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I've read SEVERAL threads, on TGO and elsewhere that all suggest that the tach signal wire behind the cluster is white. My question is where does that wire come from??? From the ECM? The ignition module? The dual connector coil?
Re: Where does the white/tach signal wire come from?
It comes from the ignition system in the engine bay. Wire passes thru the C100 connector and goes directly to the dash. You should be able to find the wire in the engine bay and check wire continuity with a multi-meter to make sure you have the correct wire.
Re: Where does the white/tach signal wire come from?
So the coil has those two connectors, one gray, one black. Can't remember off hand, but they both have pink and white wires. One connector goes to the ignition module, ....aaand presumably the "other" white wire would then go to the tach?
Re: Where does the white/tach signal wire come from?
QwkTrip, you're running the Holley HP EFI too, right? ....I'm using that, as well as the MSD 6a. Tach was working fine (or at least close enough to not notice) with the MSD and stock ECM, but once I hooked up the Holley, it's way off. Trying to do some trouble shooting before I start looking at the tach itself.
Re: Where does the white/tach signal wire come from?
Yes, I have Holley HP EFI but I don't have GM HEI ignition like you (and therefore no experience with it). I have an LS engine so the tachometer is driven by one of the outputs of the Holley ECM.
Are you trying to drive the tach from the MSD ignition or the Holley ECM? If the tach moves at all then I would guess you already tapped in to the correct wire.
Re: Where does the white/tach signal wire come from?
Right now the ignition is hooked up bone stock, except with the MSD wired in. And of course the Holley driving the timing instead of stock, but up at the motor, it's the same. No tapping into anything for the tach. ....it doesn't read accurately anymore, and I was going to drive the tach right off the MSD. Just wanted to find that wire to do so.
Re: Where does the white/tach signal wire come from?
Stock should be white wire from IGN module at distributor (two wire plug, pink white wires), to coil. From coil to tach. The terminals inside the coil are common respective to their color. As long as you twist white to white, pink to pink, only one plug is actually needed on the coil.
IIRC the white wire to the tach is on the grey connector. To the IGN module is on the black. Dunno if that helps any.
Re: Where does the white/tach signal wire come from?
This is what is was trying to figure out last week. Someone put an aftermarket tach in the 91 and ran the feed from the white wire. On my untouched IROC, the white and pink come out of the factory harness to the grey connector. Then they come out the black connector and go to the distributor. But whoever put the tach in cut the white wire and reconnected it opposite...
Re: Where does the white/tach signal wire come from?
Deadbird, Yes, that helps! ...I do NOT see that white wire going through the c100/firewall, but I'm sure it's in there. I'm going to connect the tach directly to the MSD 6a "tach signal" output, which currently is NOT being used. HOPEFULLY.....this brings the tach readings back into acceptable parameters. Next step would be a tach filter.
Re: Where does the white/tach signal wire come from?
The tach connects to the - side of the coil. That's why there's 2 white wires in that pair of connectors. One goes toward the coil, one toward the tach.
If memory serves, MSD supplies an adapter to connect the stock tach to the place on the box that it needs to go. If you don't have that, the tach is hooked up straight to the coil like stock, and gets "multiple spark discharges" (what "MSD" stands for) per firing event, which will make it read STOOOOPID high when it reads anything at all.
Re: Where does the white/tach signal wire come from?
Originally Posted by Abubaca
..I do NOT see that white wire going through the c100/firewall, but I'm sure it's in there.
Welcome. On my 'bird, I can only guess is in the same spot for a Camarrow... , is literally next to the small bolt that holds the C100 plug to the socket. If you dig under the dash, it will be on the left (towards the hinge pillar, hood release, etc)
Re: Where does the white/tach signal wire come from?
Sofa, I used to have the stock ECM with MSD, and all was well. At least, close enough to not notice. When I installed the Holley EFI, ignition is the same except of course the Holley now connects to the ICM, it now reads almost 7000 while the Holley data is showing at 4500. ...and trust me, the 383 TPI isn't at 7000!! I've got the rev limiter set, and the Holley unit reports accurate on data logs, but it just irks me seeing it rev to 7000 LOL!!!!!
I've read about that MSD adapter, and I'm considering that too. Some seem to need it, some don't. Not really sure why. It could just be a bad tach, but I want to try and see if it's something else first. Relatively inexpensive items, and others have had success, so we'll see. The speedo is all freshened up and reads dead on, and all the other vitals are handled through the Holley LCD screen, no reason I can't leave the stock gauge set up, but the tach just drives me nuts!
Re: Where does the white/tach signal wire come from?
So for what it's worth, I found it. -at least at the coil side of things. It's the white wire coming out of the gray connector. There's actually another gray connector that quickly disconnects. I attached a quick jumper wire directly to the MSD tach output and it reads exactly the same. Stock reads 1000 while it idles at 800. Stock reads 2000 while at 1500.
I'm gonna try that MSD adapter/filter part. Gonna google and read up on that a little now.
Re: Where does the white/tach signal wire come from?
Have you posted this same question at the Holley HP EFI forum? Info you get over there might complete the rest of the puzzle.
And have you referenced the HP EFI use manual? I remember there being a diagram for GM HEI with MSD distributor.
Re: Where does the white/tach signal wire come from?
I have NOT posted this over on the Holley forum. Couldn't hurt.
That diagram you're referring to is correct, but it's really no different than how you would normally hook up an MSD box. Literally the only difference is that the 4 wire connector going into the Ignition Module now comes from the Holley, and not the stock ECU. Trust me, I stared at it for about 30 minutes until I realized it's just the same as how I had it!
I really think the issue is somewhere with the MSD box, not incorrect wiring, or the Holley unit. ...and I know I said it was working/reading ok before I got the Holley system, but the more I think about it, the less sure I am. I have an extra tach, and I'm gonna plug it in this weekend. If it works, I'm gonna get my regular tach fixed. If it doesn't, I'm gonna get one of those MSD adapters.
Re: Where does the white/tach signal wire come from?
So I checked another tachometer from an 88 Iroc and functions the same. Reads high. I'm guessing it's MSD related. As I write this it occurs to me that it's EASY to go back to stock, since I have the MSD plug and play harness. 30 seconds. THAT would surely point to whether or not it's the MSD. ...but assuming it is, I'm gonna call them and get their input on my best course of action. Filter is on the list, but they also have part # 8918, which looks a little simpler. We shall see.
Re: Where does the white/tach signal wire come from?
So that's actually the MSD harness I'm using already. I've isolated the factory tach wiring, and it's either running of the MSD box "white" wire, or the box's dedicated "tach output" port. I've run it both ways, and both ways read high. This would make sense as they're both coming out of the MSD box.
HOWEVER....as you can see by that schematic, my coil now only has the black connector. It does NOT have the grey connector as that connector plugs into the MSD harness. I attached a pic from the googles, and this is how mine is now. Just the black connector from the MSD harness. That circled port is where the WHITE wire from the GREY connector came out and went directly to the tach. .....sooooo it would stand to reason that I can still connect it directly to the tach, and bypass the MSD. ....stands to reason this would work. Still....maybe the signal is still not gonna work since it's all still wired into the MSD box. ....still may have to pick up that filter.
...and even if this doesn't work, I can still bypass the MSD all together for testing purposes and confirm whether it IS or is NOT the MSD box.
Re: Where does the white/tach signal wire come from?
The coil, if you're using the MSD harness, is now triggering the coil. The white terminals and pink terminals share a common buss *inside* the coil.
Yeah, so basically the tach signal is STILL coming from the MSD, regardless of how I wire it, right? I was thinking that was probably the case. However, I CAN still bypass the box all together, and test. Then at least I'd know if it was the box causing the bad reading.
Any chance you have a 6 cylinder tach?
Did your tach ever read correctly?
My one main tach is my original tach. I think the second tach is from an 88 Iroc, although I can't verify other than the redline. But regardless, yes, the main tach did work, at least as good as a factory tach every did.
Re: Where does the white/tach signal wire come from?
I'm wondering also.
Did you actually make sure that your stock tach was actually disconnected from the factory wiring harness (white wire), getting no reading at all before running a separate wire to the tach terminal output directly on the side of the MSD box?
In other words, tach directly to MSD tach terminal.
Re: Where does the white/tach signal wire come from?
I've wired up the tach 2 ways since adding the MSD. Literally going directly to the box output/spade connector, and via the MSD harness that connects to the factory wiring. Both read high, which would make sense IF it's the MSD causing the issue.
What I said I was going to do, but have NOT done...was to bypass the MSD and return to factory ignition. Not just for the tach, but for the entire ignition. Remove the MSD completely. ...I just haven't done it. I don't think about it until I crank it up to drive and then I don't feel like doing it. I keep saying I'll do it later...and then....I forget again. I'll try and remember to check it tonight.
Re: Where does the white/tach signal wire come from?
Originally Posted by Abubaca
I've wired up the tach 2 ways since adding the MSD. Literally going directly to the box output/spade connector, and via the MSD harness that connects to the factory wiring. Both read high, which would make sense IF it's the MSD causing the issue.
What I said I was going to do, but have NOT done...was to bypass the MSD and return to factory ignition. Not just for the tach, but for the entire ignition. Remove the MSD completely. ...I just haven't done it. I don't think about it until I crank it up to drive and then I don't feel like doing it. I keep saying I'll do it later...and then....I forget again. I'll try and remember to check it tonight.
I ended up just runniung an aftermarket gauge hooked into the grey tach output wire from the MSD box. I have heard the stock tach isn't as accurate as an aftermarket one anyway
Re: Where does the white/tach signal wire come from?
Wicky, I agree with you, about accuracy.
The Holley ECU keeps accurate RPM data for datalogs, and I can even display it on the dash via the LCD screen when needed for racing or tech/tune reasons. For day to day though, I prefer the dash to remain pretty close to stock appearing. Just wish that factory tach was a little better. That being said, I don't think my issue is run of the mill "factory tach" issues being slightly off. I think it's MSD box related.
05-29-2017, 09:36 PM
...I do NOT see that white wire going through the c100/firewall, but I'm sure it's in there. I'm going to connect the tach directly to the MSD 6a "tach signal" output, which currently is NOT being used. HOPEFULLY.....this brings the tach readings back into acceptable parameters. Next step would be a tach filter. 
