Adjust reading on tach, how?

[jb86]

I posted a couple of months back that my tach was too low
I have had a fair number of issues with the new engine
Some related some not
Oil pressure light and buzzer coming on on the highway for 1
I had the white sending unit instead of the black 1 for the turbo diesel
Changed it and it's fine
Oil pressure reads correctly
The tach still reads low
At idle it's about 400 - about 50%
As I increase speed it gets closer to correct
At 2000 it's maybe 2500 or 3000, I think
I read that (I believe) fatmobil had a fix by replacing a resistor
And another member used an adjustable pot
I looked over the pix but couldn't nail it down
Could I get a more specific instruction?
Please tell me the location of the pot to install, what rating, ect
What I could do is run a lead wire off the terminals so I can remotely locate the pot to adjust while the cluster is in place
Any help is appreciated
Jon

[CRSMP5]

tach has to be out of the cluster to get to the pot... its hidden behind the white plastic that holds it into the cluster too.. so not as simple as you think...

[jb86]

I'll take it out
I saw a pic of the resistor
Just hoping for a little more info
I haven't put the bezel back on
Wires and plugs just hanging
I expect to take it out
I'm still working out bugs
This just happens to be one of them
I always hated trying to hook the speedo cable back up
But recently replaced the cable in a friends 85 turbo diesel
I found all I need to do is push up the plastic end to line up
He pushed from the engine compartment to push cable back in
It pops right in in a second
I've fought with that darn thing for so long doing it alone
Now I can get it in pretty quick
Jon

[92EcoDiesel Jetta]

I have a photo tach. If you are in CT or NYC I can calibrate the tach for you.

[jb86]

I'm in central mass north of worcestor
I found "rpm by sound"
A free program online used to calibrate aircraft tachs
I figure I could get it close by trial and error
I wouldnt mind taking a trip
Thanks for the offer
I may take you up on that
But
I need to know the resistor to change
Jon

[92EcoDiesel Jetta]

Technically it's a resistor but is called a trim pot which is a variable resistor. Do you know what a variable resistor look like? IIRC, the one in my tach was round with a small slotted screwdriver adjustment. Please post a link to the rpm by ear app. Just curious.

[jb86]

http://www.tunelab-world.com/rpmsound.html

They have an app for iPhone that you have to pay for
But this one is free
I put it in my laptop I figure I'll try that

I have some understanding of electronics
Years ago I bought a radio shack book "getting started in electronics"
It goes over all the basics
I've practiced all sorts of things on my bread board
I've installed LEDs in my car - trunk, backseat area, ect and used the formulas for resistors
I've got trays full of pots, resistors, transistors, ect
I can do it
I
Just need some specs
Jon

[92EcoDiesel Jetta]

That's a cool app. It'd be interesting to compare its accuracy to the photo tach!

[92EcoDiesel Jetta]

Here's on for Android for $1.30

https://play.google.com/store/apps/details?id=org.rcteam.rcheliheadspeed&hl=en

[jb86]

I've got the cluster out
I took the mylar film off
Pulled speedo
Where is the resistor?
It's inside some where
I can't get the tach off the board
Can someone tell me what I'm looking for?
Any help is appreciated
Jon

[92EcoDiesel Jetta]

trace the wiring from the tach movement to the ic chip. the pot should be near this chip if there is one. I forget the part number of this chip but it should be LM xxxx. If you search my post title "a real tach" I posted the chip part number towards the end of the thread.

[92EcoDiesel Jetta]

The chip on my tach is SAK215, not LM xxxx as I said earlier