Tach Question - Troubleshooting - VWDiesel.net The IDI, TDI, and mTDI source.

• Tach Question by dubdiesel1 on 03 May, 2009 07:58
• Guys, here is another tach question.  The tach in my 91 Jetta TD will only come up to 250 RPMs, when engine is running, when the key is turned off the needle falls back to the peg.  Every now and then it will begin to read accurately for 20 or 30 miles, before it falls back to 250 RPMs again.
  
  I have checked and cleaned all the wiring from the "w" connection into the car and replaced the spad connections in the harness which attachs to the altenator and the wiring harness to insure clean connections.
  
  The tach worked fine until the altenator stopped charging and I had to replace it. I have tried two new rebuild altenators, which both charge ( around 13.7 volts), but with both new altenators I am having this same tach problem.
  
  Is it possible that I could have gotten two rebuild altenators from the local parts store which will charge, but will not be sending the correct signal to the tach?  
  
  I dont think the problem is in the dash itself.  All lights, speedo, fuel gauge, etc works perfect except for the tach.  
  
  Any thoughts?
• #1 by jettabrendan on 03 May, 2009 09:40
• My cars do this while the belt is squealing. Your belt might be toast or too loose. Rev it while its in neutral and are if it catches. My money is on your belt.
• #2 by jettabrendan on 03 May, 2009 09:48
• And make sure your blue exciter wire is hooked up to the right prong.
• #3 by dubdiesel1 on 04 May, 2009 05:13
• Belts is new an tight.  Problem is in the Altenators, both new and neither one is charging.  Drove the car 30 miles yesterday and the tach fell to 0, checked the new altenator with a mulitmeter when I got home and it is not charging at all.
  
  Should have the third replacement on tuesday.
• #4 by Vincent Waldon on 04 May, 2009 08:00
• Two dead ones in a row does sound a little strange though.
  
  Does the "Batt" light on the dash come on when you turn the key to "on", and then go off when you start the car ?
  
  If not, your alternator is not receiving the necessary excitation voltage from the cluster and won't be able to start generating electricity on its own.  This will also mean that the tach won't work.
• #5 by dubdiesel1 on 04 May, 2009 11:43
• The battery light does come on when the key is turned on and it stays on  after the engine starts until the normal throttle blip.
• #6 by Vincent Waldon on 04 May, 2009 11:59
• Okey dokey.... that means the wiring to the alternator is fine.
  
  Hmmmm... third time's a charm ?!  :wink: