TACH back from NH Speedo, still noworking..

[Porsche Head]

I spoke with the guy who rebuilt it (Ed) and he said to connect it to the panel in the back, straight to the 12+/-/TD (on 6 pin). I did all that and no tach.

I don't want to ship the dran thing out again! Any ideas? What if the 12+ and the - were crossed at some point could that ever have shorted it out?

Can I test the TD terminal with an ohm meter, say once that it should read zero on Permatune? Ed says that if it didn't work attached to the back panel it was the CD unit.

The tach did jump around a bit while attached at the front and moved more when I put the lights on, weird, yet I got nothing in the back.

Thanks.

[Porsche Head]

Man I hate this electronic S___t......

Has anyone had the TACH module part of the Bosch CD fail, if so how can we test for this?

I really don't want to buy a new CD Unit If I don't have too and don't want to send out the TACH again (getting frequent air miles).

Thanks!

[MotoSook]

I see you have a 3.0 in an early car...did you use a later tach? Did the early tachs have a TD terminal?

Put a scope on the TD signal wire..

[Porsche Head]

Hi Souk,

I took your advice (sort of) and got the tach modified at NH Speedo with a new board to match the "004" last digits of the ser# on the CD box. I had a nice 8k tach and wanted to keep it.

So according to NHS, all should be well now (like using late model tach). They think it's the CD unit not giving outr a tach signal, but I can't figure out how to tesst it, electrics are not my strength.

Any ideas? I heard that 0 ohms for a permatune TD (2) to (5) terminal is good anything about 1 ohm is not good, but that's not a Bosch unit.

Early tach did have a TD post and that was re-stamped by NHS.

[MotoSook]

This is a Warren "come save the day" thing as I hate electrons too, but if you put a voltage meter on the TD terminal at the CDI box (+=CDI box, -=gnd) do you get anything? With the engine running, you should get some voltage, it's not a substitute for a scope, but it'll tell you if the CDI box is sending a signal.

Do the same to the wire at the dash.

If you have a bad lead or shorted that lead from the CDI box, to the tach..forget it.

Also, did you tap the CDI output to the correct wire (the one leading to the tach)?

[RallyJon]

I did the same thing. Bottom line is that NH Speedo doesn't have a clue. Their "special conversion to work with an 004 CD box" is nothing of the sort. Give it a conventional tach signal and it'll work fine.

Read this:
Bosch CD upgrade compatibility?

[jpnovak]

You should look for a signal coming from the dist to the CD and then from the CD to the tach. These will be 12V square waves with a small duty cycle. Set your MM to AC and look for about 3V on each signal. If you see no reading then you have a wiring issue or a CD issue. Easiest way to check the CD (assuming the engine is running) is to pop the cover off the 14 pin connector. Look for a signal on the black/purple wire (pin 9 IIRC). Then trace this wire to the next 14pin connector. Also look for continuity through the main harness to the tach. You will have to make a long jumper lead to connect up your MM.


I had a similar problem on my 3.0 swap. The CDI was had no tach output signal. Days later the unit failed. I have a new Crane HI6 system and it works great. I even converted the mag trigger in the dist to optical. Tach works great and I shouldn't have to touch it for the next hundred years or so.

Best of luck!

[Early_S_Man]

Cliff,

I am beginning to wonder about those NH Speedo folks ...

There is a 1N4148/1N914 diode D8 at the output terminal 'TD' of the Bosch CDI unit that could have failed. put a scope on the TD lead while the engine is running and hooked up to the tach ... pulse activity should be present if the output from the CDI is working and if the tach input circuit isn't open.

[Porsche Head]

Thanks guys, I too am beginning to wonder about NHS, sounds like Metric Mechanic thread

Warren I don't have a scope, but a friend of mine is an EE and offered to look at it so maybe I can borrow his scope for this.

RallyJon, seems like wiring it straight from the Dist to the tach is an easy solution but then what was done to my tach at NHS? If this is the way to go then what is an "OP-AMP" as indicated in your thread, can I buy one?

So at this point I still don't know if the CD or tach is not working, but I am getting closer....

[RallyJon]

All I did in EE212 was set black boxes on fire, so I'm not sure exactly what Bert did. His exact quote: "I ended up not using the tach output from the cdi at all. I took the signal from the distributor pickup and then used that as the input to an op-amp, which then provides the tach signal."

I think Early_S_Man posted earlier that he had something in the works, too.

[MotoSook]

OH! Jon! You just set me up for a sleepless night...op-amp! What are you thinking man...never say that outside of an EE lab...there are scarred individuals like me out there

[Early_S_Man]

Well ... the real problem out there for this board is that they just built too many kinds of 911s for too many years to make any generalizations about tachs ...

There are, excluding the cable-driven 356/550 tachs:

1. The real early tachs, before CDI ignition ... 356 and 911/914 ...

2. Early CDI tachs for use with ballast units ... '69 - '70/'71

3. The ~'71 or '72 - '77 tachs for use with point-triggered CDI ...

4. The '78 - '83 SC tachs for use with 6-pin CDI units with 'TD' output terminals ...

5. Turbo tachs for use with 8-pin CDI units, and later, 6-pin CDI units ...

6. 3.2 Carrera DME tachs ...

7. 964 DME tachs ...

Ideas I have been working on for adaptations to 3-pin and 6-pin Bosch and Permatune boxes ... using a variety of chips or discrete devices, including ... HexFETs, Darlingtons, CD4049UB, CD4050UB, SN75450B, and a few old, oddball op-amps like the Fairchild uA740 ...

[Porsche Head]

Great summary Warren.

Because I am away all week I can only test the CD Unit this weekend, I am still getting a feeling that it could possibly be the CD...can't wait to find out!

I may also run the tach over to Palo-Alto speed to pay a few buckeroos to get it checked there rather than shipping it out to the boys at NHS.

I did a Google search on OP-AMP, be verwwwy afraid, you EE guys are scary



I only realized after I sent the tach out that PP also will rebuild it, sorry guys.

[Lorenfb]

Pelican Parts has many sources for rebuilt parts and also provides this web site
which I don't think NH Speedo has. Do they?

"I did a Google search on OP-AMP, be verwwwy afraid, you EE guys are scary"

There are NO op-amps in any Bosch/Porsche CDI unit, so let's not make
the problem more complex than it is.

If appears that you have a '72 911 which used a 3-pin CDI that didn't
have a TD output. The early tach requires a high voltage pulse ( 250+ volts & 200us).
The later 911SC tach uses a 0-6 violts negative pulse. Hopefully, NH Speedo
did the proper convertion for your CDI.

Check out this web site (www.systemsc.com) on the Diagnostics & Technical
pages for additional info about CDIs & tachs.

The tach output of the 6 pin CDI rarely, if ever, fails.

If you have a tach which interfaces with a 6 pin CDI, you can use a rough-cut
file connected to ground and "run" a jumper across it connected to the TD input.
This will simulate a TD grounding pulse from a Bosch CDI. If the tach needle doesn't
move proportional to the rate of movement across the file, the tach won't work
with a 6 pin CDI.

[Porsche Head]

---UPDATE---

I have been working on this tach issue (yes still) offline, though it's always nice to provide feedback, plus I am not 100% there yet

--- here is an extract to NH Speedo on my status thus far ---

Hi Ed,

it's Cliff from San Jose, I had the 72 911E w/3.0 and what seemed to be a faulty CD TD output.

Well I could not get a new Permatune Coiless unit to work till this weekend, seems the TD wire (BLK/PUR) to the 14pin connector was not properly placed. I saw the blk/pur wire enter the body on pin#8 (chassis side) and the TD was going to #12 (engine side) which turned out to be a ground to alt, so the CD failed till I pull off the TD wire.

With the TD wire disconnected at the 14 pin (before it gets anywhere), I still had a tach signal and it was pretty good, a little slow to respond, and when I turned the headlights on the tach would drop to 0. Why even a signal at all if the TD wire is disconnected basically at the 6 pin on the CD unit?

Now I placed the TD to the #8 pin (engine side) and the car runs great but the tach seems to work some times fine then when I idle the needle just drops to 0. It also will swing with the turn indicators from 0 to 3k and seems to be effected by the turning on/off of the lights as well. Any ideas, I think we are getting closer, maybe a bad ground so whenever the signal is too weak?

Regards,
Cliff

[Lorenfb]

You should have sent the tach to Pelican which would've also provided better followup support.

[Porsche Head]

Lorenfb, I know, I know.....

I realized after when I was looking through the online catalogue that Pelican does that. There are still plenty more purchase opportunities and I have purchased quite a few parts already...amn you guys are a tough crowd

You see it could still be a wiring issue so I am not going to lay blame till I flush it out properly, that's fair.

[TRE Cup]

hi
went thru all this with a tach on Friday and we worked it out with Ed at No Ho speedo. Send your tach back and he will fix it. Ours would work on the bench but not in the car. They come over and test Porsche guages on cars when bench testing does not yield results

[Porsche Head]

Hi TRE,

thanks, not sure how to read your statement however. I trust it's me not you, I've seen your posts for quite a while now and you always provide valuable input

If I send it back and they said it bench tested okay then what will bench testing it again do? It's a bit far to pop over and have em look at it, however it's soon becoming an option.

I will take one last look at some old posts on which pin on the engine side to put the blk/pur TD wire, I have it on #8 now cuz the chassis 14 pin the blk/pur going to the tach is #8 there, seems I remember some comment about #9 when you switch over. hmmmm

[Porsche Head]

Hi TRE,

just got off the phone with Ed, seems your problem was the 3 pin CD, I have the Permatune 6pin and the upgraded relay (thingy with the fuses and CD attached

I am going to go back to test the tach at the engine and go from there, unfortunately gotta wait till the weekend again.

Thanks for all your help