Can't figure out what's wrong with my tach

[watsondenmark]

I have a stock 911T 2,2 from 1971.

My tach is working fine the first 30 seconds when the car is cold. Suddenly it goes down, and will just twitch/jump once in a while.

First i thought it was my contact points in the distributor that needed adjustment, so I did. I bougth new contacts, adjusted them 0,40mm (we use the metric system here ), and I adjusted the timing all perfect (The distributor is Magneti Marelli).

No luck with my tach though.

Here's what I did next:
1. Tested for + and ground in the back of the tach. No problems there.
2. Cleaned all the connections behind the tach, on the distributor and in the speed switch (the black/purple wire between tach and distributor is looped to the speed switch to make fuel cut off).
3. I checked for connection in the black/purple wire. No problems there.
4. I have a small stock of spare parts, so I found 3 other tachs that I tried in the car. That didn't work. I have never tried them in my car before so I don't know if they even work.
5. When that didn't work, I had to be sure there was nothing wrong with the black/purple wire, so I made a new one directly from distributor to tach. Still the same problem.

Now I don't know what to do. I would like to test my 4 tachs in some way, and I would like to messure something on the signal from the distributor.

Can anyone give me a hint, how I:
- Test the rpm signal on the distributor?
- Bench test my tachs?

Any other ideas/hints? Have I misset anything?

Please don't post that NHspeedo can fix my tach. I know. They are going to fix it as soon as I know that the fault is in the tach.

Thanks (and sorry for my English skills)

[drleah]

Check your voltage regulator. Mine is an '82 and inside the fan but I think yours is external in the back. Mine did the same thing. Brushes were really worn down. Swapped and no more bouncy tach.
HTH,
Leah

[Trackrash]

Have you checked the RPM sensor?

[watsondenmark]

Oh - I forgot to write that!

My Alternator and my voltage regulator is completely refurbished 4 months ago and my ignition coil is new!

[S2GART]

I agree, check your VR. The quickest way to check it out is to put a volt meter on your battery with the engine running. It should read no more than 14 volts and be steady. Rev the engine and it should stay around 14 volts. If it jumps up higher, than your VR is most likely shot.

[watsondenmark]

The RPM sensor? Where is that located? Maybe I allready have been around it, but the english terms and names for the parts is not my best skills

[watsondenmark]

Just tested the charging!

14,1V

No bouncing!

[Trackrash]

[watsondenmark]

I don't have that one on my car.
Correct me if I'm wrong, but a RPM sensor, isn't that just in the E-models?

[Trackrash]

My understanding is it is used on 69 to early 71 models to adapt the old ignition points style tach for use with the CDI ignition. Your car may have one of the newer style tachs that don't need the adapter. This may help. Tachometer Compatibility Chart

[watsondenmark]

Thanks trackrash. It seems that I dont have a compatible tach for my car. I have my own (that doesnt work), 2 from 1969, and one from late 1973.

Maybe the problem IS my tach.

[Trackrash]

Can anyone give me a hint, how I:
- Test the rpm signal on the distributor?
- Bench test my tachs?

I seem to remember this being discussed in the past. I don't remember if anyone ever figured it out. I guess my car has the older style so I can't help.

Maybe someone in Denmark has a tach like yours. You could compare them and tell if yours is good or bad. Good luck.

[bfunke]

What reading do you get when you connect a dwell/tach meter to the coil?

[racicotzhe]

The quickest way to check it out is to put a volt meter on your battery with the engine running.

[watsondenmark]

racicotzhe > I allready did - it's constant 14,1V

bfunke > I didnt know that instrument. I will buy it right away!

[wwest]

Originally Posted by racicotzhe

The quickest way to check it out is to put a volt meter on your battery with the engine running.

Battery post voltage is often not a reflection of what's going on with 12 volt power all the way back in the engine compartment where the alternator be.....

[bobolsn]

For what it's worth, my 71T (USA) has what I believe is an RPM sensor. Bosch part no. 0 227 990 01. If you need any info on the hook up let me know. The car is not running, but I could potentially help with wiring, etc. I have a VDO tach.

Good Luck

Bob

[watsondenmark]

Hey Bob. They stopped using the RPM sensor in some point in 71. Mine doesn't have it, and have the Tach (911 641 301 01) that should be the only one that works directly with the signal from the distributor.

My tach is from 5/71.

[bobolsn]

Watson,

I was looking at the rev limiter and found the words "Einbau Senkrecht Klemmen Unten" So I googled it and found this. I am not sure if this will help.



03-19-2009, 05:45 PM

I am seeking information and ideas re. the above. I have done a search and found some interesting tips but I need additional help.

The restoration of my 1969 911S is almost complete and the tach worked perfectly before the restoration. The engine was removed and the wiring in the engine compartment (relay board) was cleaned but not disturbed, I think. The gauges were restored by North Hollywod and reinstalled. The CD is the original Bosch 0227 200 001. The tach worked but had the infamous floating or irradic reading. Thinking that there was a problem with the gauge, the tach was returned to N H and after it was returned and installed there was no reading, as in dead. In an attempt to isolate the problem I installed a 1966 tach which does have a reading but it floats as before. I then took the original tach to a local speedometer shop and they reported the restored tach had normal readings. I believe the tach is wired correctly since there are only three wires. The parts book shows a "ballast unit", #901 602 601 00 which Stoddard says is NLA and there is no superceded #, I thouhgt perhaps this could be a problem. I have cleaned the points but not replaced them.

Any ideas or suggestions greatly appreciated.

Thanks,

Tom Scott

Tom, let me explain in non- EE terms what is going on. I will also refer you to my thread in which I explain in greater detail what these things are.

From 1964 until approximately October of 1971, the VDO tachs were driven off a 24 volt square wave signal. The tach wire came directly off the coil positive terminal until 1969 and ran up to the /1 contact on the back of the tach. When the points closed, the coil charged. When the points opened, the charging current broke down, inducing a current in the coil secondary, creating the spark, but also sent one MOTHER of a voltage up the tacho wire to trigger the tach. VDO says 24V, try a couple hundred!

Now, in 1969 they introduced the CDI. The CDI was driven off of the points terminal again, but the amount of current flowing from the CDI box through the points to ground was very small, on the order of 420 milliamps, which is a very small amount of current, at about 11 volts. We know this because we know the size of the "pullup" resistor inside the CDI box, in case you were interested.

Now, the problem was, in 1969 the factory didn't change the tachs. Maybe they didn't want to pay VDO to change them or VDO had already built a huge run of the boards, nobody knows. So the factory came up with a workaround, called the "intermediate unit." This is a little silver can with three terminals on it and the language, "einbau senkrecht klemmen unten" which means "install vertically, contacts underneath." Inside is another coil that is basically a little mini version of the big coil- it charges when the points are closed, and when they open it fires a MOTHER of a charge up to the tach.

Now, in October 1971 Porsche changed the tachs, they connect directly to the points terminal and don't use the "intermediate unit."

So you see why the '66 tach works. The floating is probably because the '66 tach has old capacitors, either that or your system voltage is too high. All these tachs are a circuit known to EEs as a monostable multivibrator or "one-shot"-- the arriving tach signal switches a transistor off, which causes another transistor to turn ON for a certain amount of time, charging a capacitor which then turns the first one ON and the second one off and the system returns to its stable state. While the capacitor is charging the circuit sends current to the needle, which is really a meter movement with a fixed magnet and little coil of wire-- the coil gets magnetized and the needle moves away from the fixed magnet. The pulses from the ignition bounce the needle higher and higher the more frequently they arrive, corresponding to RPM, and they are "integrated" by the weight of the movement to look like a smooth sweep. Too high a system voltage will bounce the needle all over the place.

Anyway, to fix your problem you need the little silver can. Can you see where Chuck Stoddard put one on his Perma-tune to drive the old tach in his 67S? They are not all that uncommon but be careful they haven't rusted out inside. Here is a wiring diagram for when you find one.

Does that help? Here are a couple threads that give more info (as if that were possible)

Good luck!

Tach - oh - woe (pre '74) - Page 2 - Pelican Parts Technical BBS

Before there was BHKZ: SWB engine electrics - Pelican Parts Technical BBS


Bob

[watsondenmark]

Please - someone help me with this. Bob, I didnt find the "magic words" in your links

I need to know how I measure the signal directly om the distributor. I dont't know the correct way to measure it. Some says i have to measure frequency, others say that I should measure "pulse".

What is the correct way? And how do I do it. I need a good explaination, if possible. I am not good at technical abbreviations (because of my bad english).


I have tried everything but that now.

-New refurbished CDI.
-New refurbished tach.
-New ignition points, and adjusted correct.
-All connections cleaned or changed (in the electrics board left of the engine, behind the tach, at the ignition coil, at the distributor).
-All wires messured through (in the electrics board ledt of the engine, between the tach/ignition coil/distributor).

It still only works for a minute when the car is cold, and then the tach dies.