Factory tach mystery

[Eastonspeedworks]

So I have a rather odd issue with a factory tach/air pressure gauge I recently purchased. I previously had a sunpro tach I added into a stock tin that worked very well but didn't look quite factory. So tonight I installed the new dash everything went smoothly I got it all hooked up and everything was testing good until I started it to check the tach. It said it was running 500 rpm idle. I knew this wasn't right and the problem became more apparent as I revved the engine. The tachometer wasn't climbing like I was expecting so I hooked up my hp tuners software as its an ls engine and I confirmed my fear it was reading unproportional. Initially I thought I was sold a 6 cylinder tachometer not an 8 cylinder one. But looking at the numbers it's backwards the tachometer says 500 and the computer says 760 ish. The tachometer says 4500 and the computer says 6000. I did some research and found they can be calibrated with a screw on the backside but that seems to only change a set rpm. Anyone have any ideas? I'm a bit bummed out I spent an awful lot on this dash to go back to my old one

[randy500]

Let's see what others have to say...in my experience I have never thought the tachs were accurate at all. They seem very lazy, I have had dozens of them and thought that about most of them.
I tested 3 this week and one did seem more responsive than the others.

[custom10nut]

You should be able to get it closer by adjusting the screw on the back of your tach. It might not get perfect, but you should get close.

[Shinysideup]

Maybe the needle wasnt put back on in right position.

[Eastonspeedworks]

The trouble I have is the tachometer isn't just off 270 rpm at all ranges its off 1500+ at 6000 which doesn't work for me this thing rips and I don't want it stuck on the rev limiter all the time trying to guess what rpm I'm running.

[randy500]

Well that LS revs way higher than 5k so your gonna need an aux tach anyway...

[Eastonspeedworks]

That's a fair point I was hoping to shift it on the zero or the x at the bottom 😂

[randy500]

Originally Posted by Eastonspeedworks

That's a fair point I was hoping to shift it on the zero or the x at the bottom 😂

I guess that COULD have worked....

[demian5]

Originally Posted by custom10nut

You should be able to get it closer by adjusting the screw on the back of your tach. It might not get perfect, but you should get close.

Adjust it and see if its different through the range. Maybe the signal wire is not putting out true RPM?

[toolboxchev]

Originally Posted by randy500

Well that LS revs way higher than 5k so your gonna need an aux tach anyway...

Your using modern technology with a 50 year old tach. So my thought is the signal coming from the distributor is the real issue.

Or just where is the signal for the tach generated from?

[clay68c10]

I had to adjust the tach on a former truck of mine, and as I recall, it was off similar to yours. A little at idle, and more as the RPM increased. I adjusted it to be close at 2500 and that seemed to fix it at all RPMs. It was never REALLY accurate, but that's what the laptop is for.

[demian5]

Originally Posted by toolboxchev

So my thought is the signal coming from the distributor is the real issue.

Or just where is the signal for the tach generated from?

I thought the same thing, but he says the previous Sunpro Tach worked fine. Adjust and report back. Something is buffering the signal somewhere.

[Eastonspeedworks]

I should have a chance to try it out this weekend and see if the screw will tune it up. I'm wondering if it isn't a 6 cylinder tachometer though I seem to remember the signal from the ls computer is 4 cylinder but I don't remember switching the tachometer when I did the efi conversion. Guess I need to make a book on all my brilliant little ideas.

[AussieinNC]

I have seen this happen when the ECM was either setup to run the other tach within the software, or a voltage up resistor was added to the wiring to get the other tach to run on the reduced voltage output from the ECM...

Check in your tuner software to see if the Voltage Up setting in the ECM is set to On.

[Eastonspeedworks]

I did have to add a 10k resistor with power to the signal wire to boost the signal but never found any option to boost it in the software. Do I need to eliminate it for this tachometer?

[AussieinNC]

I usually use 1 K resistors...not 10K...

Your readings are approx 75% of actual...(using a rounding value...) so to me, I would suspect the tach itself is reporting too low...

I know you really want that original tach look, but, as randy500 pointed out, that LS will easily spin more than that...that 5000 will peak out real quick...

Its your choice...I believe under HP Tuner, you can set the tach resolution value from 6 to 4...then recheck the tach output value...changing it from 6 to 3 would double the output, but you dont need to double it. Try the software first...

[Eastonspeedworks]

That's a really good idea I had no idea what the tachometer resolution was for. I'm self taught on the hp tuners so I'm not the best. 10k is what I got when I did some research but I used a potentiometer so I could change the resistance if I needed to. I bet between that and the screw I can dial it in pretty good

[AussieinNC]

I would leave the resistor alone for now,,,

try the tach resolution value change,,,I believe it will get you much closer...

"tach resolution" is essentially a fancy name for a rate conversion factor value...

Have fun !

[AussieinNC]

BTW...LMC does a sweet 8 K full dash unit...38-7794...but it does run 7 Benjamins

http://www.lmctruck.com/icatalog/cb/full.aspx?Page=81

[Eastonspeedworks]

Thanks a ton for the help! Have yourself a good one

[AussieinNC]

No Worries Mate !

havagoodweegend

[Eastonspeedworks]

I don't want to change this one out as its an air pressure dash that's the entire reason I changed in hindsight I should have just put the gauge in my cluster but now that I built a complete weather pack harness I'm not to excited about doing that again

[clay68c10]

Is there a shift light output option for your ECU? I'm running EBL flash on a couple of my dinosaurs with TBI and that is an option I've used. Then you can hide a light, or repurpose one of the idiot lights in your cluster.

[Eastonspeedworks]

There is an option for a shift light but I don't have any cluster wiring with the aftermarket harness. I'd have to pin it out to find it but that would be a pretty cool idea to have an idiot light for a shift light I'll do a bit of research on that one thanks for the idea!

[Eastonspeedworks]

I owe a huge thanks to aussieinnc. That fixed the tach within 200 rpm through the whole sweep if it didn't require a whole number I'd have it perfect but its darn close anyways. I'll add a shift light like clay68c10 suggested when I replace the seat. All the wiring is underneath it. Thanks for everyone's help!