I have confirmed via gps that my speedometer is accurate to a T. However, my digital dash tach is way off. It must be. Warm idling at 2k, and when I'm cruising at 60mph... it shows 4,000 rpms... 1987 Trans am GTA factory digital dash car, with stock tpi 305 and a 700r4 odometer is only ahowing 67k miles. What should I check first? I would like the tach to be accurate. Everything is bone stock on the car outside of headers.

 

I spy that your 1987 GTA has the digital dash with a tach that has a 5000 RPM yellowline and a 5500 RPM redline. Your GTA being a LB9/MX0/MD8 car should have the "Peanut" roller camshaft (same roller camshaft in an 87 LG4 engine and 88+ L03 engines) version of the LB9 engine that runs out of breath at 4000 RPM and comes with a tach having a 4500 RPM yellow line and a 5000 RPM redline.

It's highly likely the digital dash in your car had some sort of fault at some time in its past and was swapped with another one from a car with a LB9/MM5/ M39 or MK6 5 speed manual transmission or B2L/L98/MX0/MDB car which has an engine with the "Hot" roller camshaft that runs out of breath between 4400 RPM to 4600 RPM.

The true mileage of your GTA would be unknown unless whoever swapped it found one with a very close mileage or was able to reprogram the chip with the actual mileage the car had before swapping the digital dash into it like the guys below have figured out. Probably not likely they did though.

Also with the upgraded tach in there now you just can't leave that little "Peanut" camshaft in there. LoL

On topic part of my reply below.

This is a great thread about the Digital Dash cars. The guy that started it figured out how to hack it and flash the chips to correct the odometer and fix a few things then another guy toward the bottom of the thread figured out a bunch of other stuff and designed and built his own circuit board to bench test the digital dashes with. This is a super cool thread below if you haven't checked it out yet.

Hacking into the 85-88 Firebird Trans Am / GTA Digital Cluster & Driver Information Center - Third Generation F-Body Message Boards

I edited the title in my link above to what the thread should be named now since it has morphed into something new.

 

That actually makes ALOT of sense... I imagine that finding a proper digital dash would be near impossible, however I do have plans to possibly 6 speed swap the car in the future, maybe a 5 speed.... Is it possible that someone could tune the prom to get a proper reading? Like If I crack open the diff to get gearing, tire size, etc...

 

honestly, Id start with looking at a schematic and seeing where the tach signal is coming from and going to exactly, then make sure you have the rights parts for a v8car before swapping clusters. I don't think a "tune" will change your tach readout. I have a 88 GTA with digital dash with a 5 speed and 2.77 rear gears. The transmission was changed out some point in its life speedo never read right. someone had put a v6 trans in place of the v8 trans, different input shafts. anyways it now has a t56 and the tach reads fine. I had another 88 GTA with digital dash with a 350, someone did a auto to manual swap and I know that digital dash was swapped around between other digital dash's. it still maintained correct rpm. so just because one digi dash has a lower redline than the other, I don't understand or see why that would affect actual rpm readout on the tach. typically the tach signal comes off the distributor and then goes to ecm and gauge. If you had a scan tool that hooks up to obd1 you could see what the ecm is reading for RPMs. that would help with troubleshooting. If the ecm is seeing the same rpm readout as your tach, then that might rule out a faulty tach. Dunno, just spit balling ideas while on lunch break. DO NOT take my word as gospel. I'm human.

 

Unfortunately I only have a newer scanner and i don't think it reads obd1.

My plan is to manual swap it, with an LT1 T56 if i can hands on one at a decent price. You're only 2 hours away in Jacksonville, let me know if you know about a deal! may have goods to trade too if you got something yourself. I would need everything for an auto to manual swap.

 

Isn't it possible - however likely - that the original odometer chip was removed from the original cluster and installed into the replacement cluster ??



I know the Berlinetta odometer chip is similar - not the same - and I seem to remember the GTA chip could also be swapped like the Berlinetta chip.


Wasn't that WHITE socket bulb "special" somehow too ??