Installed new coil, now the tach acts funny

-
Righty Tighty

Righty Tighty

Blame it on the dog
FABO Gold Member
Joined
Aug 16, 2019
Messages
3,200
Reaction score
3,927
Location
Sahuarita, AZ
Just as the title says, I installed a new ignition coil, and now the tach is acting funny.

Fresh 318 build, electronic ignition, new Edit: Auto Meter tach. Old coil was unbranded and who knows how old, new coil is an MSD Blaster 2.

Here's the deal. I literally removed my old coil and installed a new one. That's it. That's all I did. The tach was reading perfectly before, and now it reads zero upon first startup. It'll register some RPMs when I pop the gas pedal, but still seems "off." Driving down the road, the tach will register RPMs, but seems 500-1000 below what they should be. The new coil came with a ballast resistor, initially I didn't install it until I noticed the tach acting like this. I installed the new resistor and the problem persists. The engine is running the best it ever has, but now I've got the tach issue.

What did I miss or not do correctly?
 
Last edited:
It should be fine you did hook it up to the negative side of the coil?
 
halifaxhops said:
It should be fine you did hook it up to the negative side of the coil?
Click to expand...
I’ll double check, but I thought I was being careful to connect everything exactly as it was before. There’s always that chance that I crossed something somewhere along the way.
 
I double checked the wires, tach is wired to the negative coil terminal. Ground looks good.

pntastar69 said:
Did you reinstall the original coil to see if the results are the same?
Click to expand...
Just tried that, same problem. So that tells me that it’s not the coil, rather something with the tach. Which is weird, because I didn’t do anything to the tach besides disconnect it from the coil.
 
What ignition box? Most of those boxes have a terminal or a wire to trigger the tach. You don’t hook the tach to the coil on those. Usually.
 
yellow rose said:
What ignition box? Most of those boxes have a terminal or a wire to trigger the tach. You don’t hook the tach to the coil on those. Usually.
Click to expand...
It’s the orange ignition box, I don’t recall the manufacturer. However, I used my old tach with no problems (other than the occasional bouncy needle, which I could never figure out), and then installed the new tach with zero problems. Swapped coils and now this.

I should add that I have a non-Rallye cluster with no factory tach. I’m using an external aftermarket tach.
 
does your tach use a power, signal and ground (and maybe a dash light feed)? I'd try a straight feed to the battery and the negative coil terminal. Maybe you got a bad power feed or janky ground.
 
I just don’t understand why it would be working perfectly before disconnecting it from the old coil, and after hooking it up to the new coil, issues arise. Literally the only thing I did that involved the tach was to disconnect the negative feed from the coil, and reconnect it.
 
I remember reading a few threads on MSD causing tach issues.
"I think" the multi spark throws the tach off
 
4spdragtop said:
I remember reading a few threads on MSD causing tach issues.
"I think" the multi spark throws the tach off
Click to expand...
I put my old coil back on and had the same issue. Maybe I yanked a wire somewhere along the process and now I have a bad connection?
 
4spdragtop said:
Or you could have buggered the tach? Hopefully not. Borrow another known good tach and try it with stock coil.
Click to expand...
Luckily I still have another tach I’d been using with no problems, I’ll try that.

Out of curiosity, what could screw a tach like that? All I did was hook it up
 
Righty Tighty said:
It’s the orange ignition box, I don’t recall the manufacturer. However, I used my old tach with no problems (other than the occasional bouncy needle, which I could never figure out), and then installed the new tach with zero problems. Swapped coils and now this.

I should add that I have a non-Rallye cluster with no factory tach. I’m using an external aftermarket tach.
Click to expand...


If it’s the orange box than yes, the tach wire gets hooked to the coil. An MSD or Mallory box has an output on the box for the tach.
 
Well I consulted the Autometer website, and learned that with an orange ignition module, the green wire from the tach needs to go to the gray wire that goes from the module to the distributor, not the negative terminal of the distributor. The problem is that there are TWO gray wires going from the module to the distributor.

I called Autometer, and they weren’t much help. Basically they said I need to find out which of the wires “pulses.” I’m assuming the wire I need to find is the negative? The two wires coming from the distributor are red and black, and it’s my understanding that negative leads are usually black, especially when the other is red. Is this generally a good rule of thumb?

7E3E67A2-19B2-418A-AE6B-C7A8A9AF6CF6.jpeg
 
^^NO!!^^ That I doubt very much will work. One of the wires in the dist. harness goes to the "system ground" in the module, and the other is the "hot" or signal line for the pulse. The way to figure that is to disconnect the connector and take up the bay harness end, not the dist. end. The terminal that is the signal is the "bare" terminal. You could try it, I guess....."Rig" wrap a small gauge wire around the bare terminal and temorarily re-connect it, hook that to the tach

"It might be" you are suffering EMI/ RFI---same as magnetic or radio interference--that is falsely triggering the tach

MAKE CERTAIN you have the correct wires from the igntion connected to the coil. Make CERTAIN that if the coil has a radio supression cap, that it is connected properly to the coil+

What color are the two coil wires? If you are unsure, DISconnect the coil negative. Hook a voltmeter to coil + With the key in "run" the coil + should show 12V if correct.
 
Okay, so Autometer is saying I need to connect the tach wire to the signal wire of the distributor. I think I incorrectly assumed the signal wire would be negative, since usually the tach wire is connected to the negative terminal on the coil. I'm not quite understanding what you mean by "bare" terminal. Do you mean the male end on the harness end of the connection?

I'm glad you mentioned the radio suppression capacitor, because several weeks ago, I thought for some reason I had it wired backwards and swapped terminals. It was on the +, I swapped it to the - terminal. Car ran great, tach worked fine with no changes, I thought everything was hunky dory. And I'm wondering if I even need it at all?
 
Radio condenser belongs on the + PLUS

"Bare end" Look at the photo you posted. Hard for me to see, but I believe the upper end goes to the engine bay, the bottom goes to the dist. If you separate those two, the engine bay end will have a bare male end sticking out, and the other will be recessed, female, insulated The bare end is the "signal." This is in fact a test point for troubleshooting. If you lose spark, you can tap that bare end onto engine ground (with key in "run") and it should generate a spark each time you do so.

To repeat, make certain the proper wires from ECU are actually connected to coil +

So..........Coil + PLUS should have proper wire from ECU, the radio cap if used
Coil NEG should have tach wire (normally) and the proper wire from ECU, normally black

And if that is correct and tach does not work, "try" the jerry rig to the dist. connector I've never heard of this, ever. Just did a quick search, tho, and found some references from MoparChat and B bodies only
 
Last edited:
The two wires exiting the dist are low voltage AC. There is no + or - with AC, but sometimes you see the colours listed as + or - to ensure the correct wires get hooked up. Cannot see how either of these wires would trigger a tach. There were some tachs that triggered off the coil + terminal, but the vast majority trigger off the coil - terminal. SOME tachs will not trigger from elec ign [ Mopar, HEI, etc ] & might require one of the MSD adapters, 8910 or 8920 from memory. Above applies to inductive ign, not
 
I did what @67Dart273 suggested and rigged a small gauge wire to the bare terminal on the harness end of the distributor connection, then connected the tach to that. Also corrected the capacitor wiring so that it's wired on the + terminal. Same problem, tach works but only reads above ~1000 or so. Anything below 1000 it reads zero. Since it worked fine prior to swapping coils, and since it now won't work with the old coil, I'm starting to think that @4spdragtop was right and that I'd buggered it somehow with the new coil.

Ordered a replacement, let's see what happens.
 
You must log in or register to reply here.
-

Back
Top