is normal I believe?
2 prong ballast resistor measured 0.7 ohms, unhooked.

 

[RustyRatRod]

Thanks Al, I can safely say the more exposure the "wall art" gets, the better the mood lol.
I have same board in our tach, as well as NOS MP orange ecu, with 70 up VR.
The MP "stickered" one, I have no idea if it is an MP one, but i doubt it?? I posted the resistance readings for info.
I'll put new SMP one in after work and see.
Anyone got coil specs for 70+ with elec ignition?
Sounds stupid, but which grounds are you referring to Rob?

Anything associated with the tach and ignition. Not stupid at all.

 

[Super-Cuda]

I thought all was good, swapped in new SMP UC12 coil. Steady rpm reading, started to dial in carb and timing with tach and vac gauge. Grabbed vac gauge and tach, jumped in car to chk readings in gear and I noticed the tach started to jump again...ffs. I "thought " I heard a "tick" (electrical??) Under the hood, but I was in drivers seat. I shut car off, tried to restart, no spark. Pulled new coil and specs are good/same as outta the box.
Could alternator be causing this? Dist gapped at .08. Double checked va didnt interfere with reluctor under vacuum.
With dist out, spun shaft while hooked to continuity tester and it beeped.
SMP readings 1.9 ohms and 10.7 ohms. A little higher than cold, which is normal I believe?
2 prong ballast resistor measured 0.7 ohms, unhooked.

Hey Steve,

Seems like you have checked most of the thing that could cause your problem.

Is the engine misfiring as well?

Is it possible you have cut through the tach wiring harness somewhere and it is grounding out to the body intermittently? Maybe where the Tach wiring harness comes through the bulkhead?

I'm just guessing too!!

Electrical gremlins are a *****!!

 

[67Dart273]

Steve try disconnecting one field wire on the alternator, which will disable it. See if the tach settles down. I'm wondering if you have a weak/ bad diode that is causing ripple in the charging system.

I've thought about your post, just not sure what to think. Examine grounding, wire routing, and power. I don't know if a bad ECU could cause such a thing or not.

 

[4spdragtop]

Thanks guys, appreciate all the help(much needed) lol.
Whatever is happening, it is affecting both my test tach and the one in the car. I checked all 3 wires(grey, blk and blue)physically and visually.
When I shut off last night and attempted to restart, I discovered the no spark(no flash on timing light), no start. Only thing i did was pull coil and tested, bang my head, had a lefthander and walked away lol. After work, I will install same coil and see?? Maybe some stupid heat issue??
I have the square back style alternator, 70+ and matching VR.
I'm gonna try starting again after work after reinstalling coil.
If it starts, then it's a "heat issue".
I will pull a field wire also Del, to see what that does.
Appreciate it.

Hey Steve,

Seems like you have checked most of the thing that could cause your problem.

Is the engine misfiring as well?

Is it possible you have cut through the tach wiring harness somewhere and it is grounding out to the body intermittently? Maybe where the Tach wiring harness comes through the bulkhead?

I'm just guessing too!!

Electrical gremlins are a *****!!

Steve try disconnecting one field wire on the alternator, which will disable it. See if the tach settles down. I'm wondering if you have a weak/ bad diode that is causing ripple in the charging system.

I've thought about your post, just not sure what to think. Examine grounding, wire routing, and power. I don't know if a bad ECU could cause such a thing or not.

 

[4spdragtop]

Ok so reinstalled SMP coil after sitting a few days. Fired right up, steady tach around 759. Timing(if it makes a difference) is approx 17*.
Battery is 14v.
Shut car off, checked continuity(none) between each alt terminal and alt case.
I replaced the terminal on green wire. I had soldered an extension on there previously and was fine for years. But i got rid of "splice " to be sure.
Attempted a restart and intermittent spark. Pulled green wire off of back of alternator and there was a spark?? So I figured I would take blk plastic connector cover off so i could get a better look. Same thing, no start/spark, but when I removed green wire from alternator, big spark again.
When these sparks happen, it is with key on. I am using my remote push button starter.
# on alternator is 7024111. I went with 73+ i believe charging system as an upgrade a few years back. Matched the VR to 73 also.
The green splice wasnt as bad as it looks lol, it's actually a black wire and I put green heat shrink over top.

 

[4spdragtop]

So it looks like a Vision?? Alternator. Not sure if the alt is bad or not.
Amazon product ASIN B000EZZUVK

 

[67Dart273]

This is starting to sound like poor connections in the ignition power circuit, because the very same "run" power wire ALSO branches off and feeds the VR and the blue field wire. This means that amperage load by the total circuits will try and load down the voltage if there is a bad connection in there.

The basic path for "run" is from key....through bulkhead connector.........out into the bay.........branch off to feed VR and alternator field, and feed ignition system --originally to the ballast.

However, no spark at start is the separate IGN2 bypass circuit, normally brown. That goes from the key, through the bulkhead, and to the coil + side of the ballast. It is the only power to the ignition system with the key twisted to "start."

And of course it must be intermittent which just adds to the difficulty

 

[67Dart273]

Forgot to mention, Steve, you mentioned sparking. Anytime the key is in "run" the blue is powered, and the VR is active. This means that if you disconnect either the blue or green field wires, they are drawing current through the field, and you will normally see a spark. This is in part due to the current drawn, as will as the "inductive kick" of the field itself.

 

[Mattax]

Steve, How and where are the new blue and green fiueld wires spliced in?

 

[4spdragtop]

Ok, so heres a video of it not acting up and i pulled the green field wire. Initially I dont think there was a difference, but once it warmed up the rpms appeared to drop when I hooked it up. Me leaving VR unhooked shows why I call it voodoo lol
I will post another video once it starts acting up.

 

[4spdragtop]

Heres video with it acting up, took about 13 minutes of run time. Matt, both blue and green wires all appear in great shape and are crimped/soldered with heat shrink.

 

[fishmens67]

Ok so reinstalled SMP coil after sitting a few days. Fired right up, steady tach around 759. Timing(if it makes a difference) is approx 17*.
Battery is 14v.
Shut car off, checked continuity(none) between each alt terminal and alt case.
I replaced the terminal on green wire. I had soldered an extension on there previously and was fine for years. But i got rid of "splice " to be sure.
Attempted a restart and intermittent spark. Pulled green wire off of back of alternator and there was a spark?? So I figured I would take blk plastic connector cover off so i could get a better look. Same thing, no start/spark, but when I removed green wire from alternator, big spark again.
When these sparks happen, it is with key on. I am using my remote push button starter.
# on alternator is 7024111. I went with 73+ i believe charging system as an upgrade a few years back. Matched the VR to 73 also.
The green splice wasnt as bad as it looks lol, it's actually a black wire and I put green heat shrink over top.

View attachment 1716136103

View attachment 1716136104

It sounds like you have an intermittent short

 

[Bewy]

I would:
- disconnect the tach from the harness. Ground the grd wire [ or case ], whatever it uses for the grd connection.
- connect tach + wire direct to battery +
- connect tach signal wire to coil - terminal
- run engine. If tach is normal, you have a wiring issue; if it is erratic, you have a tach problem.