VDO Tach Upgrade, replacement for internal electronics Options

[914GT]

I'd like your comments about a project I'm working on. I have made a single integrated circuit design that replaces the old-technology internal VDO tach circuit. This is a more modern frequency-to-voltage converter (actually it's freq.-to-current) and it has several advantages over the original. Here are the key improvements:

• Internal voltage regulation and noise transient/surge protection
• Jumper selectable for 4, 6, and 8 cylinder engines
• Stable with wide temperature and supply voltage variations
• Better performance over stock units that have been modified for 6 or 8 cylinder engines (improved response, etc.)
• Simple calibration adjustment
• Wide range of input voltages, from standard points ignition and *should* work on most electronic ignitions (only needs a couple volts input signal)
• High input impedance

I've made a prototype circuit board to finalize the design for bench-test, and I want to lay out the final board next. I planned to design it to replace the original circuit board. This does make it harder to get at the cylinder selection or calibration. It would be nice to keep the board inside the tach and not have something mounted on the back. Would most people be willing to remove the bezel to attempt doing this? That's the hardest part, using a small screwdriver to carefully pry the rim off. The rest is pretty easy, small screwdriver and soldering iron are needed.

So, any interest in this? I'll build one for myself, but if this is something that would be popular it's possible for me to build up more as a kit. This would be for others like me who don't want an aftermarket tach and to keep it original-looking. Thanks for any suggestions.

[mightyohm]

Can you drill a couple holes in the back of the tach (with plugs to keep water out) and have adjustments for # cyls and calibration? That would be the ideal solution.

[914GT]

Can you drill a couple holes in the back of the tach (with plugs to keep water out) and have adjustments for # cyls and calibration? That would be the ideal solution.

I'll give that some thought...
I like that - 'keep water out' - only a 914 owner would think of that.

[machina]

I'd be interested.

I went thru alot of trouble to extend the rpm range on my stock gauge, i really wanted to keep all the gauges with the silver point at the center.

Unfortunately I could not get the stock tach to work with my motor. Has CD box, tried the tach lead from that, the trigger lead from the dizzy, and the coil trigger but still couldn't get it to work.

Ended up getting this autometer at the local flaps, fit the hole and I have to say that the modern movement of the tach is really nice. Also has rev lite. Think the call it an air-core.

It would be great to have modern internals in the stock case.

[mightyohm]

I'd be interested.

I went thru alot of trouble to extend the rpm range on my stock gauge, i really wanted to keep all the gauges with the silver point at the center.

Unfortunately I could not get the stock tach to work with my motor. Has CD box, tried the tach lead from that, the trigger lead from the dizzy, and the coil trigger but still couldn't get it to work.

Ended up getting this autometer at the local flaps, fit the hole and I have to say that the modern movement of the tach is really nice. Also has rev lite. Think the call it an air-core.

It would be great to have modern internals in the stock case.

You need a tach adapter (like the MSD coil type) to make CD work with the stock tach. That is what I am using and my tach works fine. I don't like it because it is not an elegant solution but it works, for now.

[914GT]

I won't swear to this, but what I'm seeing is that it would be possible to calibrate this to higher RPM with the appropriate overlay on the bezel. That wasn't my intention though but I can simulate it. If your ignition systems has any tach output specs (volt. swing, etc.) I'd be interested. That is something else I can simulate on the bench.

By the way, the stock tach circuit is finicky on what signal it gets from the ignition. It likes the big voltage spike and ringing typical of a points system. I've read about some folks not having any luck with the tach adapters also. I don't remember what they were trying to hook up to.

[scruz914]

Can you drill a couple holes in the back of the tach (with plugs to keep water out) and have adjustments for # cyls and calibration? That would be the ideal solution.

(IMG:style_emoticons/default/agree.gif)

I don't like the idea of prying off the rim and removing the bezel. Adjustment from the back would be much better.

I have also been thinking of changing to a tach with a wider RPM range, not because I need the range, but to make the useable range more visible inside the steering wheel. If one wanted to go to the trouble of screening a different scale on the face plate, could your board be adjusted to match?

I also have a "bouncy" needle which I think is due to lack of mechanical dampening. Or is it from a problem in the electronics?

-Jeff

[914GT]

Jeff

The bezel is the PITA on this. There's no way around not taking it apart to install the new board, but I agree, once it's in there it would be nice to get at the adjustments.

Good point on the extra range. I'll investigate how well it will work with a different scale.

The mechanical movement is naturally bouncy as it has a lot of mass. Nothing much can be done about that. It has some counterweights but those appear to adjust its linearity. However, the electronics can compensate for this. It's a matter of a compromise between either too much overshoot or too sluggish a response. It's never going to be as good as a newer tach though.

[andys]

Guy,

I'd be interested in your conversion if it is adaptable my particular combination. I will be using a VSS and an electronic speedo, and would prefer to keep all the guages the same. If the 911 speedo will adapt, then I'd like to retain the same look tach as well. My planned V8 conversion will be an LSx Gen III Chevy with PCM. Since the PCM drives the tach, will your conversion accomodate the output signal?

Thanks,

Andy

[914GT]

Andy

The tach will drive with a logic-level input signal and won't load the signal source. But if you have any info on the PCM let me know I can test it. That's my problem as I have no easy way to test it on actual ignitions, but I can come close to simulating it if I know what they are putting out.

[lapuwali]

I've been batting around the idea of making an assembly that used a geared servo and a microcontroller instead of a freq-volt convertor and the stock meter guts. This *should* eliminate the bounce, and would allow easy adaptability to number of cylinders, range of sweep, peak RPM capability. You could also add a peak recall and even run the tach backwards if you like (would allow the guts to be adapted to some older British and Italian cars).

I've even considered adding a small 8x2 LCD display to the bottom third of the tach, which would have enough room for a number of other "gauges", like CHT, EGT, etc. This would all be substantially more complex than what's being proposed here, of course.

If no one wants to wait for me to get off my duff (and that's advisable), there's some outfit in the UK fitting "classic" gauges with new servo-driven guts. Good for speedos as well as tachs. I'm sure it's not cheap, esp. with such a weak dollar, but if you want stock looking gauges with new internals, look east.

[914GT]

That really looks like the way to go especially for serious driving and racing. My goal is mainly for a modest improvement at a low cost, and take care of some known issues with V8 recalibration and ignition compatibility.

[Qarl]

Or you could just sent it to a speedo shop and they can replace the internals. Palo Alto speedo replaced my circuitry to be compatible with the signal from my Electromotive ignition.

[michel richard]

Not sure how this all goes together, physically, or what the "bezel" actually is. IMHO, the real b*tch is if you have to remove the needle. Other than that, I think most stuff is easy. (Or is "bezel" another word for needle)

Michel Richard

[914GT]

I consider the bezel to be the trim ring crimped around the edge. The one I'm working on, the needle popped off the shaft without much effort. I'm sure a speedo shop will do whatever you want. All it takes is money. I can do these cheap.

I can lay out the board where the calibration and 4/6/8 jumpers are accessible through a small hole drilled or punched in the side. The rubber seal will cover the hole when the tach is installed in the instrument panel.

[scruz914]

I too consider the bezel to be the ring around the tach.

My tach is currently out of the car. I "carefully" experimented with removing the bezel. Bad idea. The paint cracked where I tweeked the rim to get the bezel off. My first thought was that there has to be a cleaner way to get the thing off. My second thought was to stop.

There has to be a better way to get that bezel off. I agree with Qarl, Palo Alto would be the place to go. Send them the tach and your circuit. The cost would be worth avoiding the PITA, and damage, of getting the bezel off. Someone who buys your IC can work with them to have the needed adjustment holes drilled, which may not be required.

-Jeff

[Jake Raby]

Make sure you cure the bouncing issues!

My stock tach is a joke in my car, it revs so fast the tach can't keep up. I'll toss a rev and bump the limiter at 7800 and the tach will bounce of 8K well over a second later..

[McMark]

What's the possibility off adding auxiliary gauges into the tach. I know people are always looking for a way to add oil pressure, cylinder head temp, etc. It'd be a really bid seller if you could drill a small hole in the face and add in a couple of other needles "underneath" the sweep of the main needle. Big winner!

Regardless I think you've got a great thing going here.

[Steve]

What's the possibility off adding auxiliary gauges into the tach.  I know people are always looking for a way to add oil pressure, cylinder head temp, etc.  It'd be a really bid seller if you could drill a small hole in the face and add in a couple of other needles "underneath" the sweep of the main needle.  Big winner!

If you get a late model 911 Turbo Tach it already has a boost gauge in the tach.
Allot of teeners buy this tach just to remove the boost gauge and put in the fuel gauge.

[McMark]

How much does it cost, how much does this replacement tach cost? That's the breaking point. I've heard about the conversion though.