VDO tach mod
 

What year is that tach? It has an IC in it!

HAPPY BIRTHDAY TACH!

whatever you are...


M

Wow. I am really impressed. Good work.

Must be Tach Day. I did my tach a a few months ago and also used a function generator to check. All I did was add a 15k resistor in parallel with the 15k that was in series with the calibation pot. This allowed me to adjust for the V8. Worked great and cost about $.15 to do.

Can I talk tach too?

I installed a Mallory ignition - Hyfire 6AL, and my tach would not trigger off the hyfire box wher connected per the instructions. Wouldn't trigger off the optical (mallory also) dizzy either. They (Mallory) ditzed around making suggestions, sold me a big air cored choke for big bucks, and in the end I got it to work by running 12VDC through the choke into the tach trigger wire. I haven't installed it permanently, but when I do, I will measure the current draw as the choke got warm even during the brief time I ran it.
So, what's going on? Is there a better way? I'm quite comfortable working on PCBs. I'd really like it to trigger off my optical dizzy or the hyfire box, and not use that choke at all. TIA for any help you may be able to offer.

Dave

I have two places the tach can be connected, right off the dizzy or a deicated wire on the Mallory box. I'll see what I can find out about each.

This whole thread is kind of Tachy:D

how do you guys know all this stuff? are you all electrical engineers or something?

Biggy72, what gave it away?? If it's any consolation the 914's electrical system will challenge the best of any EE ...

well ya make it sound so simple, but I have no idea what in the heck you're talkin bout.

OK, the mallory tach wire provides a "12v square wave with a 20% duty cycle" . What does the tach want to see? Does the stock setup privide more than that? Does their choke provide a means to provide a current limited bias voltage or something? (it didn't work)
Any help would be greatly appreciated, I'd like to get this working!

DAve

The tach normally sees 12 volts except when the points are closed, which is for only a short time compared to when it's at 12 volts. But I've tried even driving square waves into mine and it works, so I don't think the duty cycle is that critical. What could be the problem is the Mallory was designed to drive a relatively 'high impedance' input and the old analog VDO tach simply needs more current than it can put out. The tach is loading it too much. A scope on the line would tell you right away. I don't know what the choke is for except maybe to help block noise or RFI on the line so it doesn't cause radio interference. Chokes let DC and low frequencies go through but block higher frequencies. The Mallory might need a transistor to buffer its output in order to drive the tach. A cheap transistor and resistor from R.S. could do the trick. It's pretty straightforward if you want to know how to wire it up.

I'm generally ignorant of electronics, but not completely so, I build and repair tube gear, so I know how inductors filter.
This is an air cored inductor connected in series between straight 12VDC and the tach input which itself is concurrently connected to the trigger signal from the box or the dizzy. I haven't measured the voltage drop over the coil yet, it did run warm suggesting that there's a bit of current happening. The guy from Mallory described its function as 'bleeding a few volts into the tach input' - hence my thought about biasing. Another bit in the mallory manual refers to some tachs requiring a voltage spike over 12V from the -ve side of the coil.
If you still think its an impedence mismatch, and would be willing to detail the buffer design, I'd be very grateful. I've always struggled with a visualizaion of impedence, and have relied on rules of thumb re: damping typically, to keep me out of trouble. Just shows that a little knowledge is a dangerous thing.
Thanks! Dave

PS - how does the tach actually work?

Dave
Your thought about biasing is absolutely correct when wired between 12V to the tach output, and I should have caught that statement earlier as you said you had it hooked up that way. The inductor does a couple things. It increases current 'bias' for the output by pulling it up to 12 volts and it generates inductive voltage spikes everytime the current is switched through it. So it's a cheap way (well, maybe not in your case) to make the output voltage bigger by making spikes when it switches. It will get warm as the DC current through the inductor coil resistance wastes some power. And I was wrong earlier about most of the time the tach (or points) being at 12 volts - it should be ground since the points only open briefly when the plugs fire. If the Mallory works about the same then that coil is 'cooking' with 12 volts across it most of the time too. I could draw up a little circuit for you but I think what I'll do is take a look at mine tomorrow and find out what voltage swing is needed to trigger the VDO. That'll give me a better idea what to tell you.

The hell if I know! I'd like a circuit diagram for it. I thought it might just average out the pulses to make an analog voltage based on the input 'frequency' of the points contacting. But I saw an old TI chip inside that I think was used in the first calculators. That leads me to believe they have some kind of counter circuit inside - maybe the 914 actually contains digital logic! Advanced technology for 1974. Now look at all the electronics crap in cars...

Will you EEs help out a civil engineer? I installed my VDO with a compufire ignition module in the dizzy. The tach reads approximately 50% of the correct RPM. It appears to be not enough triggering voltage or current. I am not sure if Compufires put out 12v or not, I thought they did. It does not appear to be a totally linear error. At idle it reads about 50%, at about 6000 rpm it reads about 4000 rpm. The tach seems to respond slow.

Here's some stuff that might help or might confuse depending on how familiar you are with electronics. I scoped an actual 914 distributor and coil (taken off the engine and spun by a drill). The top trace is the voltage waveform at the points. Sorry for the quality but had to do this quick, and no time to fool with shutter speeds. Upper trace is 10V/div vertical and bottom two are 5 volts/div. with ground at the center of the screen. The middle photo is what the tach really likes, a nice clean pulse out of a signal generator going almost 10V swing. The bottom is the output of a single transistor buffer stage shown at bottom. It cleans up the noise from the coil. The output of an electronic ignition should be quieter so the 1 uF cap probably isn't necessary. This transistor stage will drive the VDO tach and should accept any reasonable level pulse out of an ignition module. It won't dissipate much power either. All the parts should be available at R.S. or electronics supply store.
http://forums.pelicanparts.com/uploa...1067042294.jpg http://forums.pelicanparts.com/uploa...1067042304.gif
You'd have to build this on some perf board and mount it somewhere maybe near the tach. Of course there's no guarantee it's going to work with everything, but it drives my tach OK after I modified it for a V8.

Spikey. so 12VDC, 50% duty cycle?
WRT the schematic, Yup, I get it, looks like a gain stage. I forget the name of the topology where the plate (anode) drives the next stage, but I do indeed get the idea. I assume that the 12V is a DC supply voltage to the anode (is that called the emitter?), not simply a notation of voltage present there. So, how does that cap work? Does it store a charge to help level out the supply voltage? What is unusual to me is that it would shunt AC noise to the tach drive rather than ground, but then I'm stuck thinking about PS filtering...
I have to go and buy the transistor, (tons of surplus houses here in Toronto) and can thenput that together for $2 in 10 minutes. I wonder if it will fit in the tach housing.... wouldn't that be slick.
Thanks very much Guy. I will be travelling next week and won't be able to try it till I return, but will post my results here when I do. Something like this is so much more elegant than that choke. Thanks again.

Dave

So with a Compufire, I could probably leave the 1uF cap out. I would need a 10k and 470 ohm resistor and the transistor only to clean up the signal.