TheSamba.com :: View topic - Correct Dip Switch Settings VDO Tachometer

Mon May 04, 2015 8:37 pm

kuleinc wrote: I'm having the "tach is reading too high and swinging problem". However if I hookup my handheld tach at the engine, the tach in the dash works properly. I wonder if I just need to change my condensor? or should the diode fix this?

Did you find a fix for this?

I just realized the same issue is going on with my own today. I hooked my multimeter up by the dash using the same wire coming all the way up from the coil. The multimeter reads the signal without any issues up at the dash using the same wire as the VDO gauge, and while the multimeter is connected - the VDO works fine. Once I disconnect the multimeter, the VDO starts reading crazy high all the time.

Tomorrow im going to run a wire, as a test, from the coil up to the VDO (without routing it under the bus or anything, just the shortest wire I can do from A to B to see if it CAN work properly.

Mon May 04, 2015 9:33 pm

airkooledchris wrote: kuleinc wrote: I'm having the "tach is reading too high and swinging problem". However if I hookup my handheld tach at the engine, the tach in the dash works properly. I wonder if I just need to change my condensor? or should the diode fix this?

Did you find a fix for this?

I just realized the same issue is going on with my own today. I hooked my multimeter up by the dash using the same wire coming all the way up from the coil. The multimeter reads the signal without any issues up at the dash using the same wire as the VDO gauge, and while the multimeter is connected - the VDO works fine. Once I disconnect the multimeter, the VDO starts reading crazy high all the time.

After confirming my dip switches were correct, one of the typical "VDO Tach Diodes" cured mine of the high-sided error. My tach functioned perfectly with the dwell meter in the engine bay, but the dash tach read high without the dwell meter hooked up.

Just completely insulate the diode if you do install it; an accidental grounding of the signal wire will stall the engine.

Robbie

Thu May 07, 2015 4:50 pm

Thanks for the reply asiab3, the diode really did fix it for me also.

It bugs me that I needed it as I ran this tach before, in a bus, and it didn't need it at that time. Oh well. The diodes were so cheap that I now have 99 spares and they still cost me less than $3 shipped. :P

Thu May 07, 2015 4:54 pm

Same here; my tach worked fine sans diode on the old engine. Oh well.

Robbie

Thu May 07, 2015 5:55 pm

One would think that, after a while, VDO would wise up and fix their circuit design. If it were my company, I'd find it embarrassing to keep churning out a flaky design...

Thu May 07, 2015 6:10 pm

telford dorr wrote: One would think that, after a while, VDO would wise up and fix their circuit design. If it were my company, I'd find it embarrassing to keep churning out a flaky design...

I think SGKent reported, and I noticed too, that the tach variance goes away with the tach in the engine compartment. It's possible the rear-engined VW market is the only subculture with this issue... Maybe a front-engined car has a short enough run to not need it. (Does VDO even make instruments for front-engine cars, now that I think about it?)

Has anyone tried contacting VDO?
Robbie

Thu May 07, 2015 11:32 pm

Quote: Has anyone tried contacting VDO?
Robbie
vdo recommends a diode when that happens. I believe it is even in the instruction sheet but that is from memory. I believe the problem is that the tach is reading signals that are bouncing back and forth in the long wire going forwards from the engine. Also - it would be interesting to know if it is carb and FI buses that do it or only FI buses with an ECU.

Thu May 07, 2015 11:34 pm

SGKent wrote: Quote: Has anyone tried contacting VDO?
Robbie
vdo recommends a diode when that happens. I believe it is even in the instruction sheet but that is from memory. I believe the problem is that the tach is reading signals that are bouncing back and forth in the long wire going forwards from the engine. Also - it would be interesting to know if it is carb and FI buses that do it or only FI buses with an ECU.

Huh, my instructions were half a page, and in five languages. No mention of a diode. But there are different models being made currently.

My Mallory electronic distributor never needed the diode with the same tach. With points the diode is needed. There's another variable for 'ya… :?

Robbie

Thu May 07, 2015 11:50 pm

asiab3 wrote: SGKent wrote: Quote: Has anyone tried contacting VDO?
Robbie
vdo recommends a diode when that happens. I believe it is even in the instruction sheet but that is from memory. I believe the problem is that the tach is reading signals that are bouncing back and forth in the long wire going forwards from the engine. Also - it would be interesting to know if it is carb and FI buses that do it or only FI buses with an ECU.

Huh, my instructions were half a page, and in five languages. No mention of a diode. But there are different models being made currently.

My Mallory electronic distributor never needed the diode with the same tach. With points the diode is needed. There's another variable for 'ya… :?

Robbie

There is a VDO tech sheet out there that mentions it. I have seen it and others refer to it but I can't find an original. Here is a length article Telford wrote on his 1971 where he studied wave forms and things like that. http://flowcon.us/td/vdotachometerfix/

Thu May 07, 2015 11:53 pm

Telford's post was how I was clued into this being a real issue, not just forum fodder. :D

Hot Spark (gag) also publishes this:
http://www.hot-spark.com/tachometer-fix.pdf

An online boat forum turned up this:
Quote: From the VDO website:
"Will VDO tachs work with points ignition or Pertronix?
Answer: Yes, with diode in line on the signal lead. Instructions are as follows:
Purchase a diode #1N4005 from a local electronics store such as Radio Shack or Circuit City. Cut both ends of the diode so each is approximately 3/4” long. Crimp 1/4” female spade connector on the end of the diode with the silver band. Crimp butt-splice connector on the other end of the diode. Crimp the opposite end of butt-splice connector to the wire connected to the ignition signal source. Connect 1/4” female spade connector used in #3 above to terminal #4 on the back of the tachometer."

So, the answer to the riddle lies in the polarity you get with the silver band of the diode towards the tach. Diodes have a band painted on the direction of allowed flow, so based on VDO's instructions it would appear the signal flow is from the coil to the tach. The offending spike is towards the coil and the diode would block that. I think.

So it seems VDO needs to tighten up their whole product line, not just the VW tachs…

Mon May 11, 2015 4:36 am

Is anyone able to make me up a wiring loom to fit this?

I've done it twice, and I can't get it to work.

I'm not very good at wiring up the block.

Wed Feb 24, 2016 10:10 pm

I've completely traced the circuitry inside the VDO 333051D. I can post a schematic, but I'm not sure how to do that on this forum. It's a PDF.

Just to be clear, the switches S1, S2, S3 need to be On, Off, Off for a 4 cylinder engine. The VDO instructions are correct, if you hold them upside down. :roll:

Back story: I do not have a VW or Porsche. My involvement here has been on Blaupunkt radios. I do have a vintage race car that I purchased the VDO tach for. I bought it because it's the same brand as the original mechanical tach, and it is 80mm, which fits the hole in the dash without mods. It also says it works with points ignitions. As noted here, this is BS. I think Telford's additional circuitry is excellent. I plan to try that in the race car this year. Why did I end up tracing all the internals of the tach? My race car has a monstrous Mallory VoltMaster II coil in it that toasted the tach after 3 laps. The dealer that sold it to me and VDO were both unwilling to replace it. I set it aside for a while, then decided to go nuclear on it. I opened it up and found the chip was toast. I replaced it. It died again! That's why Telford's circuit is brilliant. It should isolate the chip from that evil monster in my engine bay. :lol:

My race car uses a Pre-A Porsche 356 splitcase transaxle, so perhaps I qualify for buying SOME Porsche parts...

Thu Feb 25, 2016 8:48 am

So with all these issues with a VDO tach should I consider buying a different one? I was going to be purchasing one ASAP. Now I may consider another if it will have to be messed with to work.

Thu Feb 25, 2016 9:36 am

khalimadeath wrote: So with all these issues with a VDO tach should I consider buying a different one? I was going to be purchasing one ASAP. Now I may consider another if it will have to be messed with to work.

Putting gasoline in a late bus is considerably harder than adding a diode. Many tachs have the same issue so it is not limited to VDO.

Thu Feb 25, 2016 9:53 am

Few years ago had VDO tach with switches on back. Tack needle bouncing up & down. Played with different switch settings, but did not help bounce. Put in a diode and solved the bounce problem. A month later I discovered the voltage regulator was intermittently bad, by watching the voltmeter on dash. Replaced voltmeter but suspect the bounce was caused by voltage regulator.

Maybe check the consistence of the voltage when the engine is running. Yes diodes are cheep and easy to install.

Thu Feb 25, 2016 8:54 pm

This tach is reasonably accurate, once you get the filtering sorted out.

Update: The internal schematics are uploaded in the Miscellaneous section:
http://www.thesamba.com/vw/archives/manuals/VDO_333051D.php

Thu Oct 18, 2018 11:20 pm

Hi Telford

Thanks for sharing your buffer circuit. I was wondering if you could provide a little guidance in regards to changing the values for C1 and C2 you mentioned?

I ordered this tach for my bus
https://www.vwispwest.com/211018533GY/Bay-Window-Tachometer.html

I'd like to make your buffer circuit as I have a low threshold for alternator whine in the stereo. Even if this tach doesn't have a bouncing needle problem, I still don't want to send the coil voltage through the bus.

Do you think this circuit is generic enough that it should work as is, or should I consider the changes you mentioned? My engine is a 1915cc T1, but it has hydraulic lifters, so it's not a high revving engine.

I already have an order from Mouser started. I'd like to add some different capacitors for C1 and C2 if needed, but I'm not sure how much smaller you mean, ie 5nf to 5pf?

Another detail I'm not finding is what gauge wire to run for the signal? Any advice? is 20 enough, or maybe 18?

and last clarification... with the buffer circuit added at the coil end, I don't need a 1N4007 at the tach, right?

Thanks again Telford. btw, poked around your site, you have excellent taste in comics ;)

Fri Oct 19, 2018 10:35 am

Hikelite wrote: Hi Telford

Thanks for sharing your buffer circuit. I was wondering if you could provide a little guidance in regards to changing the values for C1 and C2 you mentioned?

I ordered this tach for my bus
https://www.vwispwest.com/211018533GY/Bay-Window-Tachometer.html

I'd like to make your buffer circuit as I have a low threshold for alternator whine in the stereo. Even if this tach doesn't have a bouncing needle problem, I still don't want to send the coil voltage through the bus.

Do you think this circuit is generic enough that it should work as is, or should I consider the changes you mentioned? My engine is a 1915cc T1, but it has hydraulic lifters, so it's not a high revving engine.

I already have an order from Mouser started. I'd like to add some different capacitors for C1 and C2 if needed, but I'm not sure how much smaller you mean, ie 5nf to 5pf?
The values shown work up to 6000 RPM. For faster, decrease the caps a little - say, 5 nf down to 3.3 nf. Value should be roughly inversely proportional to max engine speed.

Quote: Another detail I'm not finding is what gauge wire to run for the signal? Any advice? is 20 enough, or maybe 18?
Wire size isn't critical. It's more an issue of mechanical integrity. 20 AWG would be fine.

Quote: and last clarification... with the buffer circuit added at the coil end, I don't need a 1N4007 at the tach, right?
Right.

By the way, I finally reverse-engineered VDO's front end circuitry. It's a design compromise intended to also work with an alternator as a signal source (diesel applications). Unfortunately, it's a bad compromise. They really need to have two input pins - one for ignition signals and a second for alternator signals. I did work up an internal circuit fix which solves the problem. It's shown on my website.

Quote: Thanks again Telford. btw, poked around your site, you have excellent taste in comics ;)
XKCD!

Fri Oct 19, 2018 5:15 pm

telford dorr wrote: By the way, I finally reverse-engineered VDO's front end circuitry. It's a design compromise intended to also work with an alternator as a signal source (diesel applications). Unfortunately, it's a bad compromise. They really need to have two input pins - one for ignition signals and a second for alternator signals. I did work up an internal circuit fix which solves the problem. It's shown on my website.

Yep. I saw that.

I'm not sure what ISP makes their gauges from. It could be a VDO inside. I'm not sure. I'm mostly wanting to avoid hum. I've had that problem in other VWs.

hmmmm 6000 RPMs is fast I think. I suspect I would never rev it that fast. It has hydraulic lifters, so I understood it wouldn't be an RPM monster. I wanted extra umph to get up the hills without having adjust valves every few weeks. I honestly am not sure how fast it revs having never had a tachometer connected to it. :oops:

I'll trust your testing will be better than mine and just use your circuit as posted.

Thanks again. I was still in San Diego when this bus was my daily driver. When I finally get her back there, I'll buy you a beer or three. 8)

Fri Oct 19, 2018 6:50 pm

IIRC, stock type 1 engines redline at 4600; type IV at (around) 5500 (shorter stroke). Thus, 6000 limit at the tach should be fine.