Tacho is crazy, or perhaps the owner?

[Stefan]

Prerequisite: Everything in enginebay put together after been dismantled for a long time ago. I'm the new owner and have done the job to put the car together again :?

Problem: When ignition on, the tacho runs to max. Engine started (idle...), tacho still at max.

I've checked the cables and looks to be correct according to diagram, at least until beneath dashboard.
I've done some serious attempt to remove the dashboard together with tacho, but realized that will be the last solution for this problem.

Dashboard removal = "PITA" according to this forum = Great E9
knowledge.

In my donator car (E3 Bavaria) there is only 1 (one) screw underneath speaker to remove the dashboard instruments.

Back to my E9, I want to be the logical "faultfinder" and blame yourselves for all problems

If a tacho runs to max, is it a ground or "signal" fault, or in worst case, a broken tacho? :idea:

There are a few earlier topics about "tachos" with similar problem, but none with the "solution" included.

Well, everything else on my dear E9 works excellent

 

[DJSimca]

I would probably try to disconnect the tacho drive cable from the gearbox and turn it with a power drill (needs to turn counter clockwise) to see if the tacho works mechanically.

The dismantling it is not that bad at all ... in fact I swapped tachos not so long ago .. and it is possible without removing the instrument cluster.

Here's what you do... open the small compartnent to the left of the steering wheel. Through there you are able to easily remove the combi instrument (petrol, water temperature) by disconnecting the wiring and loosening 2 finger screws. Once the screws are loose the instrument needs to be turned just a little bit and it comes out.

Once this instrument is out you will gain access to the left main bolt for the instrument cluster. Loosen that a bit and pull the instrument cluster a bit toward you to give yourself more space. Through the glove compartment and the hole for the combi instrument you can now remove the tacho drive cable and the instrument.

To make it real easy remove the steering wheel first

 

[Tierfreund]

I believe by "tacho" the OP means the rev counter. Not the speedo....

I don´t think there is a fault in any component possible that would make a speedometer go to max on a stationary car unless it got stuck while traveling at VMax (how likely is that...)(on our cabledrive oltimers that is, , not digital bus driven speedos of these days)

So please, don´t chekc or replace your speedo rive for nothing unless you are a natural masochist. And even if you are: You have an E9, so go looking for rust, there allways is enough for some serious pain...

 

[Tierfreund]

As for the revcounter fault:
Go looking at the coil. Try unplugging cables until your revcounter goes to zero while the ignition is on. Looks like the signal cable for the revcounter is connected to the "ignition" power lead instead of the "ignition signal" lead. Once you´ve found you´re signal cable going to the rev counter, all that´s left is finding the right place to plug it in. (don´t ask me where though, mine works and I´m not going to mess around with it...)

 

[Arde]

The tacho input is electrical of course. It goes from the coil, through the
diagnostic plug pin 9 and on to the tachometer instrument. The tachometer gets two other inputs: ground and 12V. If you remove the instrument you
should verify good continuity for each of those, specially ground.

 

[Stefan]

Broken tacho?

I've managed to crawl underneath dashboard and measure direct on tacho inputs. (Similar position as the most difficult in gymnastics I saw on Olympic Games recently).

Supply and ground is OK. When disconnecting signal cable the tacho still shows max.
As soon 12V is connected to tacho, with or without ground cable connected, the tacho still runs to max.

I cant remember that a portable tacho act like this.
My guess now is a broken tacho

Anyone dismantled a tacho before, trying to repair it?

 

[Arde]

Probably broken, if you want to make sure the connectors are not swapped, the signal is the middle contact, ground is near signal, and 12V is further away on the other side (towards the passenger side on LHD car).

I would open it out of curiosity, and also try to swap it with another tacho and see if the other one works.

 

[wazza.csi]

Hi there I can almost garantee the tacho has blowen the SAK215 IC located on the circuit board inside the tacho itself, It is the standard IC used in almost all VDO Tachometers from the late sixties until the early ninties, You maybee able to source one from a local instrument repair shop or just have them repair the tacho! cheers Warren

 

[Stefan]

Well, I've dismantled the tacho. Much more easier than I thought. I think the speedo is worse.

However, this tacho doesn't include such a sophistic component such SAK215. It's only built with single components.

I could get a serious try to exchange the most suspicious components by myself, but there is a lot of mechanics for me to perhaps damage before....

I've been in touch with a local (one of the few) qualified instrument repair guy, and he will fix it.

I will end this topic with a final report of this question.

 

[wazza.csi]

Hi there, You must have one of the earlier tacho's that have with trasistors instead, I am an instrument tech by trade but a bit far away from you, your local man should have no problem as the tacho was built by the best instrument co possible!! regards Warren

 

[Honolulu]

I got a spare tachometer

email me at cgshawaii (at) netscape (dot) net perhaps we can bargain...

 

[Stefan]

A final post that could help others of E9 forum

After dismantled the tacho: A bit tricky not to damage the perfect electromechanical design. In the early 70's there were some limitations compared today to design a tacho. The mechanical design in the tacho is almost pornographic.

I was able to measure all the electrical components. There was a transistor (with non-standard identity) which was broken. Non standard means it was a kind of secret component only for the VDO people at that time. Most of them are not with us today.....
With some help from the local electronical store I found a replacement component ($ 1/2 USD). For more detail info, please mail or "mess" me.

Otherwise I'm learning by my own mistakes. But in the end, I will be the only responsible for all the works. My attempt is to use the original parts from the car (unless the worn out pieces)

Regards from Sweden (the country with (per capita) the technical nerds....)) Could it be the local water???

//Stefan