Tach - oh - woe (pre '74)

[carreraplanes]

I was wondering if there were any Pelicans out there who could help me with my tachometer problem, which goes something like this. My RS ’73 Rep blew its spark box (CDI unit) after a specialist had rebuilt the motor. The box was replaced and the engine now runs fine but the tachometer ceased to function. The specialist said this often happens when the CDI blows as it is in line with the tacho. So I sent the Tachometer itself away for testing but the instrument shop said there was no fault found.

This led me to believe that it must be the trigger unit. I thought to swap it with that in my 1970 year model 2.2 litre 911 T but this appears to be a slightly different system. I am referring to the tin box units that reside behind the voltage regulator. I believe that the one immediately behind the regulator controls the heated rear window. It is the box behind that which is confusing me. My Haynes workshop manual suggests that this is an RPM Transducer.

I thought I could swap the transducer boxes between the cars to see if it rectified the problem. The connections are exactly the same but the tin boxes have different part numbers. The one on the RS rep (’73) gives a Bosch number 0 333 400 001 (I think that is correct as it is partly obscured) and reads 12v/13,5/6. The ’70 T Bosch part number is 0 336 611 006 and reads 12v/15,0/6 which might suggest that the boxes are of different values. However on the T there is an additional unit behind the transducer that the manual refers to as an RPM Sensor, The Bosch number on this item is 0227 990 001. The '73 does not have one of these.

So my questions are can the boxes be swapped, what is the difference between the two systems, anybody know how to check/test the boxes, and has anybody had any experience on troubleshooting this system?

Some pics for additional guidance:



Above is the system in the '73



This is the '70



And these are the transducer boxes the '70 T one is on the left. The other is the '73. Note that the connections are identical.


Thanks in anticipation,

Keith. (Stressed Pelican).

70 911T

73 911 RS rep.

[Porsche_monkey]

Have you checked the wiring up the tach for continuity?

[Early_S_Man]

Kieth,

The RPM Transducer/Speed Relays can be exchanged, but the problem with your '73 isn't the RPM Tansducer/Speed Relay, as that box shares the same input signal from the points as the CDI unit!!! To test for the proper signal at the tach ... remove from dash, and put a probe end of a 12 Volt test lamp connected to +12 Volts at the same time as cranking. You should see a blinking test lamp as the engine is turning over ... if not, then the problem is probably at one of the two 14-pin connectors that the Black/Violet wire has to travel through to get to the tach!

The large, tall, round aluminum can on the electrical panel of your 2.2 is probably the Bosch 0.227.990.027 Ballast Unit ... a resistive divider that feeds the tach. Please remove that unit and test for continuity and resistance values between all three terminals after thorough cleaning. It would help several on the board with early tach problems to post those values ... my suspicion is that it is a 1:30 divider [15 Volt: 460 Volt] but have no hard data.

Here is a simplified diagram of the CDI, Ballast Unit, and Tach circuit:

[carreraplanes]

Warren,

Thanks very much for taking the time to sort out that wiring diagram for my car and for your advice. I'm going to try to check it out this w/e if I have time, if not it may have to wait until the following one. I will post on here when I do and will also check out the resistance values on the T box and post them.

BR

Keith.

[KevinS]

Keith,

the same exact thing happen to me on my 1971 T. I never was able to figure the problem out. I'd be real interested and hearing how it goes.

KevinS

[carreraplanes]

Warren,

You were of course right, the fault was merely a break in the black/purple wire. Thanks again for your help. TYour wiring diagram is I think for the earlier cars with the ballast unit. The RS thingy does not have that unit.

The ballast unit on my T has a different part no. to the one you suggest : Bosch 0 227 990 001

The resistance values are A to B: 16 ohms B to C: 40 ohms. so the ratios would vary with your hoped for value. Perhaps this is down to the part no difference.

Keith.

[304065]

Interesting.

So the Kettering ignition tachometer (like in my '66) takes its signal from the points circuit. I believe, but I am not certain, that this version of the tach works by averaging the voltage that is produced by grounding the +12v input from the ignition through the points.

Now in 1969, Porsche went to the CDI. The points closing allow the capacitor to charge and their opening fires the capacitor, which dumps 460 volts into the coil 15 terminal. So the resistive divider was introduced, which allows the new high-voltage system to work with the old tachometer.

Attached is a picture of the Bosch 0 227 990 001 (Porsche 901 602 601 00) can on Chuck Stoddard's 67 911S which has been converted to Permatune.



You can clearly see three connections on the can.

* terminal A, which is the ground (a very strange labeling, because terminal A is the voltage output from the CDI, and you can also see the letter A instead of a "15" on the coil, but that's the way Bosch did it, this is confirmed by both Warren's diagram and the factory SL35 diagram

-- and someone has used RED wire which is exactly the WRONG color to use);

* terminal B, which is clearly a blue wire running from the Coil 15 terminal; and

* terminal C, which is a black wire with purple stripe running forward to the tachometer).

So, B is the input voltage, C is the output voltage, and A is ground.



Now, if A:B is 16 ohms and B:C is 40 ohms, that must mean that R2 is -24 ohms, but you can't have a negative resistance value. Something must be amiss. (edited to correct the negative value)

If you use the equation, you get values of more like 18K ohms and 500 ohms to bring a 460 volts down to 12.4. I must be missing something, or the design of the Bosch unit is other than a simple resistive divider.

The diagram shows a "transistor" on the box, but I wonder if that's just Bosch-speak for "No user-serviceable parts inside."

Does anyone still use one of these ballast units? Those of you with 1969 911's are you out there?

By 1971, the ballast had been eliminated, and the tachometer is shown connected directly to the points. This is the "internal tach modification" as done by the speedo shops.

[carreraplanes]

Hi John,

I think my '70 911 T uses the ballast unit.

[Early_S_Man]

John and Keith,

I suspect an error in transcribing the measurements, as the values just 'don't compute' ...

I suggest a retest, obtaining three measurements, A - B, A - C, and B - C. If one test value equals the sum of the other two, then you know everything is right ...

If A - C is 16 Ohms, and B - C is 40 Ohms, then I would expect the A - B measurement to be 56 Ohms.

My suspicion about the transistor symbol on some ballast unit is that label was to identify the correct ballast unit for CDI-equipped engines in 1969, because there is another ballast with a different pn for 911T engines with Kettering ignitions.

Here is a revised/edited diagram for the 1969 - 70 engines with CDI and MFI using the ballast unit between coil lead 'A' from CDI and tach:

[bigchillcar]

i'm just a visitor passing through the 'electronics olympics'.. good work, fellas.
ryan

[304065]

Warren, thank you for catching my transcription error, I meant to say -24 ohms, which is impossible (at least with the level of physics I studied)! Thanks also for the revised InterEurope diagram.

I have ordered a NOS ballast unit for testing 901 602 853 00 and will report the resistance values here. I'm not above cutting it open either.

It just kills me that people will pay $150 to have their tach opened up for a modification that's probably nothing more than $1.00 worth of commonly-available discrete components.

I also have an old 912 tachometer on the way, this should be fun.

[jstobo]

You pay for what they know not what they do. Not all of us are electrical experts. Now if the car ran on a chemical reaction(i know besides oxidation of gasoline) I may be able to do more. Just so you know my tongue is planted firmly in cheek. I wish I could fix electical problems at the componet level.

[RSBob]

Which brings us to our hosts 'Easy, Inexpensive Ignition Kill Switch" involving adding a simple switch to the tach wires. kill switch

I added one in 15 minutes and hid the switch. Even if some one pulls your ignition and tries to hot-wire the car, it won't start. However your starter might get toasted if they keep at it for a while. But what would you rather have, your car or a new starter?

[Early_S_Man]

John,

I didn't notice any error in your numbers ... it was Keith's test numbers that looked bogus. I must have missed them when originally posted!

Yep, I agree about the $150+ conversion fee, too! An entirely new DIY board could be fabricated using an LM2917, precision 1% metal film resistors, polypropylene caps, precision Bourns trimpots, and FR-4 epoxy pc board ... for way less than $150, and a superior performing tach [after calibration to better specs than the typical VDO production line tolerances] would be the result for a fee comparable to the level conversion.

Rick,

Another variation discussed here before, perhaps even more slick, would be for the hidden switch to activate a time-delay relay that kills the igniition or fuel pump after 30 - 45 seconds.

BTW, if anyone out there is wondering why I post some diagrams rotated 90° ... it is so rectangular diagrams can be maximized and sized with the short dimension at 800 pixels. The image should be saved on your computer, where it can be rotated and saved in the correct orientation with Irfanview ... an extremely useful Freeware program available for download at:

www.irfanview.com

[304065]

How could I have missed this thread?

electrical stumper question

(BTW, the German writing means install vertically with connections below)

OK, back to the picture of Chuck's car. Since we know the orientation of the terminals and what's inside, and the orientation of the lettering on the can, we can put together what is connected to what. The resistor is the first thing connected to can terminal B (from the BHKZ). The resistor and the choke in series connect to can terminal A, the ground. And finally, the polyester-film capacitor is connected between terminal A and C, the only one left.

[304065]

[304065]

The resistor: Gold for 5% tolerance, then brown, green, brown, or 150 ohms, but I am neither certain of the colors nor the code.

So if R1 is 150 ohms, then measuring from B to A is 189 ohms, which means L1 is 39 ohms impedance. If A to C is 253, that's measuring across the cap, which would eventually show an open if you were measuring it with DC, once the cap charges. And if B to C is 443, well, that's just the sum (nearly so) of all the resistances. From the photo it looks like C1 is .01 microfarads, 630 volt specification.

Ok, so is this an "RLC" circuit, such as used to filter? Be gentle, I went to Law School, not Engineering School. The resistor will lower the voltage. The inductor will pass certain frequencies, as will the capacitor. One may filter the high and the other the low. But that's only worthwhile for an alternating current. Isn't the output of the BHKZ a 460 volt DC pulse?

edit. . . researching low-pass and high-pass filters. . . this looks like a high-pass filter with an additional inductor in the input leg. . . stay tuned. . .(pun INTENDED)

[Early_S_Man]

John,

This ballast unit is a bit more complicated than I thought!!!

I don't think that is a coil ... probably a NI [non-inductive wound] wire wound resistor, and it has a third connection that is connected either as a center tap, or maybe somewhere else in between the end points ...

The end points of the NI resistor R2 seem to be connected to the 0.01 mF 630 V cap C1 at terminals A and C. The center tap seems to be connected to R1 at the end farthest electrically from terminal B.

It may be impossible to construct the circuit precisely and accurately without the measurements from the three terminals of the wirewound resistor or coil ...

I can, of course, make some estimates based on a SWAG ...

ASSUMPTIONS:

1. W.W. resistor is center-tapped ... WRONG, resistance measurements don't compute!!!

Of course, the R1 resistor color-coded at 150 Ohms may have changed due to heat and vibration over the years!

2. A - B resistance is 189 Ohms, implying that center tap is only 39 Ohms from the 'A' end of NI R2, assuming 150 Ohms for R1.

3. But, B - C resistance of 443 Ohms implies that either R1 is greater than 150 Ohms or leg 2 of R2 is almost 300 Ohms!!!

At this point ... the resistance measurements on that particular ballast unit seem questionable!!! Assumptions and SWAGs don't seem to help get very far into this 'swampy' area ...

[304065]

Well, if you order a 901 602 852 00 here is what you get. . . WRONG PART!



For posterity it says BERU Germany WZ09-2 and on the mounting tab it says 6.0 (which I assume is ohms).

Anyway, this looks just like an external ballast resistor such as one would use with a hot coil. I had a similar one on my BMW 2002. It is NOT, however, a choke, cap and resistor!

Back it goes!

[Early_S_Man]

John,

That is certainly the same pn I posted back in 2004, and I am beginning to wonder if the 901.602.853.00 isn't just the equivalent part made by Bosch!

Is it possible that the tach designed for use with Kettering ignition of the 1969 911T ... never used a resistive divider ballast unit??? I don't remember whether it was the paper parts catalog ot PET that I looked up those two part numbers in, but there is now some confusion over the use of 'ballast' as a descriptor for two entirely different circuit components attached to a coil ... one for the starter circuit in a car with Kettering ignition, and the other for the tach circuit in a car with a BHKZ unit!

Does anyone out there have a factory schematic for the 1969 911T???

When I look at the factory schematic for a 1970 914-6 ... it shows a CDI unit and no ballast unit to the tach ... anybody know if this is correct???