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1.6 TD tach over-reading on AAZ engine - FIXED
Otis2:
FYI
I have an '85 Jetta TD tachometer in my AAZ-powered vehicle. I have the original 90 amp alternator and stock belts (no A/C) from the '97 Jetta this engine comes from.
For a while it read close enough to what I expected, that I thought it was working properly.
But recently I had to re-wire the tach, and after that it was overreading by more than 1000 rpms, even though I had the potentiometer screw bottomed out to the lowest needle reading possible on the tach.
I read the epic regcheesman & fatmobile threads on gasser tach modification, and asked those folks some questions, and now I have the overread fixed.
I de-soldered and removed the 13K resistor from the 1.6 TD tach. Its the "brown orange orange" resistor on the right-hand side of Fatmobile's photo:
I then wired a resistance decade box into the two vacant poles left by that 13k resistor. Then I went for a test drive, using a GPS and adjusting the switches on the decade box to force the tach needle to show exactly what RPM should be shown for any given speed. It is so much easier to flick switches on a decade box than to play with that potentiometer screw on the back of the tach.
Instead of a 13K resistor, the value that most closely corresponded to the accurate RPM was 8260 ohms.
So I post that value for the benefit of anyone who is doing the same project (1.6TD tach into an AAZ powered vehicle, with 90 amp alternator & W signal).
Now I will buy resistors to equal 8260 ohms, and put that into the circuit instead of the decade box.
Here's a photo of the decade box I was using:
Remember that number, 8260 ohms. The alternator pulley on the AAZ cars is obviously a different diameter than the one used on the 1.6TD cars.
regcheeseman:
I'll guess that the pot was on 0 ohm and not maxed, But the total resistance would be good.
8.2K would be the nearest resistor to use
Though I'd probably go for a 6.8K with 2.5K pot
fatmobile:
Glad to hear you got it figured out.
That's a lower value than I would have guessed, the pully probably is larger, like you said.
Did you have the pot set to center?
jspurr:
i have a 1.6 tach in my aaz and so what your saying is if i find a 8.2k resistor and switch it with the "brown orange orange" resistor that will fix my issue with the tach?
Otis2:
--- Quote from: "jspurr" ---i have a 1.6 tach in my aaz and so what your saying is if i find a 8.2k resistor and switch it with the "brown orange orange" resistor that will fix my issue with the tach?
--- End quote ---
If you are grossly overreading like I was, then I think "yes", that will be pretty close, but read on.
When I bought some 8.2K resistors and tested them, they were all around 7.9K on my multimeter, so that wasn't going to work precisely, just as regcheeseman warned me. And although my decade box was set for 8260, when I tested the decade box with one multimeter it showed 8200, and with another multimeter it showed 8300. So who knows which of the three ohm values was "exact". So I could see that I could not just throw in a regular store-bought resistor and have it be precise for the tach needle position.
Instead, following regcheeseman's advice, I bought a little "10-turn adjustable 10K resistor" at my local electronics shop. It has a tiny potentiometer screw on the top that allows ten 360 degree turns between 0 and 10K ohms. That's far finer resolution than the pot screw on the back of the VDO tachometer. This was exactly what I needed.
I soldered this thing to the long speaker wires which I already had soldered into the empty poles on the tach circuitboard, where I had previously removed the 13K B-O-O resistor.
This 10-turn 10K resistor had enough resolution with its pot screw that I could get it pretty much exactly to 8260 ohms (or at least, to what my decade box said was 8260 ohms). Much easier to adjust in my lap on the end of those speaker wires, than fighting to get a tiny screwdriver in to the back of the tachometer's pot screw.
Once I got it set to match the decade box settings, I just folded up all the excess speaker wire, tucked it under the instrument cover, and snapped it all back down.
If you were really fussy for originality you could solder the 10-turn 10K resistor directly to the circuit board on the tach. It's small enough that it would probably fit in the space vacated by the 13K resistor and be completely hidden, but it would be impossible to adjust on the fly in that spot.
In answer to regcheeseman's point, I'm pretty sure my pot on the tach circuitboard was bottomed out to 0 ohms, but it was still massively overreading before removing (and replacing) that 13K B-O-O resistor. The original tach pot screw remains bottomed out now, with the new (approx.) 8.2K resistor arrangement in place.
Obviously the way these tachs work, the greater the resistance value between those poles where the 13K resistor lives, then the higher the needle will read on the scale. 13K was way too high for my alternator pulley, but 8.2K (or thereabouts) was just right. If somebody had the opposite problem (ie. tach needle reading too low) then I expect that they could probably solve it by putting a higher value of resistor into that position.
Final note - my tach's circuit board was very slightly different than the one in Fatmobile's picture, even though I think it's from the same year 1.6TD car (it shows "Diesel 2,22" on the face). The 13K B-O-O resistor on mine was mounted at a 90 degree angle relative to his.
If you're going to do this mod, just use an ohmmeter and test the resistor that you think is the correct one, and see if it registers 13K. If so, that's it, pull it out and replace with a 10-turn 10K resistor, and dial it in to about 8.2K to start (as tested on your multimeter). That should be really, really close. If you want "perfect", then you'll have to play with the tiny pot screw on the 10-turn 10K resistor while using a GPS to hold a known road & engine speed in gear.
Here's a photo of the kind of adjustable resistor I used (although this one is a 10-turn 20K model). Looks huge in the photo, but actual size is about the size of a fingernail.
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