tachometer problems (long) - one works, one does not

racemor · 05:04 PM

Ok, here's the scenerio. I recently did engine work and when re-installing, some idiot (ok, it was me.

) re-installed the coil backwards. Without realizing this as the reason it would not start, the car was trucked back to the shop that did the motor work where things were taken care of and everything works and runs great.

Obviously there was electrical damage and I ended up buying a used cdi box from the shop and borrowing a 7k rpm tach from a T to use while my 8k rpm version was sent off to be repaired, since it was not working now. So, off to North Hollywood Speedometer it goes for fixing. And since it's there, might as well do the cool rotated face offset conversion.

Now is where things get strange. When the 8k tach was returned it would not work. There was some confusion as to if I may have asked them to convert it to use with a 6-pin cdi. So, I sent it back, they made sure it was set up for a 3-pin and returned it to me. Still not working. Kevin at north Hollywood said they'd take another look at it. When it got back to them they checked it on the bench tester - worked fine. They even took it to a local shop and installed it in another '73 with a 3-pin box. Worked with no problems.

All this time, the 7k tach has been working perfectly.

So Kevin and I started comparing notes. We found that the part numbers on the cdi boxes were not matching up. The one from his test car matched my original burned unit - Bosch 0 227 200 001. But the used box I bought has the number 0 227 200 008. That must be the problem! So I borrowed a working box from a friend with the 001 number. Installed that and tested with the 7k tach. Everything's good. The 8k tach comes back one last time. I install it tonight and it's dead as a door nail!!

What's the deal??? I'll list some info below on numbers from the back of the tachometers if it helps. Just not sure what to try next short of seeing if the working 7k tach could be converted to 8k. But I've alread thrown a lot of money into repairing and converting my other one with out much luck. Any help is appreciated.

racemor · 08-31-2005, 05:04 PM

Here is the part number info.

Zeke · 08-31-2005, 05:48 PM

The borrowed tach works with a transducer, a gizmo on the engine electrical (I can't think of the word) thing in the rear fender area. If you have '73 electrics, the '71 tach shouldn't work in your car. That's all I know, wait for Warren (Early_S_Man).

racemor · 08-31-2005, 06:30 PM

That's interesting Milt. Other than the cdi and tach, there were no other changes. There was some question as to wether or not the transducer was in working order. They came up with one, but I think that since the 008 cdi and the 7k tach were working together, they thought that the transducer was alright. Maybe not?

Lorenfb · 11:30 PM

The tach should have been sent to Pelican for repair because Pelican:

1. Has a rebuilding source which is very reliable that not only
knows speedos/tachs but knows Porsche wiring & electrical setups.
2. Is very price competitive in its products.
3. Provides this BBS Forum. Does North Hollywood Speedo provide this?
I think NOT.

All 3 pin Bosch CDIs function the same electronically from the standpoint of the tach signal!
A problem which occurs with some Porsche tachs is that they (low pulldown) may prevent some
3 pin CDIs (points pullup of 100 ohms) from functioning. These tachs must interface with
30 ohm pullup CDIs.

So, based on the two types of pullup CDIs, it's possible that the repaired tach may allow
a 100 ohm CDI type to run the car but not receive a positive enough signal to function
because of how the tach was repaired. Therefore, the repaired tach may require a 30 ohm
pullup CDI type.

Bottom line: You need to ask how your tech support guy hooked-up the tach to verify its'
functionality. In any case, he should be able to modify the tach to function with your
desired hookup.

Zeke · 08-31-2005, 08:11 PM

Quote:

Originally posted by Lorenfb
The tach should have been sent to Pelican for repair because Pelican:

1. Has a rebuilding source which is very reliable that not only
knows speedos/tachs but knows Porsche wiring & electrical setups.
2. Is very price competitive in its products.
3. Provides this BBS Forum. Does North Hollywood Speedo provide this?
I think NOT.

All 3 pin Bosch CDIs function the same electronically from the standpoint of the tach signal!
A problem which occurs with some Porsche tachs is that they (low pulldown) may prevent some
3 pin CDIs (points pullup of 100 ohms) from functioning. These tachs must interface with
30 ohm pullup CDIs.

So, based on the two pullup CDI types, it's possible that the repaired CDI may allow
a 100 ohm CDI type to run the car but not provide a positive enough signal for the repaired
tach because of how the tach was repaired.

Indeed, Pelican does have a rebuild service. I believe they send them over to NorthHWS.

Lorenfb · 08-31-2005, 08:15 PM

"I believe they send them over to NorthHWS." - Zeke -

Not true! Just ask Wayne or Tom. But that's not the issue, it's just NOT
good to bypass Pelican even if the above statement were true! This forum
has value & should be supported via business whenever possible.

Early_S_Man · 08-31-2005, 08:33 PM

Tom,

I believe that problematic tach was intended for use with a 'Ballast Unit' -- a three-terminal resistive divider circuit used with early tachs and both battery-coil and CDI ignitions. Here is a simplified circuit using the ballast unit:

The mystery '008' CDI unit is interesting, though, and I would love to see pics of the circuit board if you could pull the bottom cover and take a couple of close-up pics with digital camera or put it on a flatbed scanner ...

Early_S_Man · 08-31-2005, 09:10 PM

Tom,

OK, reading back over ... forget what I said about the ballast unit! It is sometimes a problem with '70/'71 tachs, but that isn't the problem if the 7K tach worked all along with both '008' and '001' CDI units.

Does your distributor still have points?

The only other issue I can think of is the date code of the CDI units ... stamped into the same side of the case as the 'Vorsicht!' sticker with lightning bolts! The number are small, and at the other end where the 3-pin plugs connects. Early CDIs before '73 used 33 Ohm pullup resistors, and later ones used 120 Ohm.

racemor · 09-01-2005, 03:03 AM

Warren, Yes, I'm still using points. The code stamped into the side of my non-working original just says 226. I'll need to pull the working 001 box in the car to see what it says, but may not be able to get a chance until Friday evening. the 008 box is stamped 632 and hand writting on the back of the box is "77 Euro". I'll get you a digital shot of the 008 box today.

racemor · 09-01-2005, 05:49 AM

Maybe the solution is to rebuild the original?

Here are the shots of Bosch 0 227 200 008:

Lorenfb · 08:30 AM

The image of the 008 is basically the same as all other 3 pin Bosch CDIs.
The input pullup uses the later value of about 100 ohms & not the early
value of about 30 ohms. The added component with the heat sheilding
is a power input resistor which prevents the 22 volt zener from being
destroyed by overvoltaging, e.g. bad voltage regulator or battery charger.
This component was added when Porsche discovered that CDIs were
being damaged when the alt. regulator failed and caused excessive
overvoltage (> 22 volts).

Again: Mostly likely the tach has been repaired/setup for the lower CDI
pullup resistor. As a test, you can get a 50 to 60 ohm 1 watt resistor
and connect one end to +12 & the other end to the tach input. The 1 watt
is ONLY for test & a 3 watt or more is necessary for a permanent mod.
This will determine if the tach requires a lower CDI pullup.

Zeke · 09-01-2005, 08:27 AM

Quote:

Originally posted by Zeke
The borrowed tach works with a transducer, a gizmo on the engine electrical (I can't think of the word) thing in the rear fender area. If you have '73 electrics, the '71 tach shouldn't work in your car. That's all I know, wait for Warren (Early_S_Man).

Again, I'm out of my league.

I've been reading here a long time and this is the first I've seen of the difference between 3 pin CDI's. Goes to show you what a great resource this is. Now I'm gonna go out and look at my spare CDI to see if it's compatable with what's in the car.

To tell you the truth, I couldn't follow the testing of the tachs from here to there and with this and that. I just know that tachs are specific for different years. I have a '71 T tach in my '71 S and it works only because it came from the same year, AFAIC. I also have an E tach from a '69 which I haven't tried, now I probably won't. My S tach died when I was welding on the car.

Lorenfb · 09-01-2005, 08:33 AM

"I just know that tachs are specific for different years." - Zeke -

You need to ask more questions of your Porsche techs, e.g. Dieter @ Lomita 911,
or John @ Dayton Ent.

racemor · 09-01-2005, 08:45 AM

I'm with you Milt, this stuff is over my head too. But it helps a lot to have people around that know how to talk you through the problems.

So then are we sure the transducer is not the problem? Is it just a matter of getting the right cdi matched up to the right tach?

Lorenfb · 11:22 AM

"Is it just a matter of getting the right cdi matched up to the right tach?" - racemor -

Sounds good, like one of many solutions. Check the Pelican parts list, as they
sell Bosch CDIs via their many supplier sources.

Early_S_Man · 09-01-2005, 11:31 AM

Tom,

If you have any friends or coworkers that know or tinker with electronics ... I'm sure one could easily repair your old CDI unit, or modify the '008' unit to be compatible with your tach!

Below is a pic of the board in your '73 unit ... you will notice just how close it is to the '77 unit date-coded 32nd week of 1976! Below that is a board layout updated to the '77 layout ... which can be updated to the 33 Ohm input pullup resistor by just soldering a 50 Ohm 5 Watt resistor between the two flea-clips outlined in blue.

racemor · 09-01-2005, 01:56 PM

Thanks Warren, this newest diagram helps. I know I mentioned earlier that electrics are over my head sometimes, but it's mainly the big schematics that indimidate me. I've done a little of this kind of thing in the past, I may take a shot it myself.

racemor · 09-01-2005, 07:21 PM

Warren, I have one last question. I don't know understand the date codes on the Bosch boxes very well, but I have one more 001 box I can try. The date code on this last box is 422. Would this be a match internally to my original 001 with the 226 date code?

Early_S_Man · 09-01-2005, 07:56 PM

Tom,

The date code is expressed as 'YWW,' where Y is the last digit of the year, and WW is the week of production.

No, '422' would be a 1974 production model unit built in the 22nd week of the year. It would have the 120 Ohm input pullup resistor rather than the 33 Ohm resistor in your '72 model.