I have a sbc and using the holley hp ecu to run my coil near plug

I ran the tach signal wire (A28) to the stock tach.
The tach works only up to 3000 rpm, then it drops to zero.
I have an autometer shift light hooked to the same wire, and it works fine.

The holley produces a 12v square wave signal
My stock tach apparently needs a sine wave signal.

I called Autometer about part #9117, and it doesn't work with this application.

Anyone know how to fix this?

 

I guess the 91 tach is different than my 89? My 89 works fine with the holley hp

 

Is your tach the 6000 or the 8000 rpm unit?

 

Its neither lol its 7000 max

 

My Holley HP EFI delivers a square wave at system voltage (I've had it on oscilloscope). Works fine with my '89 Firebird tachometer after establishing good grounds in the gage module.

 

Do you know where the grounds are in the gauge module?

 

I've never seen a tach that requires a sine wave? That would be pretty weird. All tach's from the era in question use the same signal that drives the coil itself. And then when wasted spark and COP ignitions came on the scene the tach signal was generated by the ECM and the tach itself stayed essentially the same.

GD

 

I am using the tach wire from the Holley Hp, not the stock ecu.
So what did the stock 730 ecu from 1991 produce for a tach signal?
12v square?
5v square?
5v sine?

I found this
https://www.ebay.ca/itm/142171581596?ul_noapp=true
He claims this product would fix my issue

 

The stock ECM doesn't produce the tach signal. The signal is from the negative terminal of the (single) ignition coil.

I have an aftermarket ECM - complete stand alone. It is NOT configured to output a tach signal (though it is entirely capable) because the stock tach doesn't need one. The coil firing signal from the distributor module IS the tach signal.

GD

 

So what signal was produced from the single coil?
12v square?
5v square?

 

The coil is, of course, 12v. Actually it's system voltage so it could be anywhere from 9v to 15v or so. Same as every coil since they got rid of ballast resistors back before HEI came on the scene in the 70's.

GD

 

I was going to say what others have said. My Holley didn't run my 89 tach. It ran off the negative on the coil.

 

I'm confused. This is the responses I got about my problem.


This is what the ebay guy said
"It sounds like your tacho is an 18V type that will trigger at 12V when there's plenty of time for the input transistor to saturate, then dies when the pulses shorten at higher RPM. This booster should cure it."

This is what autometer said about part#9117
"Unfortunately, we do not have anything that will work good with the OE tach. All my devices will send out a square wave 4 pulse per revolution signal. Your gauge will be looking for a sine wave and unfortunately i do not have a good solution.
All our parts are designed for aftermarket gauges."

This is what MSD said about part #8913
"This part won't work for you. Maybe try Dakota Digital"

 

Yeah holley is a 4 pulse per rev signal i believe and the online support does mention some tach’s may need a Signal booster.

But im not sure what differences there would have been with gm tach’s over the years. I know my 89 ran fine with both the tach output from the 411 pcm and holley. Maybe gm changed something?

if it worked with a 4 pulse per rev single coil oem pickup then it should read the same signal from holley

 

https://forums.holley.com/showthread...om-Tach-for-GM

could try making a custom output

 

Okay I'm a little confused... I'm getting ready to wire in a Holley HP unit and for the tach I just figured all I had to do is leave the current tach wire from the coil alone which seems to be what some are saying but then post 14 and 15 go down a different route. The car is a 1987 IROC-Z

 

Might also be worth it to reach out to one of the tach gurus who is also a member here on this TGo forum.
Cajun Tach Supply is his company.
https://www.thirdgen.org/forums/member.php?u=120121

I have a different Holley sytem (sniper) but the tach he sold me works great and is super accurate with my setup.

not that you need to buy a new tach, but he might have some wisdom to sort out the issues

 

Are you going coil on/near plug with your holley?

 

LB9GTA, I was hoping to retain the stock cap and coil setup. Not sure how I would trigger the system on a stockish SBC without the pickup under the cap.

 

On a basic SBC,you should be able to use the original wire (white on my 89) to drive the tach.

 

So your wire from the coil would still go to the tach, as normal.

I have a crank trigger, no distributor(cam sync is in place now)

 

Okay thanks all, that makes sense.

 

So what is your holley going to manage then? You want it for the gauges? Is it going to control timing?

 

The expectation is it would run the fuel, spark, fans, TCC. I'm pretty sure all the other gauges in the dash and cruise control are separate from the ECM since I have an '87. Most of my other emissions pieces and A/C have been removed, I drive the car about 1000 miles a year and do auto-X stuff so I don't really worry about my carbon footprint with it... I've just been going down the worm hole doing research on this thing. The motor is out for a new oil pan and a head upgrade this winter and I'm considering buying the Holley HP to replace the stock ECM that runs a custom PROM. The existing tune works fine and my engine guy thinks I'm a little crazy for considering the change but I'm a little scared of 33 year old wires and I've always wanted to play around with a system I can tune vs just datalogging and getting mail order deals. I would like something I can drive out of the box but tune as I learn.

I haven't bought the Holley HP EFI yet, just planning and plotting... It does seem the wiring process won't be any harder whether the engine is in the car or not so I might just take my time as I research.

 

I'm jumping into this conversation...

I've been following this because I am doing something extremely similar.
I have an aftermarket ECU ( AEM Infinity) and Its now controlling my aftermarket coil (MSD coil near) and my aftermarket distributor (Holley Dual Sync).
So just like the others in this post I am trying to find what I need to do to connect my stock Tach back into this aftermarket ECU setup I did.

If I'm reading this thread correctly, it looks like youre saying that I need to connect a wire from the stock tach (87 GTA) and run it to the negative side of my coil (MSD)??
Which wire? The stock tach only requires 1 wire to function correctly?
Thanks in advance guys!

 

Try installing a 10Kohm resistor in series on the tach signal.

 

I have a similar setup (but with a LINK ExtremeX) and there's no need to change anything about the stock tach wiring. It runs off the coil negative just as it always did. The Dual Sync just gives it a cam signal - you are still running a single coil system.

Watch out for those AEM Infinity units. They have a lot of timing drift and virtually every one has had to go back to AEM (who has to send it to the real manufacturer) to get un-bricked at some point - they lock up frequently. AEM doesn't make them - their contractor (Engine Labs) kinda blows. Although the fact that AEM isn't making their own ECU's is either a good thing or a bad thing depending on who you talk to.

GD

 

Thanks!,
Yes I am still running a single coil setup. So just come off the negative side of that coil and go to which wire on the stock wiring harness? I cut the plug off when I swapped distributors so I need wire color to try and find it.