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Thread: Digital Dash Project Modification -- Redoing the Tach

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    Default Digital Dash Project Modification -- Redoing the Tach

    Some of you may remember my digital dash project that I posted about a year and a half ago. With this project, I made a digital dash for my ZX2 that uses programmable digital tach modules for the speedometer and the tach. I wanted to avoid tapping into the car's systems as much as I could to avoid any computer issues, so for the tach display, I used a DEI 454t tach-signal generator module, and it has worked quite well... until now.

    Recently, while driving, I have noticed that the tach occasionally reads about 200-300 RPM lower than it should. Then, it goes back to normal. It's not that the engine is actually running slower -- it's just the readout. I'm not sure why -- maybe the DEI 454t is going bad. In fact, I have also noticed the tach display occasionally reading a low RPM, such as 100 RPM -- with the ENGINE OFF! Again, maybe the 454t is going bad. The 454t picks up pulses from the vehicles electrical system to simulate a tach signal, so with the engine off, and the alternator not running, I don't see why there would be any pulses.

    So, I figure its now time to actually try to tap into a tach wire in the vehicle. I used information from this forum to find a tach wire at the coil pack, and I figured I would be able to just rewire my digital dash tach wire to this coil pack tach wire, and then recalibrate the tach display module in my dash project. For safety, I even put a 3300 ohm resistor in line with the wire that I'm tapping into at the coil pack. Easy, right? Not quite...

    The first WTF came when I hooked up that wire and started my car. Not only was I getting a reading on the tach, but also on my digital SPEEDOMETER! And yes, the car was parked and not moving. Now, the sensor for my digital speedometer is completely isolated from the car's systems, except for power and ground. I use a magnetic reed switch mounted near a drive shaft, and I mounted a magnet to the drive shaft. The reed switch is hooked to 12 volts, and the magnet whizzing past the reed switch as the drive shaft spins sends 12 volt pulses to my speedometer module. So, what the &*$! was causing a speed reading?! I checked my wiring and found no evidence of any bad wiring or "cross talk" between my wires. Finally, I figured I'd put a bigger resistor in line from the coil pack wire to my tach display module -- I upped it from 3300 ohms to 330 k-ohms (330,000 ohms). Fortunately, that took care of the phantom speedometer readings. I guess the pulses from the coil pack wire were so strong that they were resonating throughout my whole digital dash!

    So now, attention turns to the tach readout. It was somewhat proportional to engine RPM, but "jumpy", meaning that, even when the engine was running at a constant speed, the RPM readout would jump up and down within a range of a few hundred RPM. Now, I had a very similar problem with the speedometer when I first built this project, and I accidentally discovered that putting a high-value resistance parallel to the reed switch that I mentioned above cured the problem. So, I figured that doing something similar would work here.

    So, after experimentation, I determined that it appears that there are positive pulses coming from that coil pack wire (as opposed to ground pulses). So, again, after experimentation, I put a diode in line with the wire coming from the coil pack (in addition to the 330 k-ohm resistor) to make sure that, not only that just positive pulses get through to my tach, but also that any positive electricity that I deal with at the tach won't back-feed to the coil pack. And then, I put a high-value resistance between battery+ and the wire coming from the coil pack that goes to my tach -- making the connection after the 330-k ohm resistor and diode, of course.

    This seems to be working somewhat. When driving, the tach reading is pretty steady. However, at idle, it gets jumpy and inaccurate. So, I still have more work to do. Right now, my theory is that maybe the 330 k-ohm resistor in line with the wire from the coil pack is a bit too large, so that the signal that gets to the tach is just not strong enough at lower RPM's. So I'm thinking that the next step is to try a smaller resistor there -- maybe in the range of about 100 K-ohm -- to see what happens.

    So, that is where I'm at with my digital tach. If anyone here is familiar with the wiring of the car and the coil pack, and has more information about the signal coming from that wire at the coil pack and has any suggestions, I am "all ears"!
    And when I came to, I had a VISION! A REVELATION! A PICTURE IN MY HEAD! A picture of THIS! The FLUX CAPACITOR!

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    Could you piggy back a signal off of the crank position sensor? It's a 32-1 configuration using the back of the flywheel. What about using a hall effect switch/sensor that triggers off an existing hole or notch on the crankshaft pulley? Or an optical switch and a piece of foil tape on crankshaft pulley. Or if you can program the pulse rate, maybe put a hall sensor near the cam sprocket and configure for #of teeth

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    Those also sound like good ideas. But for a Hall-effect sensor, doesn't that require mounting a magnet on the moving surface? Or can it just simply detect changes in position of the metal as it moves by it, like detecting the teeth of a gear whizzing by without actually having to touch the gear or have anything attached to the gear? If so, I guess it works like the pick-ups of an electric guitar, which respond to the motion of the guitar strings.

    I've thought about the foil solution, as well. But it seems that would be prone to wear. If I put a piece of reflective tape on a pulley, I would think it would wear quickly due to the belt rubbing against it. And mounting a light and photocell (to pick up the reflections) could also be difficult.

    I just bought a fistful of parts at Skycraft, my local parts and surplus store, and I'm hoping I can perfect what I've started. If not, I may go to a "Plan B."

    Speaking of Skycraft, it looks like that is one of my few options now for parts in my area, other than ordering online. Just about all of the Radio Shacks have closed up shop here. Sad to see. It was always nice having them nearby to pick up some resistors, LED's, project boxes, and the like without having to go to Skycraft (which is a great store but is closed Sundays and only open to 6:00pm M-F and 5:00pm Saturday) or having to wait for an online delivery. My Dad would take me there when I was a kid for me to pick up parts for whatever I was building at the time or to check out the newest electronic products, which at the time often involved CB radios, to give you an idea of how long ago that was!

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    Depending on type of hall effect it may or may not need a flying magnet. Crank and cam position sensor have built in magnet and when it's close to a surface that has teeth, those teeth change the magnetic field and cause the hall sensor inside to send a signal. Other kinds are activated when exposed to a magnetic field.

    If the reflective tape were put on the face of the pulley, front or back, the belt would not ride on it. I have done this using a handheld IR tach. The tape is still on the pulley after about 7 years

    1 running Ford out of 5 isn't bad, right?

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    Okay, so here's an update. I bought a neat little project helper called a breadboard -- it's just a little circuit board about 1.5" x 2" that has multiple holes in it, with each hole being a contact, and rows of contacts hooked together. It lets you design basic circuits by placing the leads of components (like resistors and diodes) into the contact holes, and adding jumper wires where needed, so that you can easily test the circuit, and then remove components and pop in different ones (such as testing resistors of different values), to "fine tune" a circuit. Once I am happy with the way it works, then I can make a more permanent circuit using a PC board and by soldering the connections.

    So, I think I got it set pretty good now, though I still have to drive it around some more. However, it still doesn't read accurately at idle. It seems as long as I'm giving the throttle even a little bit of gas, it works well, but at idle, it reads high and the values jump around. Then again, even the readings when using the DEI 454t seemed high at idle, at least when first starting the car.

    Another important thing. That tach wire coming from the coil -- the one I found by reading remote start notes on this site -- seems to have quite a bit more than 12 volts coming from it! When touching that bare wire, I can feel mild electrical shock/pulses, and I don't think that would be the case with just 12 volts. That must be why that even my speedometer was picking up pulses resonating through the dashboard (and I'm talking about my digital speedometer in the dashboard I have made) when I first tested that wire. And I first tested it through a 3300 ohm resistor! It's a good thing that was there, or else I would've probably fried my tach display module. Now, I have the tach signal going through a 150,000 ohm resistor, as well as a diode.

    If this doesn't work well enough, I may very well try an optical solution. I see a pulley where I may be able to attach some foil tape, and perhaps doing that, and shining a light on it and picking up the reflections with a photocell, may work. Alternately, I also wonder if a fuel injector wire may work.

    EDIT:

    I did some research, and I found that a photocell may not respond quickly enough for this purpose. And I also found that in some vehicles, fuel injectors can be triggered in different patterns, depending on things such as the load on the engine, which would mean that using a fuel injector wire for this purpose could produce erratic readings. Does anyone know if that is the case with the ZX2? In other words, with the ZX2, do the fuel injectors always trigger proportionally to RPM, or is it the case that the rate of trigging of the injectors can vary with engine load or other conditions?
    Last edited by Excelsior; 08-07-2017 at 01:29 AM.

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    Do you have a picture of what it looks like in the dash?

    John

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    Check out this thread...

    http://teamzx2.com/threads/44057-Digital-Dash-Update

    Post Numbers 12 and 26. Those show the cluster installed in my car. The speedometer is in the middle, toward the top of the cluster, and the tach is on the right. Those posts also show a few other goodies in the car !
    Last edited by Excelsior; 08-08-2017 at 01:02 AM.

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    I'm not sure if I love it or I hate it! Definitely a cool project for those that like that look. Just not my taste but cool nonetheless.

    John

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    Yep, it's definitely not for everyone. Back in 1982, when I saw the original Knight Rider show for the first time, I would say that it literally changed my life. At the time, I did not have a significant interest in cars. But once I saw the dash on KITT, the Knight Rider car, I became inspired to try to make the interior of my cars as close to that car as I could. I've been doing that for 35 years. And it's very possible that I would not have been a professional car stereo installer for 12 years of my life had I not developed that initial interest.

    As for the tach, I am waving the white flag. I cannot get this system to work reliably. I thought I had it designed pretty well, but during the day, when driving 35-50 mph, the tach display reading just keeps jumping all over the place, and it's not related to dramatic changes in actual engine RPM. That wire I've been using just does not appear to be a good wire for a tach. Either stray pulses are getting in there, or maybe the actual voltage of the pulses from that wire changes under different conditions, which would mean that the values of the resistors I'm using would also have to change to make the display work properly. So, I just ordered a new DEI 454t from Amazon, and hopefully that will work.

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    Okay, here's an update. I ordered a new DEI 454t, and finally got a chance to wire it up. I haven't soldered the connections yet, but I've got it buttoned up enough to test it out, and so far, it's working! I tried something different. The instructions indicate to mount the unit on a power cable close to the alternator, but the wires coming from the alternator are a bit difficult to get to (at least on my car), so with the original unit, I mounted it on the (+) battery cable, and it worked fine for a year and a half until recently. This time, I have it mounted it to part of the intake manifold, right above the alternator. I put some foam insulation in between the intake manifold tube and the 454t to help damp some of the vibrations it could receive. Since it is not right on a wire/cable, I had to wire the unit to what I believe is its maximum sensitivity (to adjust the sensitivity, you wire one wire to one of three other wires that come from the 454t -- the instructions imply but do not explicitly state which wire is for maximum sensitivity). But so far, it seems pretty accurate, and if fact may be even more accurate than before at idle.

    I do have one question, for anyone who may know. Is it the case that, for a 2002 ZX2 with an ATX, when driving 60 mph in top gear, overdrive gear with lock-up torque converter engaged, that the engine is spinning at 2400 RPM? Or, put another way, in top gear overdrive with lock-up torque converter engaged, would the engine RPM's be 40 per every 1 mph of speed? I thought I had seen this somewhere, but I haven't been able to find it. Just want to make sure I'm calibrating this bugger properly.

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