Skip to main content
Topic: The stock digital tach is apparently very inaccurate (Read 4400 times) previous topic - next topic

Re: The stock digital tach is apparently very inaccurate

Reply #30
Cool. Keep us posted.
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: The stock digital tach is apparently very inaccurate

Reply #31
 :smile:

Yay. It worked like a charm, see the attached pic. You'll have to zoom in but it's tracking perfectly with the Holley EFI dash. I free-revved it to the yellow and it was nice & smooth. Can't drive the car tonight because it's wet out there and I'd kill myself, but I was more than pleased to know my tach is calibrated now.  :headbang:  :roxor:  :headbang:
11.96 @ 118 MPH old 306 KB; 428W coming soon.

Re: The stock digital tach is apparently very inaccurate

Reply #32
That's awesome! It's accurate all the way to 5000 RPM correct (then the tach just turns red)?

Can you list the parts you used and a write up so I can duplicate this with my tach?
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

Re: The stock digital tach is apparently very inaccurate

Reply #33
That's awesome! It's accurate all the way to 5000 RPM correct (then the tach just turns red)?

Can you list the parts you used and a write up so I can duplicate this with my tach?

Sure, I bought this 5.355 MHz crystal from Mouser and replaced the one on the PCB (silver can shown in the picture below.) That's it.  :grinno:

Disassembly.

I'm guessing some know how to remove the tach module from the dash, but for those that don't:

Remove the clear plastic face from the dash (6 screws.) Flip the dash over and remove the 3 screws which hold the tach module to the dash, one is in the middle center (behind the tach) and two are at the top on either side of the illumination bubs for the tach. Next remove the screw holding the ribbon cable to the dash, then carefully separate the ribbon cable from the dash which is held in place by two white friction-fit posts. Now flip the dash again. Remove the Lamp Out and Check Engine light sockets (twist type) from the lower left face of the black bezel, then pull the ribbon cable off the two black friction posts. Lift the top of the bezel off the 3 white posts and note that there's a rubber pad at the bottom that fits over the steering column. This pad ties the top black bezel to the white dash body and can be rather annoying to separate from the bezel as just when you think it's going to go, you note there's a tongue on the bezel that interlocks with a slot in the dash body and this dumb rubber pad is stuck to that tongue. I probably should have just used a razor to make clean cuts to separate the two, but one way or the other you need to un-stick this very sticky rubber pad that likes to tear apart under stress.  :beatyoass: Once you can fully lift the bezel, you'll note that there are two multi-wire cables that connect to either side of the dash from the bezel. You'll need to disconnect the 5 pin one (only 4 wires) that connects to the tach (left side.) It's taped so you'll need to cut or remove the tape first. Once that cable is unplugged, fold the bezel back and grab the tach module and lift it. Note the ribbon cable will require you to 'slide right' to remove the tach module cleanly. Fold the bezel back onto the dash and put it somewhere safe.

Disassembling tach module and replacing crystal:

Firstly the back part of the plastic clam shell must be removed from the tach module.  This part stressed me the **** out and I'm assuming it'll be the same for others. There are two tabs that hold the (probably yellowing) back of the tach to the rest of it. Both tabs must be pushed in to then rotate the back piece out of the way. I used a screwdriver to push one side in (easy), then try not to pull it out to far while working on the other side. It takes patience and you may wonder if you're going to break the plastic tab off, but it'll even eventually pop off. Once the back is off you'll need to carefully slide the blue connectors on the ends of the kapton (brown/orange) ribbon cables from the right angle connectors. There's two per side. Try to slide them off evenly so the pins don't get bent. Once all 4 are removed, use your thumb and forefinger to hold the ribbon cables out of the way while you lift and remove the stacked printed circuit board assembly from the tach. Turn it over and you see this:



The final step before desoldering and replacing the crystal (oval shaped shiny metal can) is to separate the two printed circuit boards (PCBs) that are held together with little plastic stand-offs. Carefully wedge something between the PCBs where no components are and pop the boards apart. Start at the end away from the little white ribbon cable that connects the two boards together. The boards can then be "unfolded" being careful not to damage the ribbon cable connecting them.  Now you have access to both sides of the PCB so you can desolder and remove the old crystal, then solder the new one back in.

Reassembly is much easier, just be careful when finagling the ribbon cables back into place and don't forget to plug the 5-pin cable back together also. Might be a good time to clean up the sockets and replace some of the finicky dash bulbs. I recommend getting a can of CRC QD Electronic Cleaner and using it on all bare copper/tinned connections.

Oh, and try doing a post like this on short attention span Facebook  :giggle:

I did buy 10 crystals just in case I'm tasked with helping out someone who isn't comfortable doing this (for a fee, it's a lot of work to do as a favor.)
11.96 @ 118 MPH old 306 KB; 428W coming soon.

Re: The stock digital tach is apparently very inaccurate

Reply #34
Wow, this is so helpful! Definitely deserves a write-up on coolcats.net. Just for the sake of documentation. I have a spare (semi-broken) cluster that I may need to try this out before I go messing with the one in the car.
1986 Ford Thunderchicken, 5.0 AOD w/ Shift kit,  354,XXX miles. 1-Family owned. Original engine+trans.
8.8 Disc Rear w/ 3.73 Posi. CHE Control Arms. '04 Cobra brakes all around. 2000 Cobra R wheels. Tubular front LCA's. MM Steering Shaft. Unlocked Speedo, Lowering springs, Eibach sway bars front and rear. Ram air intake.

Re: The stock digital tach is apparently very inaccurate

Reply #35
So, I made a little video of the modified tach in action. I even free-revved it to >5k RPM so y'all could see. Seeing as it reads accurate now, I had to really stab it hard to get it to kiss 5k RPM.

https://youtu.be/PWwLk0xy6g8
11.96 @ 118 MPH old 306 KB; 428W coming soon.

Re: The stock digital tach is apparently very inaccurate

Reply #36
Thank you for going through the trouble of writing all this up and testing everything.  :bowdown: You've gone above and beyond.

I'm going to do this modification in the spring when the car comes back out. It's cheap and pretty easy to boot. It's too bad that the tach still just goes solid red above 5000 rpm. If that could be fixed it would be perfect. I couldn't even begin figuring out how to change that.
88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, bunch of other stuff.

 

Re: The stock digital tach is apparently very inaccurate

Reply #37
Wow. Great work, above & beyond for sure.
It always amazes me the amount of knowledge on this forum. Thank you sir.
87 Tbird LX w/Factory floor shifter:D  3G upgrade. Tinted Windows...85 Mustang GT steering wheel(non-cruise) 17'' Saleen SC style wheels,Front/Rear TC sway bars/poly bushings & Mustang GT steering rack...'05 Mustang V6 springs...93 Cobra MC & booster, MM adjustable C/C plates,  Work  In  Progress.......  ( On The Shelf---HO computer, 19lb injectors, HO cam, BBK headers, Explorer Intake, Cold Air Intake ,Phantom Gauges, Stinger stainless exhaust pipes )