When another 914 owner who was a professional mechanic with years of experience with VW engines drove my 914, the first thing he said was, "Your tach is off - reading too high."

He gave the car high marks on everything else, which put a smile on my face.

So I've been doing a lot of testing and investigation, and have pretty much concluded that the tach is reading 7% higher than actual RPM. At 74 actual MPH it reads 3,600RPM. Tires are 195/60R15's that are 95% of the original P195/15 diameter. At 74mph, Owners Manual says RPM should be 3206 (3250 at 75). Adjusting for my smaller tires comes out with 3,366 RPM, when I'm reading 3,600 RPM.

From experience, I guess the only solutions are to replace it with a good tach from another 914 or have it rebuilt. I have a top source I've used to rebuild Smith's tachs & speedo's on my old Triumph motorcycles – Nisonger.

Is there a company that does good rebuilds of VDO speedos? If so, what is the cost? I could not find one on the web.

My concern with replacing my existing tach with another 914 is that the replacement might be off as well.

So how common is it for a 914 VDO tach to be this far off?

Seeing advice from the 914world Brain Trust......

TIA,

GN

Just out of curiosity, do you have access to a clamp on Dwell/Tach meter to compare/assess the poor calibration of the VDO Tach?

Personally, I think I tend to shift more by ear than by tach, and I don't think that much of a difference would be a deal breaker for me, but if you go the rebuild route, I think both of the following are well respected:

North Hollywood Speedo or Palo Alto Speedo.

First off, I agree with the suggestion of checking tach reading against a dwell/rpm measurement - that would confirm/deny the tach reading being high (I guess your speedo could be reading low...? )

Once the tach has been established as the culprit, then it would be interesting to check if the tach error is proportional to revs (ie error increases as RPM increase), or its an offset (ie tach always adds 234 RPM at all revs). If the latter case, then the needle registration is off, which CAN be a DIY fix if you're willing to wrestle with the VDO bezel. If the former case (proportional), you should get a shop to look at it.

Hopefully helped and not confused,

- Tony

My speedo is 1mph high checked by GPS - reads 75 at an actual 74.

My old bench/testing tach has died, and I've been wanting to find a good tach to test mine with. I am good friends with a nearby local shop, so guess I'll drop by to see them and have them check it.

I'm a stickler ("anal retentive"?) about instruments being accurate and can' live with them being off. But actually I shift pretty much by ear as well. But I need something accurate to set my ear by.

The tach reads right on zero with engine off, and I wouldn't want to mess with the bezel and try to adjust things that way. "Been there, done that" There are very few things I can't repair/rebuild, but I've found enclosed instruments is one of them.

I'll call North Hollywood Speedo & Palo Alto Speedo after confirming the tach error with a testing tach.

Thanks guys for the help.

GN

Forgot to ask: are VDO tachs adjustable? Is there some adjustment on back where it can be calibrated?

I assume not as no one suggested that.

GN

Sees like there SHOULD be some sort of internal adjustment for calibration.

GN - So yes, they are adjustable if you know what you're doing with 50 year old analog electronics design Here's a thread that contains some analysis of the electronics in the tach, although bear in mind a) there are some "less than informed" comments in there, and b) it doesn't touch on how the electronics deal with the electromechanical movement and its subtleties (you can adjust that too):

https://forums.pelicanparts.com/porsche-911...ter-guts-2.html

The other approach (which can be found in some threads here on 914World) to "adjust" the tach is to replace the innards with a more up to date movement and electronics. This is not trivial though, so your best bet is one of the CA Speedometer shops for a refurb?

- Tony

Very informative explanations there, thanks for the link. The statement was made that tach error is likely caused by failing electrolytic capacitors, which makes sense. They do tend to fail over time. Unfortunately many newer ones are poor quality, but sometimes there's just no other choice. If you have a meter that can measure capacitance, you could unsolder one leg of each cap and see if they show out of tolerance. Ain't saying it'll be easy, I can vaguely remember opening up a speedometer on our 914 many years ago and prying the bezel wasn't fun. Hopefully some online searching will reveal some easy tricks for this, haven't started digging around this problem yet.

Dan

Tony and Dan -

Wow. Thank you. I am a very old electrical engineer who has repaired circuit boards, replaced diodes, resistors, etc. and tested them all many moons ago. So testing and replacing a capacitor (yes some fail with age) is easy. Fixing the electronics is the easy part. I have test equipment to test a lot of components, but not all. It totally makes sense that an old capacitor could be the cause of the problem.

Removing the bezel without damaging it is the hard part. At least the cover is plastic and not glass from what I'm told.

I think I'd rather send the tach out to pros and pay them to do it.

But I look forward to reading the article - thanks for it!

GN

Great article! (Is there anything not on Pelican somewhare?).

First, "No, the early tachs are not adjustable, nor is there an internal variable resistor that allows for the adjustment. The reason for the tach drift is the old electrolytic capacitors are failing. They can be replaced using the data from this thread, but only if you are experienced with discrete components troubleshooting."

Second, while I am NOT good at troubleshooting circuits, I can troubleshoot most individual components.

Third, it does seem to me that capacitor C2 is the likely culprit.

Fourth, the circuit on this article is for a 912, and there seems to be some question as to its accuracy - so I don't trust it.

Leading me to Fifth - best thing is to send tach to the experts!

Thanks guys!

GN

Palo Alto Speedometer told me this:

"Yes, Pertronix can cause a problem. The early tachometer circuit boards do not “process” the signal properly, and will cause the rpms to read inaccurately. Does Pertronix offer an adapter or filter to “clean” this up?"

So, checking in with Pertronix - their tech support has been great before....

But normally I'm running MSD and the tach signal comes directly from it. MSD may be bad, need to retry it. But the Pertronix drives the MSD signal, so I don't know.

GN

Might check here.
https://ashlocktech.com
Several things that you might be able to use to test or fix your issue.
I use their tach adapter on my 6 conversion and it has performed well on multiple ignition systems (have had a few different engines in car) including pertronix. That said I have never check the accuracy to the level you have.

john