ok, I put a new tach in my car today. It's a V8 tach, I got tired of my V6 tach reading 4000 RPM on the highway. I know a V8 tach will read too low, but since these things get higher with age, I'm wondering just how far off it is?

Right now, according to the tach, my car is idling at 750RPM when warm. Haven't taken it out on the highway with the new tach yet though

If you have a 5 speed, please tell me what your car idles at at 70MPH and warm idle

thanks

 

a v8 tach will be way the hell of, take it from me(I have one on my car atm) and it actually reads high not low, I redline that tach rediculously fast, which sucks cause I have no idea when I'm hitting actual redline...without ever having had an accurate tach to go by I can't go by engine sound either, because I simply don't know what the engine sounds like at redline, I have an auto, but its still annoying because I can't manual shift it, and I bought the cluster specifically for the tach, but the seller was an idiot and told me it was a v6 one, and his only solution was shipping it back at my expense....



All that being said I believe there are shops that can readjust one to read like it should by swapping in the correct resistors, but idk how much it costs, and the v8 tach doesn't quite go as high as I wish it did.

 

so far it reads low, about 33% low, which is what I heard a V8 tach is supposed to in a V6.

it's not high, that's for sure. I drove to the store, and never hit 3000, even when I pushed it.

Sounds like you have one that has the effects of age, they always read high when old

 

Oh and to make matters worse because my trans leaks it has issues getting into 2nd gear, would sure be nice to know what rpm it gets up to when it starts sounding dangerously high, I had to calc what mph 1st gear tops out at for redline so I don't over rev the engine...bleh

 

if I can determine that this V8 tach is working exactly like it's supposed to, I would trade it for a working V6 tach is one came up, or sell it to pay for one

I can imagine a "testing working" tach is worth a nice penny to somebody

 

750 RPM is just about right for idle, depending on the speed screw setting. If you have a 3.42 rear gear, you should be running about 2400 at 70 (2360 or something like that... have to look that up). On the TGO home page, go to the Tech Articles section and there should be a link to the Tech Equations where you can figure this stuff out for yourself if you have a different gear in the rear. You need your tire diameter as well.

Here: https://www.thirdgen.org/calculations

 

It said about 1700 @ 70 this morning
It reads low, not sure which is better now, low or high

Wish i could find a good v6 tach

 

At least yours is in the ballpark, cringing while your transmission tries to find 2nd and you know everyone is staring at you cause your engine is built and loud as hell damn near redlining while you're topping out at like 38 mph is not fun.....

 

If you search the forum, there is a way to make your tach work. Something about adding a resistor. I know you can do it to the V6 tach to use it in a V8 car, so you should be able to do something likewise to reverse the application.

Somebody here has done it, I am sure.

 

Oh, and my V6, 5 speed, 3.42 car is running just a hair under 2500 at 70 with 245/50-16 tires.

Idles near 750 when it wants to...

 

58mark Idk if you have access to the crap to do this or not(I do not), but this is how to do it, would certainly be great if someone actually did it to post up the #s so other people with the same problem who don't have as many tools or as much electronic savviness could do it. Also idk what the capacitor needs swapped to...he kind of didn't answer that part.

There is circuitry in the dash near the tach.

It’s contained on a small plug-in PCB or integrated in the main board.

On or near the PCB you’ll see a capacitor, replace it.

Also on the PCB you’ll see a resistor chip.

Desolder pins 4 & 10.

Use a 500K potentiometer.

Solder it across traces 4 & 10.

Move the potentiometer to half way.

Start the engine and compare the tach reading with a test equipment quality tach.

Dial it in, unsolder the potentiometer, measure it and install a matching resistor.

 

according to this thread, that fix only works on the 90-02 tachs... please correct me if I'm wrong

https://www.thirdgen.org/forums/inte...meter-fix.html

 

if your engines "built" it should be tapping out at 38, just because its loud doesnt mean its "built". or if it is, its built wrong

 

It actually redlines at 44 mph in first if memory serves, but you don't really want to ride an engine right at redline for an extended period, my engine is pretty built(260 grind cam, ported heads and intake manifolds), and I have an exhaust that is damn near as loud as open headers, I have a gutted TPI snorkel intake, and I have no hood insulation and a hood with a cowl, so yes, its freaking loud, and yes its built, I never said that because its loud it must be built, I built the damn engine myself, and FYI yes, a built engine IS louder, whenever you have a free flowing air filter setup vs. factory stock ones that most cars have the engine gets a lot louder, not to mention have you ever noticed an engine is louder higher up in the powerband? Last but not least generally if someone goes to the trouble of building an engine they usually have the sensibility to add a good exhaust system, which 9/10x is also louder, yes you can have a free flowing exhaust that is quiet and makes more power, but for the vast majority, exhaust upgrades make the car louder.
pretty much anyone that knows jack crap starts with their air filter setup because its by far the easiest and cheapest way to gain a couple ponies, so yes built very often does mean loud, and no I am not some idiot who assumed because my car was loud that it must be built.

 

58mark, I think it applies in general, unless for whatever reason the earlier year models have a different setup on the back, I would have to pull my cluster to check..but atm I have bigger issues I am working on(that and I actually don't know the year car my cluster is from), the tach is on the list though, its driving me nuts, but honestly from what I have found, it looks like my best bet will be just getting an aftermarket one, I would rather not have one gauge not matching the others and don't plan on doing a full aftermarket cluster swap anytime soon, but oh well, what can I do...

 

I have to amend my previous figures, those were the old ones from the last (factory installed) transmission.

I had not recorded my numbers with my new trans; I did today because of this thread and the overdrive gear is different for me now.

My new transmission came from an 86 Trans Am. I have no way of knowing if that was the original trans that came with the car. Here is what I DO know:

4th gear, 55 mph, 2550 rpm (same as the last trans)
5th gear, 70 mph, 2000 rpm (the last trans was about 2450)

Obviously, my two transmissions have different overdrive ratios. My original trans may or may not have been original to the car. At 200k miles, who knows? I only know that it was the V6 trans because it had the 14 spline input. My new trans has the V8 input shaft and I had to put a custom clutch disk in to use it behind my V6. Other than that its the same.

I just threw that out there because I don't know the difference between years and overdrive ratios. If you are comparing tach readings, you should use 4th gear because presumably everybody is running 1 to 1 in that one, right?

Good luck!!

 

thanks, I got a new "?" tach in the mail yesterday.. hopefully this one will WORK

 

Might I ask where you found a v6 tach? They are hard to come by it seems..

 

a fellow tgo'er answered the call and sold me one on a car he was parting out... even sent me a video of it working so I could see that it was accurate before he shipped it

http://s149.photobucket.com/albums/s...1-11-01-49.mp4

 

put the new tach in today! I'm not sure if it's totally correct, but its sure the HECK closer than any thing I've had in this car

60 mph 4th gear 2900RPM
60 mph 5th gear 2250rpm
70 mph 5th gear 2600rpm