tachometer signal revisted, LONG

[Bernard Littau]

Hi Richard,

If I understand the problem, your tach wants 5 pulses for every 4 pulses 
given by your ECU.  If you plug your current ECU into the tach it reads 
20% low. 

If the tach can't be switched to four cylinder mode, and a new tach is 
not an option, my sense is you'll need to build a circuit to give out 
five pulses for every four input, spaced relatively in time.

I've not done this exactly, but my starting point would be to consider 
hooking a frequency to voltage converter to a voltage to frequency 
converter, and adjust the RC values to give a 4:5 pulse ratio.  I know 
this seems like a massively complicated way to solve the problem, but 20 
years ago when I dabbled with this sort of thing it would have been an 
afternoons entertainment to cobble this together. 

I did basically the converse, on many occasions, sampling a voltage 
source, converting it to a frequency and recording the frequency on a 
cassette tape recorder.  The tape recorder was then played back through 
a frequency to voltage converter to reconstruct the sampled voltage over 
time.  Today we would do such things fully digitally into a microprocessor.

A quick search on Google using: 'frequency to voltage converter', or: 
'voltage to frequency converter' brought up lots of hits to parts and 
data sheets.  You may be able to still purchase these things at Radio 
Shack, I know they used to carry them.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq

Richard Hoffman wrote:

>Alright so now that THAT theory is out the window....how do I make the 5-cyl
>tach behave correctly for a 4-cylinder, assuming that I am not using any
>factory ECU (reprogramming with 1552 tool might not work?)
>Thanks all,
>Richard
>----- Original Message ----- 
>From: "Phil Payne" <quattro at isham-research.com>
>To: <quattro at audifans.com>
>Sent: Thursday, December 04, 2003 4:49 PM
>Subject: Re: tachometer signal revisted, LONG
>
>
>  
>
>>>>The other is a 6-position switch, which reads:
>>>>
>>>>Diesel         ,   Benzin (gas)
>>>>RW,US,EG,RW,US,EG
>>>>
>>>>with the switch under "US" on the Benzin side. I think this has more to
>>>>        
>>>>
>do
>  
>
>>>>with the MFA system, but I could be wrong.
>>>>        
>>>>
>>>That's for the Autocheck. It switches between brake symbol, 'BRAKE
>>>      
>>>
>LIGHT'
>  
>
>>>and 'BREMSLICHT' on startup, for instance. RW must be Rest of World, EG
>>>could be European Community, US is self-explanatory.
>>>      
>>>
>>It switches quite a few things in the "Bordcomputer".
>>
>>-- 
>>  Phil Payne
>>  http://www.isham-research.com
>>  +44 7785 302 803
>>_______________________________________________
>>quattro mailing list
>>quattro at audifans.com
>>http://www.audifans.com/mailman/listinfo/quattro
>>
>>    
>>
>_______________________________________________
>quattro mailing list
>quattro at audifans.com
>http://www.audifans.com/mailman/listinfo/quattro
>
>
>  
>