Stock tach that inaccurate?

jmuva44 · Apr 9, 2005

I just recently installed a 5" Autometer tach. Everything seems to work okay so far. However the 5" tach is reading about anywhere from like 500 to 1000 RPMs MORE than the stock guage. I know the stock gauges suck butt and are often off....i'm not new to that fact. But 1000 RPMs?????? I hooked the green tach wire up to the yellow/orange wire going into the MSD coil. I know it is negative cuz it says negative on the inside of the connection.

Any insight??

Thanks
Chris

*edit: removing dumb statement

poneypower89 · Apr 9, 2005

jmuva44 said:
I do have 3.73 gears and the speedometer is WAY off. This couldnt effect the stock tachometer could it???

I don't see how that would matter ... The tach moves when the car is out of gear, so it's not affiliated with speed.

willys1 · Apr 9, 2005

poneypower89 said:
I don't see how that would matter ... The tach moves when the car is out of gear, so it's not affiliated with speed.

Your tach works in and out of gear.Revelutions per minute(RPM) You speedo works off your tranny,has nothing to do with your tach!!

jmuva44 · Apr 9, 2005

Thanks....lol.....if i had actually thought about that before i said it, i would have realized that.

squeeeg · Apr 9, 2005

Ya it is off that bad. It is just about worthless for anything that incolves RPM's

HISSIN50 · Apr 9, 2005

yep, hit or miss. mine is within 100 rpm to ~3K rpm. but they are generally off by a percentage, so the higher the revs, the more the variance. good luck.

jmuva44 · Apr 10, 2005

Alright, just took the car out on a test drive. Two things are going on.

First, the tach only functions when the headlamps are on. Second, the tach light does dim with the dimmer switch but when dimmed, the gauge stops working and displays larger and larger values for each decrease in light with the dimmer switch. So basically, the tach only operates with the headlamps on and the gauge lights fully lit.

I have the black wire hooked up to the negative post on the battery.
The green wire is hooked up to the negative post on the MSD coil.
The red wire is currently and temporarily pushed into the #13 fuse.
The yellow wire is tapped into the blue/red wire going into the HVAC controls.

Any ideas on this now????

HISSIN50 · Apr 10, 2005

it has been a while since i hooked one up - are you sure you dont have the key-on power to the guage and the lighting hooked up backwards? it sounds like you might have the power to the tach hooked to the dimmer circuit, so the rheostat is messing with the reading. i cant remember the colors though.

so what happens to the tach if you disconnect the 'lighting wire'?

good luck.

5LugFoxFanatic · Apr 10, 2005

I bought a shift light/tach because I wanted to see how accurate mine was so here are some pics I took. Seems dead on. So Ford get a plus point for having a gauge in my 86 work.

View attachment 517824

View attachment 517825

Michael Yount · Apr 10, 2005

5lug - give us another picture, but this time up around 6 grand. Just because it's within 5% or so at 2 grand, doesn't mean it stays accurate across the range. Their accuracy is all over the board - well documented. Many cases of both the factory tach matching an aftermarket unit, and others where people saw 7 grand on the stock tach without having hit the ECU's 6250 rev limiter. Even if yours is accurate at higher revs too (good for you if it is - I like the look of the stock gauge package without an aftermarket add-on), it doesn't mean that they all are.

Seems like you have the tach's power wire hooked into the dimmer switch power - so when you dim the lights, reduced voltage is going to the power-side of the tach and screwing things up.

Check the red/yellow wire hookups more closely. Whichever one lights the tach should go to the variable voltage dimmer hot wire. Whichever one POWERS the tach, should go to a source off the fuse box (AFTER the fuse) that is hot when the ignition switch is in the 'on' position. Check this out -- http://hp.autometer.com/instructions/005899_instruction.pdf

PRO50SC · Apr 10, 2005

HISSIN50 said:
yep, hit or miss. mine is within 100 rpm to ~3K rpm. but they are generally off by a percentage, so the higher the revs, the more the variance. good luck.

Yup. I have that same problem.

04sleeper · Apr 10, 2005

What seems to happen is that the stock tach responds slower than the Autometer.

Every fox Mustang I have ever seen an aftermarket tach in has been this way. If you sit and hold the RPM constant, the stock tach will catch up to the aftermarket gauge. But if you rev the car up and down you will see the Autometer Tach will jump much faster than the stock tach, up and down.

So when you are racing the car, depending on how fast your car revs, it can be off due to the factory tach trying to play catch up. Every car will be a little different.

jmuva44 · Apr 10, 2005

First, i found my problem. I tapped into the wrong fuse for the power....duh.

Second, my stock tach is actually off by 1000 RPMs cuz I ran it in neutral to 2k, held it, 3k, held it and 4k and held it. The stock tach never "caught up". It stayed 500-1000 RPM behind depending on how high it was being revved.

Third, I do not have the MSD box as it is referred to. I do have the MSD coil which i tapped into. It seems to be working just fine tho.

bmorrisj · Apr 10, 2005

the dimmer wire is a white wire behind the a/c controls...

04sleeper · Apr 10, 2005

bmorrisj said:
the dimmer wire is a white wire behind the a/c controls...

Or the only 5 AMP fuse in the fuse panel.

jmuva44 · Apr 10, 2005

Weird....so today i take my car out and the stock tach is dead on with the 5" tach. Yesterday it was up to 1000 RPMs off. This of course is good, but still confuses me.

795.0pacecar · Apr 10, 2005

89sleeper said:
What seems to happen is that the stock tach responds slower than the Autometer.

Every fox Mustang I have ever seen an aftermarket tach in has been this way. If you sit and hold the RPM constant, the stock tach will catch up to the aftermarket gauge. But if you rev the car up and down you will see the Autometer Tach will jump much faster than the stock tach, up and down.

So when you are racing the car, depending on how fast your car revs, it can be off due to the factory tach trying to play catch up. Every car will be a little different.

Thats exactly how mine is.

Michael Yount · Apr 10, 2005

Them electrons is hard to corral and herd....

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