VDO tach mod

[Dave at Pelican Parts]

Bet it'd fit under the relay board cover really nicely. And the tach signal wire plugs into the 12-pin connector on the board, goes through the board, and connects to the 14-pin connector... Sounds like an opportunity for a nice clean install, there!

--DD

[914GT]

Originally posted by DDS
Spikey. so 12VDC, 50% duty cycle?

An actual 4-cyl distributor the points are open a little less than 50% I think for the normal points gap setting. I noticed the tach would work down to just a few volts swing on the input, but it got finicky on the DC level it was switching around. Maybe this has something to do with all the problems hooking it to electronic ignitions.

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.

You're pretty close. This would be a 'common-emitter' stage if working with small signal RF stuff. In this case, the transistor is just a switch and it's either mostly on (saturated) or off.

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.

Yeah, my drawing sucks but the 12V means that it's connected to the battery side of the coil or ignition switch. The resistor is between 12V and the collector of the transistor.

So, how does that cap work? Does it store a charge to help level out the supply voltage?

The cap just filters off some of the high-frequency noise that might get through. When the field on the coil collapses (when there's spark) it rattles the transistor off and on. Just trying to roll it off some.

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...

The cap could be wired to ground instead of 12v, it won't matter except for polarity if an electrolytic is used. Then the minus side of the cap would go to ground.

This isn't exactly a sophisticated circuit but it might work for you. I know there's something that would be better, like using some comparators, but then it's going to be a bit more complicated too.

[914GT]

Originally posted by bpick84
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.

You're guess is as good as mine on that one. But I think in general that electronic ignition modules don't have all the noise on the output line you'd use to hook up a tach. Mechanical points are great at bouncing and arcing and then the coil is ringing like crazy at the same time. But that's what makes the sparks.

[914GT]

Originally posted by bpick84
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.

I'm wondering what that thing is putting out. Sure be interesting to hang a scope on it to find out. I think you're probably right, it's right on the hairy edge and a lot of the pulses are dropping out and not getting counted. So it would be running low, erratic, and not responding well. Maybe it's worth trying out the little buffer circuit too.

[DDS]

Heven't yet built that little circuit but will try it this week. I have been pursuing better tech data from VDO - they have been prompt and courteous, but not yet helpful.

Here's the discussion:
Hi Bill:
As explained below, mentioned, the issue with the tach isn/t repair, it is understanding the input signal requirement so that we can make it function with modern ignition systems. These systems provide a tach output signal that does not reliabley trigger the older VDO tachs. The manufacturers of these systems are not clear on how to make it work either.
I do have a speedo I need recailbrated so for that I will contact Nichols.
If a tach can be repaired, then it follows that the the schematic and other technical data must exist, and if so it must exist somewhere at VDO. Should I try Germany, where I started?

Thanks, Dave.

BTW - here's the discussion that raised the issue this time, but it is a very common problem when adapting older VDO gauges to modern ignition systems. VDO tach mod


>>> Reichart William 11/06/03 08:41AM >>>
The take is to old to find any records or schematics.
The only thing you can do is get it repaired at Nichols Speedometer (336-273-2881)
Or replace it with a new one. Sorry. The repair shop is sponsored by Siemens VDO.

William M. Reichart
Technical Support
Siemens VDO Automotive Corporation
Service & Special Solutions
4905 Tilghman St. Suite 120
Allentown, Pa. 18104
Phone: 1-800-265-1818 Ext. 4018
Fax: 610-289-1769
bill.reichart@siemens.com




-----Original Message-----
From: Dave Syer Sent: Wednesday, November 05, 2003 6:59 PM
To: FM ABE Technical Support
Subject: RE: info@vdo.com


Hi William: Sorry for the delay. Attached are pics of the tach, thanks for
the offer of assistance.

Regards, Dave



> -----Original Message-----
> From: Reichart William [mailto:bill.reichart@siemens.com]On Behalf Of FM
> Hi Sir,

> These instruments are to old for our data base.
> Can you describe the back of the gauge so I can
> Find the instructions.
>
> William M. Reichart
> -----Original Message-----
> From: Haslop Caroline (ext)
> Sent: Monday, October 27, 2003 10:35 AM
> To: FM ABE Technical Support
> Subject: FW: info@vdo.com
>
> Hello,
>
> Here is a technical support mail from Canada for your
> attention.
>
> With kind regards,
>
> Caroline Haslop
>
> ____________________________________________
> Caroline Haslop
> Service and Special Solutions
> Marketing Communication
> Siemens VDO Trading GmbH
> Kruppstraße 105
> D- 60388 Frankfurt am Main
>
> E-mail: Caroline.Haslop.ext@siemensvdo.com
> Tel: +49 (0) 69 40 80 5 235
> Fax: +49 (0) 69 40 80 5 321
> http://www.vdo.com
> ____________________________________________
>
>
> -----Original Message-----
> From: info@vdo.com [mailto:info@vdo.com]
> Sent: Friday, October 24, 2003 5:40 PM
> To: info@vdo.com
> Subject: info@vdo.com
>
>
> Subject: info@vdo.com
> Text: This is a technical inquiry relating to 1970s VDO
> tachometers as installed in 1970-76 Porsche 914 cars.
> These tachometers have compatibilty problems with modern ignition
> system, and there seems to be difficulty in finding a solution.
> Would it be possible to email a schematic for this unit and if
> possible, specifications for the trigger signal requirement?
> Many thanks for your anticipated assistance.
> Dave
>

[red-beard]

I don't know too much about the tachs, but I do know that around 1971 or 1972, there was a design change in the Tachs. The Early models will not work with the electronic ignitions and will not work with a MSD unit.

I ended up sending my 914/6 tach out to a speedo shop and they replaced the circuit board with one from a later tach.

I think the tach in the pictures is a "later" tach.

YMMV

[DDS]

This is one of the pics I sent to VDO - the tach from my '72 parts car which I assumed was the same as the tach in my my early '73 driver. Is this an early or a late tach? Thanks!

Dave

[red-beard]

I'm not sure. But then, I'm also dealing with a 6 instead of a 4, which should only be a calibration issue.

James

[914GT]

Dave - from the back it looks exactly like the one from my '74 not that that means anything. That would be great if you could track down that schematic. I'm sure I could come up with a better circuit after seeing the whole picture.

[DDS]

Hi Guy - yup, it would be great to really solve this problem for everyone.
Just heard back from VDO USA who sent me back to VDO Germany. Stay tuned!

Dave

[maf 914]

Guys,

I've been reading the posts with interest. I want to add a Mallory HiFire VI which has a tach output, but am concerned my '76 tach won't like the signal. I guess maybe I just need to get with it and give it a try.

Mike

[DDS]

OK, both my Tachs are stamped 230/41/2 dated 2.72 and 6.73, with Porsche no 914 641 302 10 0013 silkscreened on the top of the face. Appears no change over those months anyway. Mike at Mike's 914's will let me look at his box of tachs to see if there were any changes over the production run. He says all 914 tachs appear identical and work identically in all cars. He may not have encountered the anomaly James mentions above.
Anyone have anything different?
Dave

[DDS]

Hey, what happened to the late tach part nos?
My inquiry has been directed to a second person in Germany at VDO.
Guy - I picked up the parts for that little circuit - about $4 including a small enclosure, though I will try to install circuit inside the tach housing. I will tangle with the tach shortly!

Dave

[DDS]

Hi Guy:

VDO came through with the signal req't but no schematic. Sounds like points and a coil provide a spike that black boxes don't. Here is their response:



Hello Dave,

333.230/041/002
this is a old version Tachometer with 24 Volt Square Wave signal from Ignition KL.1.

New versions Porsche use a HKZ Ignition with a 11 Volt Square Wave .These numbers are 333.230/041/007 to 333.230/041/010.

There is no compatibility within the Instruments .

Maybe Palo Alto or North Hollywood Speedometer have a fix for your customer.

Palo Alto 650-323-0243
North Hollywood 818-761-5136


William M. Reichart
Technical Support
Siemens VDO Automotive Corporation
Service & Special Solutions
4905 Tilghman St. Suite 120
Allentown, Pa. 18104
Phone: 1-800-265-1818 Ext. 4018
Fax: 610-289-1769
bill.reichart@siemens.com

[914GT]

Quite possible the old versions needed a good kick-in-the-butt voltage spike off the points to get them to count. The points give a lot more than 24V, just take a look at the scope trace. For the old version, the simple transistor circuit won't be enough but it's still possible to make a circuit simulate the points yet driven off a modern ignition. If I was doing it I'd probably change the 470 ohm resistor out with a small inductor, or a series resistor-inductor, to create a voltage spike. Might need a protection diode somewhere to keep from blowing the transistor, or a transistor that can handle the higher voltage. But without having the old tach to play with I can't give you an exact circuit guaranteed to work.

[Bob Bischoff]

Guy
I took a old tach apart last week to check the CKT board and looked at it. It had 2pnp transistors and a 3 transistor I could not find a cross reference for. The 2 pnp were old European transistors used in some audio equiment. I was going to draw out the ckt and see if I could figure it out. I looked at this and said with the price you can get a later tach for why bother with this. I did my newer style tach last year and it works great. If you send me a PM with your address I'll mail you the ckt. board.

Bob

[914GT]

Bob, Sure, might take you up on it if there's anyone (Dave??) who could benefit from it. I've 'reverse engineered' a few other circuit boards in the past (I'll leave it up to your imagination what they were). Maybe I don't need it though. If I didn't have so many other projects going on right now I'd breadboard a circuit that would simulate the old points/coil. Then this would work on either of the VDO tachs. Well I'll see what I can come up with then maybe Dave will volunteer to build it up and try it.

[304065]

914GT,

How about a 24V dc/dc converter substituted for the 12V supply? These are available surplus: one common one costs $.50 and takes a 5V input, which could easily come from a 12v-5v voltage regulator. That would provide a 24v square wave pulse for the old tach.

Have you ever tried to drive the tach with 24v pulses from a signal generator? Or maybe they only generate like 12vpp, I guess in that situation you would need a 24VDC power supply wired to the collector, no?

[914GT]

Hi John,

The old VDO tach doesn't operate well on a square wave, even with a higher voltage swing. It's been a long time since I've worked on these, but I recall that the tach was not accurate over the full RPM range when driven with a square wave. The tach works best with a fixed pulse width independent of input frequency, simulating the fixed dwell time of a points-type distributor.

For bench work I built a selectable timebase with a 555 timer. I use a miniature rotary switch to select different RC networks, each calibrated for a frequency corresponding to RPMs between 1000 and 6000 at 1000 RPM steps. The frequency is determined by whether you're using a 4, 6, or 8 cylinder engine. I use a frequency counter to adjust the trimmer pots at each step. The 555 outputs a pulse with a fixed width equal to the normal engine dwell. I buffered the output through an open-collector transistor stage with some inductance on the collector. I forgot what I used for inductance, something from my junkbox. It could be a small coil of a few mH or a winding on a small transformer. Not real critical, enough to give some good inductive voltage spikes with some ringing. I protected the transistor with a diode for protection. With a scope the waveform looks pretty close to what a stock coil ignition puts out, and it's handy for bench testing and calibration without smelling exhaust fumes.

I gave up on the old VDO tach movement and circuitry a long time ago. It's a dog compared to a modern lightweight movement and electronics. I've been installing the innards from new aftermarket tachs (like the $30 ones from Checker) into the VDO housing. Looks stock but performs like a tach in a new car, and is 4/6/8 cylinder selectable and compatible with most ignitions.

[304065]

Thank you for the update. I like your idea for a tachometer pulse generator. From the old scope traces you posted it looks like the voltage would be well above 24v anyway despite what VDO says.

So I've got a couple of LM2917 chips that I plan to fab into a modern tachometer with a minimum parts count. I got the idea from Warren Hall, aka early_s_man, and there are a couple schematics floating around out there.

Has your experience been that the intertia of the old VDO tach movement isn't as good as a modern one? I could easily source a modern tach and use both the board and the movement-- how did you attach the old needle? A picture of that or the name of the cheapo tach would be great to get started.

Thanks!