19IROC87

i have a 87 305 IROC-Z and i put a 5" auto meter tach in it but i dont know what wire to tap into on the ignition coil ther are 2 white and 2 pink wires, and i know it needs to go to the negitive but i dont know what wire needs to be taped into ????

Benm109

You are going to want to tap into the white wire that comes out of the gray coil connector and runs down to a wiring harness behind the motor (and eventually to the factory tach). The other white wire should come out of the black coil connector and just run from the ignition coil to the ICM on the back of the distributor, do not tap into this wire for your tach signal.

Purple 92 SS

I did this, but .. heres the but..

the tach, autometer, is reading about 500 or so rpm lower than the dash tach.. which I had thought was accurate prior to putting in the Autometer. However, I know I have issues with my volt meter and suspect my temp guage.... so how do i know what my tachs are reading and which is correct?

-steve

Benm109

What are the 2 tachs actually reading? I know you said the Autometer is reading 500 too low, but what is the Autometer showing at idle and what is the stock tach showing at idle? I'm more prone to believe that the Autometer tach is correct and that the stock tach is going bad and reading too high. Us 90-92 Camaro guys are plagued with this problem, but I think you already know that.

If this is your 305 TBI, it should idle at 600-650 when warm, for reference.

Purple 92 SS

well yea, im aware of the 91/92 issues too, and coincidently i have another tach for the purple car as well to put in it. Just havent done it yet though.

its on my 89, that ive installed this tach.. the car at idle is anywhere from 500-600rpm.. on the autometer and maybe 800 or so on the stock.. at most..

at 70 mph i compared the two today.
its a l03/a4 with 2.73's and this is what it read.

Dash 2500 rpm
Autometer 1800 rpm

so which is correct? hell if I know.. I know that it was under constant throttle, and I had the speedo at 70 on the nose long enough to make sure none of the 3 needles were fluctuating.. so thats about as accurate of an assesment as i can get under throttle. I'll try to double check the idle when warm levels later this evening.

thanks.
-steve

Trickster

If you want to test the tach for accuracy there are a couple of ways of doing it. 1. buy or borrow an engine analyzer that checks voltage, amperage, and rpm. Sears and several parts stores carry them rather inexpesively. 2. get a scan tool and hook it up to the ALDl, start the car and let go into closed loop mode. set the tool to read engine RPM's. Slowly press the gas pedal and compare the reading on the scan tool and your gages.

PhLaXuS

Just for reference, my 91 RS tach reads about 100 RPM more than the engine RPM at idle and about 500 RPM more than the engine when higher than 2000 RPM. The alternate RPM readings were taken from WinALDL.

crazymex83

What size tires are you running on your '89? You could check and see what the ideal RPM would be at 70mph that way to see what they should both be reading. Assuming that you are running 245/50/16 size tires and with the 2.73 rear gear, 0.70:1 4th gear ratio, you should be running about 1759RPM at 70MPH. So I would say that your Autometer tach is more accurate then your factory tach.

Regardless, I'm interested to see what you find out.

Trickster

You also forgot to mention that if he has changed to a different size tire that his speedometer would also be off.

Purple 92 SS

well unfortunately i have the cheapo rims on it.. so theyrer the old factory 15's ill check the tire size when I go to lunch today.. unfortunately been out of town until today so i havent had time to cehck or be on here

As for the gearing, Iknow its 273's and a4.. so those are definates.

The tires I'd assume to be stock replacements for teh 15 inch rims.

as for the idle when warm.. its about 500-600rpm on the autometer and about 700-800 or so on factory tach if I remember correctly.

-steve

Purple 92 SS

Tires are 215/65/15's...

ugh.

-steve

crazymex83

The 215/65/15 is a 26" diameter tire so with your 2.73 gear and 0.70:1 overdrive you still should be around 1728RPM at 70mph. So your Autometer Tach is more accurate.

crazymex83

actually, now that I think about it, did the transmission shift into overdrive before you looked at your autometer tach? If you looked at your factory tach first while the trans was in third gear and read 2500rpmit would be correct. If the trans then shifted into overdrive before you looked at your autometer tach and read 1800rpm they would both be correct.

At 70mph in third gear with your trans and gears you should read 2469rpm. In 4th gear you would read 1728rpm. If this is true you may want to confirm this info with another drive.

trkklr77

this may seem stupid and elimentry, but did you chech the tach to see if it was switched in to the 8 cyl mode. they have a small plug in the back, in it will be 2 switches. both on one side is 4cyl, one up one down is 6cyl, both on the other side is 8cyl.




my old pick up with 31" tires, 2.73 gears, 700r4 would read 1800 at 70ish

Purple 92 SS

crazymex, thanks for the info, Im pretty sure it was in 4th already, but ill double check today when i go on a 45 min trip for work. (yes, i do drive her for a daily driver and for my work trips)


Trkklr,

The tach that I have (autometer) has 2 loops of wire on the back of it, and according to the instructions for 8 cyl use, you are not supposed to cut either loop. Therefore, I didnt. Otherwise for 4 cyl use, you cut one colored loop and for 6 you cut another colored loop. But like I said I didnt cut either.

thanks for the input guys.

-steve

trkklr77

okay, i was told they didnt do it that way anymore. good enough

Purple 92 SS

ok, thanks.

Also, I checked the car at 70mph again..

as close as i can tell at 70 mph the factory tach reads about 2250, and the autometer right at 1800 or so rpm..

this is in fourth gear.
(od)

thanks
-steve

PhLaXuS

Calculations you need:

https://www.thirdgen.org/newdesign/t...ulations.shtml

Purple 92 SS

well it turns out the autometer is more accurate by about 5-600 rpm at any given time. Thanks for all your help.

I recently put one in my buddys 91, and he's in pure heaven now he knows he's idling at 600 rpm instead of 1800.

-steve

Eats5.0's

iTrader: (1)

It's not just you 90-92 guys,,, us 88 guys have them too. Totally unreliable.