The stock digital tach is apparently very inaccurate

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The stock digital tach is apparently very inaccurate

Reply #15 – March 26, 2015, 12:42:13 PM

Quote from: White85GS;445826

as much as I like these cars more than the MN-12, I will say this, the ONE thing that I feel was better was the layout of the electronic cluster in them, oh, and the HO 5.0 :-D the 85-88 just has a kooky layout that I've never really liked.

Also, I forgot about the one that could be had in the Aerostar, looks a lot like the Fox one, doesn't it?

The left panel/tach looks spot on save for some color variances. I wonder if the center would work as well or if the pinouts on the ribbon in back are not the same....

The stock digital tach is apparently very inaccurate

Reply #16 – March 26, 2015, 01:10:23 PM

Quote from: V8Demon;446535

I must have a freak then. Mine's always been pretty spot on. My speedo gear is correct for my rear gear ratio/tire size combo and it shows what it should. I've verified the accuracy with a timing light as well. Most anyone I know who knows these cars that I've shown how accurate it is are shocked.

You're pretty lucky then. I've had reports from so many people about the inaccuracy of the digital gauges that it's pretty much a given that they're wrong most of the time LOL.

The stock digital tach is apparently very inaccurate

Reply #17 – March 26, 2015, 02:47:00 PM

Lou, you said your DASH tach is telling you 3000 rpm at 70 mph?
Whether I'm running 225-60s or 205-70s (have had both on the rear while on the highway at some point in time), I see 2600-2800 rpm at 80 mph, 3.73 gears, 3000 rpm non-lockup converter.

The math tells me this is what I should be getting too. Interesting stuff. Maybe you do have a V6 tach!

The stock digital tach is apparently very inaccurate

Reply #18 – March 26, 2015, 05:52:15 PM

Quote from: ZondaC12;446545

Lou, you said your DASH tach is telling you 3000 rpm at 70 mph?
Whether I'm running 225-60s or 205-70s (have had both on the rear while on the highway at some point in time), I see 2600-2800 rpm at 80 mph, 3.73 gears, 3000 rpm non-lockup converter.

The math tells me this is what I should be getting too. Interesting stuff. Maybe you do have a V6 tach!

We actually around 2800rpm :hick:. Yeah the stock tach is around 500-600rpm off above idle. With 225/55/16 tires and a non-lockup converter the engine is actually spinning around 2300rpm at 70mph, which the Autometer tach confirms. At idle (700rpm in park) the stock tach shows between 800-1000 rpm. The same thing is displayed in drive at idle (real reading 650rpm). I knew the stock tach was off at idle as I confirmed it with two different tach/dwell meters. The Automater tach I installed has an accuracy of +/-50rpm. I couldn't wrap my brain around how off the stock tach was until I installed the Autometer tach. 2000rpm stock tach, 1500 on the Autometer. 3000rpm stock tach, 2400 on the Autometer. 4000rpm stock tach, 3400 on the Autometer.

The stock tach has been costing me at the track. I have an epoxy mod valve body in my Thunderbird so when I drag race I shift it manually 1-2-3. I used to upshift at what I thought was 5500rpm. Turns out I was up shifting around 4800rpm, which is defiantly costing the car some trap speed at et.

The stock digital tach is apparently very inaccurate

Reply #19 – March 26, 2015, 05:56:12 PM

Yeah if you're looking to get everything you can at the track, good instrumentation is a must.

Re: The stock digital tach is apparently very inaccurate

Reply #20 – November 05, 2019, 02:00:16 PM

Quote from: thunderjet302 – on March 07, 2015, 06:59:59 PM

The stock digital tach is apparently very inaccurate. I recently bought an Autometer tach/shift light so I can shift my car manually at the track (it has an epoxy mod AOD). I hooked it up and set the Pulse Per Revoluation setting to 4 (what Autometer says a single coil V8 should be set to). I knew from previously using a tach/dwell meter on my car that the stock tach is around 200-300rpm off at idle. Now you can see it in the car.

It gets worse at higher RPM. According to the stock tach the car's cruise rpm at 70mph is 3000rpm. It's actually 2300-2400rpm. The yellow shift light is on as I set the shift light to come on at 2250rpm for testing.

It gets worse. At 4000rpm on the stock digital tach the Autometer tach is displaying 3400rpm. Above idle I observed the stock digital tach reading 500-700rpm high. The Autometer tach is being fed a signal from the ignition coil wire under the hood so it's getting a good signal. So the moral of the story is the stock digital tach is way off. Don't use it for any kind of performance driving.

Man oh man - I saw this thread and decided to dig into this. I used the electrical diagram book to find the I-O's for power and the tach signal and hooked up the module in my lab which I had removed from the spare electronic cluster I have. I used a signal generator to create a square wave and used my oscilloscope to measure the frequency of the signal into the tach. Apparently Ford thought keeping it simple was better than accurate? The thing is very precise, but it's (deliberately) off by 25%!

It's all very simple - 50Hz = 1000RPM on tach display, 100Hz = 2000 RPM 150Hz = 3000 RPM and so on. The math is simple:

50Hz * 60 (convert to cycles/min)/4 (power strokes/revolution) = 750 actual RPM - which agrees with the original post.

100Hz (2k RPM on tach) = 100*15 or 1500 RPM

150Hz (3k) = 150*15 or 2250 RPM

I will be poking around this spare tach (electronics shown below) to see if I can find a way to trim this thing so it's more accurate.

Re: The stock digital tach is apparently very inaccurate

Reply #21 – November 05, 2019, 06:30:51 PM

I completely forgot about this thread.

If you come up with something let us know. I'd love to mod the stock tach so it worked accurately, and ditch the Auto Meter one.

Re: The stock digital tach is apparently very inaccurate

Reply #22 – November 05, 2019, 07:30:33 PM

It's the V6 tach used also in Aerostar. They don't reconfigure anything when they decided to use it in our cars. That's why everyone finds it off 25% use one of these to correct it. https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=127/category_id=694/mode=prod/prd127.htm

Re: The stock digital tach is apparently very inaccurate

Reply #23 – November 06, 2019, 02:00:21 PM

Quote from: TheFoeYouKnow – on November 05, 2019, 07:30:33 PM

It's the V6 tach used also in Aerostar. They don't reconfigure anything when they decided to use it in our cars. That's why everyone finds it off 25% use one of these to correct it. https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=127/category_id=694/mode=prod/prd127.htm

But if it can be fixed for a couple bucks of parts soldered to the stock tach I'd go that route. I can solder to circuit boards easily enough, and I'm cheap

Re: The stock digital tach is apparently very inaccurate

Reply #24 – November 09, 2019, 03:54:51 PM

Hmmm, I may have a solution if the microprocessor will cooperate. The silver can in the above photo is a crystal oscillator. Sets up the timing for everything, including the sampling of the pulses from the negative of the coil (or whatever tach input you're using.) It's marked 4 MHz. I think it counts the number of clock cycles between spark plug fires, the less pulses counted (higher RPM as plugs are firing more frequently) the more bars you get on the display. I needed to think of it the other way, the more clock cycles counted between spark events, the less bars you'll see. So if we have more clock cycles for the same engine speed, it should mean a higher clock speed should show less bars at the same RPM, which is what we want. So, my guess was that if we increase the frequency of the clock to 5.333 MHz (it reads 250RPM high @750RPM = 250/750 or 33% error), it would then read accurate. I wanted to verify this before ordering a crystal. I have some crystals from the "crystal upgrade" thing that I was going to do to my A9L, so I tried sticking one in there. Went crazy, 18MHz must be beyond what it can sync to. I then realized that my function generator could run to above 2 MHz. I maxed it out at 2.16 MHz and then tapped it into the crystal location. I then realized I had no way to simulate the pulses from the ignition coil

Wait! I have this old tube audio frequency generator from like the 1960's! Now I'm definitely looking like a mad scientist.

I was hoping the input for the tach was robust enough to handle a 50 Hz AC sine wave. I tested it with the original 4 MHz crystal and it worked. Sweet. I then removed the OEM crystal and injected my 2.16 MHz signal from my function generator. It powered up fine, and then when I connected the 50 Hz signal, I saw 9 bars on the tach!

I sat down at my laptop and did the math - it should have read 9.2 bars, perfect. So, assuming the micro can sync up to the higher 5.355 MHz (closest frequency available) crystal that I'm ordering, it should read pretty much on the money.

The last experiment I did was to confirm that it reads continuously up to 5000 RPM (3rd yellow bar) with anything 5200 RPM or higher being the red bars all lit. In our cars presently, the red bars all illuminate at anything higher than about 4000 RPM.

I'll post back with results using the new oscillator crystal probably within a week since Mouser has both crystals that I am going to order (5 Mhz & 5.355 Mhz) in stock.

Re: The stock digital tach is apparently very inaccurate

Reply #25 – November 11, 2019, 10:10:26 AM

Wow!!! That is some impressive work right there.

Nice job. If it works Ill send you my tach.

Re: The stock digital tach is apparently very inaccurate

Reply #26 – November 11, 2019, 02:52:43 PM

Let us know how it works out when the crystal arrives. You've really gone above and beyond here.

Re: The stock digital tach is apparently very inaccurate

Reply #27 – November 12, 2019, 12:16:19 AM

Parts will be here Thursday. Unfortunately I won't due to a death in the family. I should be back by the weekend, however.

Re: The stock digital tach is apparently very inaccurate

Reply #28 – November 12, 2019, 03:17:09 PM

My condolences. No rush at all. My Thunderbird is put away for the season. Got 2" of snow here on Halloween and 3" yesterday. Car isn't moving out of the garage for awhile.

Re: The stock digital tach is apparently very inaccurate

Reply #29 – November 20, 2019, 04:31:51 PM

Well, I finally got home Monday night and my first chance to try to new oscillator was last night.

It worked as expected. The only issue I saw is now that it's sampling up to a higher frequency (5k RPM is now 333Hz vs 250Hz), it had some issues at higher RPM when the (positive) duty cycle was less than 20%. I couldn't really measure the actual duty cycle at the coil (engine running) with the cheap portable oscilloscopes I have, I'll just have to try it with the dash hooked up to the signal from the neg of the coil and see if it lights smoothly to redline. Will update assuming the weather cooperates enough to let me try this out in the next few days.