wierd tach problems

[psyte]

since i bought my car (86 jetta 1.6L n/a diesel standard) the tach jumps all over the place and usualy hangs around 6 - 7000rpm's on the guage.  someone told me this car gets its readings from the alternator, anyone know why its doing this?  the red battery light stays on when i first fire it up and the tach is at 0, once i hit the gas the light goes off and the tach goes nuts.

[myke_w]

Try another alternator..

[burn_your_money]

The tach gets it's reading from the W terminal on the alternator. It's normal to have the rev the engine a bit to get the alternator charging, I wouldn't worry about that.
You might have a bad wire that's shorting out somewhere

My tach does not go anywhere near 7k


do you have a converted gas tach?

[myke_w]

possible it's gas tach that somebody stuck in there? Does it say diesel on the tach like the one  in the pic above?

[VWCaddy]

Yep, sounds like a gas tach has been installed.  It'll read many times the actual RPM if driven off the W terminal output:
- http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml#OEMInstall

Gas tach in a diesel will look like this:

[burn_your_money]

Deleted

[psyte]

mine looks different than both of those.  but mine deffinately doesnt say diesel on the tach anywhere so its possible it was converted.  it looks alot more like the first pic posted by "burn_your_money" exept theres a shift indicator at the top(wich also doesnt work) and the temp and gas guage is below the tach.  also theres no lights on the lower section of that little 10 LED cluster and its missing the temp light(wich is actualy in the middle of the temp guage instead)  why would anyone want to swap out the diesel guages for a gas one?

[burn_your_money]

Most diesel's did not come with a tach and for a factory look people will modify and swap in a gas cluster with a tach.

Could you get a picture of yours?

[psyte]

heres the pic

 [/img]

[burn_your_money]

That's a mk2 gas cluster. It must have been converted, or possibly just plugged into the W-terminal with no modifications, which is why you are getting the weird readings. Did it ever work?

[psyte]

nope, not since i bought it.  id really like to have a working tach, is it possible to find a diesel tach in a junk yard?  or can i just buy an aftermarket tach for this car at a parts store?

[VWCaddy]

nope, not since i bought it.  id really like to have a working tach, is it possible to find a diesel tach in a junk yard?  or can i just buy an aftermarket tach for this car at a parts store?

Yes to all the above.  Answers to how may be found on the web page I posted a link to earlier.

[Otis2]

Actually, those instructions on the www.4crawler.com page do not work.  

I'm surprised nobody has pointed you to the posts here on this forum by regcheesman before now.  

Anyway, better late than never.

http://www.vwdiesel.net/phpBB/viewtopic.php?t=3741

http://vwdiesel.net/phpBB/viewtopic.php?t=3096

Good luck.

[VWCaddy]

Actually, those instructions on the www.4crawler.com page do not work.  

I'm surprised nobody has pointed you to the posts here on this forum by regcheesman before now.  

Anyway, better late than never.

http://www.vwdiesel.net/phpBB/viewtopic.php?t=3741

http://vwdiesel.net/phpBB/viewtopic.php?t=3096

Good luck.

Added those links to the above page under the tachometer modification section:
http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml#OEMInstall

The other options still apply as well.

[Otis2]

The other options still apply as well.

Not sure that's true.  Have you performed those other options and tested them yourself?

In post 6 in the second thread I quoted above, when someone referred him to the gas-tach hacks on the 4crawler website, regcheeseman replied:

I was looking to do that to mine, and followed his instructions to the letter - they didn't work.  

I reverse engineered the board and the suggested mods and there was no reason why they would ever work. I emailed the originator of the gasser tacho mod and received no reply.

So figured - do it myself (I'm an electronics design engineer)