Dip Switch Settings on a VDO Tachometer

[aeromech]

[SGKent]

We are in the process of restoring a 1977 bus and am reinstalling the dash tomorrow.

Tellford - can you repeat the switch settings you are using on the back of the VDO tach with your device and post a photo if you have a camera that will take macro photos?

My recomendation is to put that device in a small LMB type box etc when you are done but add a switch to the circuit to lock out the incoming signal. This would allow anyone to turn off the tach if they were trying to isolate it from the ignition circuit to troubleshoot.

What amazes me is that tachometers are simple circuits, the technology is old and well known. If someone is running points, the signals are somewhat consistent between 12v cars other than variances between coils and the inductance of the wiring. One would think that VDO would not have a problem like this. I can see where aftermarket ignition systems could have differences that cause issues but that so much energy is being spent by so many people and so many posts on this subject baffle me.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[telford dorr]

[aeromech]

"By the way, I'm jealous of your engine test stand - makes my cobbed up empty tranny case look sorry. Since you're in SD, if you ever want to build a buffer circuit for your test stand tach, let me know."

I would. As I said, for some reason my VDO tach in my control box for the stand goes wacky. How should we proceed with that?

_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
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Boeing 727,737-200-300-400,757,767
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VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[telford dorr]

[telford dorr]

[SGKent]

traveling writer wrote

[telford dorr]

It would seem, from the instruction sheet, that if the switches are set correctly, the tachometer is expecting two ignition pulses per rev. If each switch is toggled to its opposite position, then the tachometer expects lots (12x ?) of pulses per rev, which should make it read extremely low when hooked up to a VW engine. Thus, if the switches are set opposite of correct, it should be obvious.

The big question is what exactly does switch #1 do? It's always 'on' for ignition connections, and 'off' for alternator (e.g. marine / diesel) connections. Does this switch also change the tach's front end filtering or gain? Or does it merely switch in a pulse pre-divider?

Switches #2 and #3 select the proper scaling. Flipping them should only change the scaling by a factor of 2, 3, or 4 pulses per rev.

Maybe the factory screwed up and mounted the switch upside down on the board for full production runs (dip switches are symmetrical and can be installed either way). This would change the 'on' direction of each switch, but not its function - that should be fixed by position.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[telford dorr]

TACH SWITCH MYSTERY SOLVED!

Compared a second VDO tach (aeromech) to mine, and found out that VDO is using different switches on different production runs of these units. On my unit, a switch must be moved down to be 'on'; on the second tach, the switch must be moved up to be 'on'. No wonder people are going nuts trying to set this unit up correctly!

So here's the rules:

1) The correct switch settings for use of this tach on a VW (4 cylinder) are: switch #1 = 'on'; switches #2 and #3 = 'off'.

2) For setup purposes, above, number the switches 1 to 3, where 1 is towards the center of the tach and 3 is towards the outside. This is true regardless of how the switch is actually labeled! [e.g. in production they might mount it upside down, which would make the printed labeling wrong; however the circuit traces on the tach's board should always be in the same place.)

3) If your switch has an 'on' label printed on the switch, 'on' is towards that label. If your switch has no 'on' label printed on it, 'on' is up (relative to any printing on the switch.)

Feedback requested! Post if these rules don't work on your unit! (Pics desirable.)
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[SGKent]

Thank you. That agrees with the paperwork they sent with mine.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[Traveling Writer]

It couldn't be simpler: No matter what the orientation of the dip switches, turn the tach until the damn things read right side up. THEN flip them the exact opposite of the VDO instructions. If your dip switches are not installed upside down vis a vis the tach body, then you still must flip them to the opposite settings contained within the instructions. The first time, I did them per the instructions. The tach barely moved, but it did almost get to 1k when I was revving quite high (4000k by the sound of it). Pulled it out (actually used a mirror and a tiny screwdriver to flip them one by one to the opposite of what they had been (the VDO instruction settings), and presto, worked like a charm. Also installed the diode in line from the coil, no jumping needle (tho I didn't test for jumpy needle without).
_________________
Cheers,
Davi

1977 2.0 FI Westy
2011 1200cc Yamaha Super Tenere
1976 Feet (they work surprisingly well)

[SteveStevieStevens]

I fired mine up for the first time this weekend and it was barely registering at close to 3K RPM, so I'll try the fixes above and report back this weekend.

[SteveStevieStevens]

It worked. VDO instructions were backwards for my tach as well.

[SteveStevieStevens]

By the way this is the working DIP switch setting that I had to use:

Works great like this, no bouncy needle or anything.

[AirSchnitz]

I realize that I am coming to the conversation a bit late, but I also have had the Jumpy Tach issues. I bought mine August of 2008 and had all of the mentioned issues. The switches were backwards and mine jumped. I just read this forum, installed the diode from radio shack, and it works great now. Not only is the jumping gone but it's more accurate too.

Thanks for the advice.

AirSchnitz

1971 Campmobile
2009 Jetta TDI

[SGKent]

we installed a diode too and that solved our bouncing.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[peteandvanessa]

Just to chime in with another data point.

I also installed a VDO tach, with the same issues as everyone else. I followed the thread to set the correct DIP switch settings. That got me close, but my idle was about 1100 rpm and the high end revs were showing 6500-7000 rpm. Now I know my engine is a tweaked 1835 cc, but it sure isn't reving to 7000 rpm

I installed the proverbial diode and guess what, idle dropped to a respectable 650 rpm and high end revs dropped to 4500 rpm (more realistic)

So if you have set the DIP switches correctly but your revs seem too high, get yourself that diode pronto
_________________
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http://www.thesamba.com/vw/forum/viewtopic.php?t=568220&postdays=0&postorder=asc&start=0

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http://www.thesamba.com/vw/forum/viewtopic.php?p=7295189#7295189

[-Rusted_Radio-]

Lord why didnt I find this years ago...? I figured I was crazy running new wires..making new grounds...scratching my head. If it wasnt 1am and cold I would head outside this moment and check this out. Ah I cant wait till morning...ive been going nuts over this for about a year...
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Wes
61 Beetle on 69 Frame
Siloam Springs, Ar

[-Rusted_Radio-]

Well...diode installed and needle isnt jumping around anymore. But hell the thing still dosen't read right. idle at about 4 and high rev oh about 11. WTF..I though't you guys had found the key to my world of problems...or maybe I'm missing something
_________________
Wes
61 Beetle on 69 Frame
Siloam Springs, Ar

[aeromech]