I have an '83 TA with a new 305 in it. It feels like a new clutch was installed, because I only get resistance in the first half of the clutch pedal. However, I'm more concerned about my trans. The car cruises in 5th gear, 55mph, at 2500rpm. Isn't this a little high? Is my tach malfunctioning? Is the new engine suspect? Is there a new trans? Is this normal? Any help would be greatly appreciated. Once I get the thing painted, I'll post pics!

 

What rear gear do you have and how tall are your tires?

 

Check your factory "trim package" thing that looks kind of like a tach against an actual instrument of known accuracy. I'll bet it reads 15-20% high.

 

The tires are near bald in the back, but new in the front. I didn't put on larger tires if that's what you're asking.

How can I check the rear gear? I know that it's either a 3.23 or 3.73 from the spec list on the site. 1G2AW87HXDL225340 is the VIN, but the decoder doesn't show me anything about gearing.

On the factory "trim package", where can I find that at? Or are you saying the tach is part of it?

Sorry for all the questions, I just purchased this TA and it didn't come with a manual .

 

You're right, the VIN does not speocifically contain gearing info. However it can often be inferred from the engine package. The 8th digit of your VIN is "H", which means you have the LG4, the base modle lowest-perf V8 for that year, which means that most likely you have 3.23s (or at least your car came that way). 3.73s would have been standard with the L69, VIN code "7" (like mine) in 83.

With 3.73s and a 5-speed and the stock tires, 55 mph should be around 2150-2200 RPM. With 3.23s, it should be around 1850 RPM.

Your factory "tach" is not an instrument, it's a "trim package". The factory did not intend for it to be an instrument, it's just eye candy, to help make it "look like" a performance car. They are notoriously inaccurate. They almost always read high when they fail, and nearly all of them read high to some extent; the one I have in my car now is the most accurate one out of all the ones I have, and it's only about 3% high.

 

watch out
it happened to me when i swap the crappy v6 to V8, the electronic tac was reading higher (made for v6) and there was nothing i could do besides, replacing it with a V8 tach or installing an aftermarket one.
one thing, it read about 25% higher (about the same as yours) so are you sure it is not an original v6 car
Fernando.

 

I'm 90% sure that it didn't have a V6 in it before. I checked the VIN and it show's it was a 305 V8 (non-HO w/carb). The tach also isn't electronic. It's analog. There is a new V8 in it though, a 305 V8, but it's the GM Goodwrench 305 if that means anything.

 

I missed the trim package reply. If my tach is inaccurate, is there some way to recalibrate it?

On the VIN id, I hate to say it, but I could also have a 3.73 gearing. Manual option G92 would put a 3.73 in it. If the tach is that off, I can't really get a good reading. If the trim package is 25% off, then I'd be around 1850rpm and reading 2350rpm. However, if the gear is a 3.73, then it's closer to accurate.

Would mileage have any affect on how "off" the tach is? The car has 48k in mileage and sat in a garage since '96.

 

Yes it can be re-calibrated, but it's not something the average car owner can do.

Very, very, very few of those cars actually came with any gears other than the std gears. Not that I would know a whole lot about these cars though, I've only been driving Z28s since the 70s, and spent alot of time on car lots when these cars (83 models) were brand-new, looking for one I would be willing to bite off on. When I bought mine back then, I didn't even see any LG4 (or LU5) cars with any gears other than the standard ones, but truth to tell I didn't really pay much attention to those, because it took me about 2 minutes to come to the conclusion that I had no use for one of those, rather I wanted a HO car. However it's been 20 years since then, and alot can be changed about a car in 20 years, including the gears (or the whole rear). Still in all, I seriously doubt that your car has gears that would produce that kind of real RPM; the fact is, your tach is off, it's just a question of how far off.

With 3.73s it should be 2150-2200 RPM at 55, not 2350. Trust me, I know, I have 3.73s and a T-5.

Mileage has little or nothing to do with the tach error. Age may; however luck of the draw seems to be the major factor.

 

Thanks for the help!

If recalibrating is difficult, where would I have to take it? Would it even be something worth doing?

If the tach is off, couldn't we find out how far it's off with an ideal idle RPM to the actual idle RPM?

The 3.73 gearing reply was extreme, meaining if the tach was way (25%) off, that it would be close to an accurate 3.73 instead of the actual 3.23 speed. 20% and it'd be real close to a 3.73 impersonator.

 

The thing that fails is a little surface-mount resistor that's on the circuit board mounted on the back of the meter movement. It's one of those laser-trimmed things that the factory often uses in such applications. IIRC they used a 50 µA meter movement along with the little circuit thing, nothing particularly special. But if you don't have a way to unsolder and replace surface-mount devices, then there's no real way to do it. As far as creating an accurate signal, a square-wave generator and a frequency counter is entirely adequate, which is how I do it, rather than using another tach and the engine. That's just too much like work, and I hate work.

There's billions of those things in the boneyards though; simplest thing to do is just go hork a handful of them and pick the one that's most accurate and put it in your dash. It takes like 10 minutes tops to swap the circuit board out.

Idle is only 10% of full scale. That's not where you'd want to test a meter for calibration accuracy. Check it at 2000, 3000, 4000 RPM, where you actually use it.

Incidentally I used to be a tech in a NIST-traceable test eqpt lab long long long ago, so things like this are sort of trivial to me (I even have the test equipment). I understand that not everyone has access to this sort of thing though.

 

You lost me at "square wave generator and frequency counter". I know of those, but I don't know how I'd implement them in the use of accuracy. Unless that's what you're talking about being too much work.

Is a soldering iron too big for this kind of replacement on my actual board? Because that's something I might be able to have done. Otherwise, it's another trip to the junkyard!

Any special cars I'm going after? 3rd gens that had LG4s or just any car?

 

I've got a tach from an 82 T/A that seemed pretty accurate. You can have it for $22.50 shipped.

 

Is it the tach or the circuit board that is suspect though?

 

Probably the tach since there is no real circuit board.

 

I think there is some circuit board. Older cars are difficult to figure out because everything's so mechanical with a little electronics thrown in for good measure.

 

There is a circuit board on the tach itself. I'll take a quick pic of the one I have. Hold on a sec.

 

 

Seems nice but 22.50 seems a little pricy. The electronics don't seem too hard to unsolder either. I'll keep you in touch and when I replace it, report on the change.

 

Well, feel free to make a reasonable offer. You can have the other gauges as well, except for the speedo, it's a 155mph speedo and my friends wants to save it.

It is a nice piece too,

 

I'll pm you about it - I'd like to keep the thread focused on high RPMs in 5th gear, and the possible (and probably found) issues on them.