Factory voltmeter issues

[driberif19]

Hi guys/gals.  I've searched and can't find anything describing this.  I swapped a cluster from an 81 k20 (gauges and tach) to an 86 k10 (no gauges, no tach).  I've done the repinning and had to modify the housing a bit for the cruise control, but other than the voltmeter, it's all working.  The volmeter reads correctly with truck on, not running.  Once the truck is started, it goes flat like the truck was off.  The truck seems to be charging fine.  Here is what I got with a multimeter....
When I check at battery it is pos and neg posts.  At alternator is using ground and the white wire, which changes to brown and ends up at pin 3 on the cluster connector.  At cluster is using Pin 3 and Pin 4, both of which are brown (or black hard to say). 
With ignition on, truck off...At Battery 12.3v...At Alternator 0.7v...Cluster 11.2.  Gauge on truck reads about 11.
With truck running...At battery 14.4v...At Alternator 15.16v...Cluster 0.53v.  Gauge points at 9 o'clock just like when the key is off.
Any help is appreciated.  Thank you.

86 Chevy K10 4wd.  350 4bbl quadrajet.  Manual transmission.  3" Pro Comp lift with 33x12.5 General Grabber AT2.

[bd]

Welcome to the site from Central California!

The volt gauge needs to have ignition power and ground.  If you are trying to ground the gauge through the alternator #1 regulator connection, therein lies your problem.  The volt gauge should have 12-volt ignition power to one post and chassis ground to the remaining post.  From your description, I think you have ignition power to one gauge post (this is good), but the other gauge post is connected to the #1 regulator 'excite' terminal of the alternator (not good).  With the key on and engine not running, the gauge "grounds" through the regulator and registers ~12 volts.  However, when the alternator begins charging, that #1 regulator post becomes 'hot' and ~14-volts is supplied to both sides of the gauge, so the gauge stops registering.

To correct this, you can reroute the alternator lead that connects to the gauge, so that the lead instead connects to a 12-volt ignition source through a 1N4007 diode and a 50+ohm, 2-watt resistor.  The diode is polarized and should be installed so the end of the diode with the silver band is closest to the alternator.  On the other hand, the resistor is not polarized and can be installed on either side of the diode, pointing in either direction.  The diode prevents the alternator from trying to charge the battery through the small lead; the resistor limits the current flow through the regulator circuit before the alternator starts charging.  Power to the #1 regulator connection is what turns the alternator ON, so you need a controlled power source to that terminal.  The #2 regulator terminal should connect directly to battery power.  Other methods can also be used.

To correct the gauge operation.  Reconnect the gauge post that went to the alternator, instead to constant ground.

[driberif19]

Thanks for the response.  I was using the wires that were for the battery light for the voltmeter.  Makes sense it wouldn't have voltage when the truck is running, or the light would have been on all the time.  I plan on reinstalling some indicator lights where the PRNDL indicator would go, so I still want to use those wires for the light.  According to the wiring diagram, it looks like it should be pink/black and black wires for the gauge.  I believe pink/black would be ignition hot and black would be ground.  I shouldn't need the resistor and diode done that way, correct?

[greygta]

Also, does anyone know where to get replacement needles?

[bd]

Thanks for the response.  I was using the wires that were for the battery light for the voltmeter.  Makes sense it wouldn't have voltage when the truck is running, or the light would have been on all the time.  I plan on reinstalling some indicator lights where the PRNDL indicator would go, so I still want to use those wires for the light.  According to the wiring diagram, it looks like it should be pink/black and black wires for the gauge.  I believe pink/black would be ignition hot and black would be ground.  I shouldn't need the resistor and diode done that way, correct?

• Pink/black is generally ignition Ckt 39; black is generally ground Ckt 150 - together, they will power the volt gauge.
  
  
• You will be able to connect the original #1 lead from the alternator to one side of a "charge light" as long as you have "12-volt ignition" to the other side of the light.  However, you'll need to employ an incandescent bulb (not LED) for the charge light in order to correctly 'turn on' the regulator, otherwise you may experience delayed alternator charging.
  
  
• Regarding your reply, "...Makes sense it wouldn't have voltage when the truck is running, or the light would have been on all the time," in fact, at the point the alternator begins charging, voltage is applied to both sides of the "factory charge light," current stops flowing through the bulb, and that's why the charge light goes out.  Until the alternator begins charging, the bulb actually "grounds" through the regulator, and the bulb illuminates.
  

[driberif19]

Wanted to thank you bd for your help.  I havn't had a chance to get back to work on the truck, but that all made sense and I think I have it figured out...in my head anyway.  Another question I have is I have an 80lb and 60lb oil pressure gauge.  Will either work?  Do they use different senders?  Any preference?

[bd]

...Another question I have is I have an 80lb and 60lb oil pressure gauge.  Will either work?  Do they use different senders?  Any preference?

Either gauge will work, but you'll need to match the sending unit to whichever gauge you choose.  The calibration pressure of the sender is stamped into the 9/16" hex nut at the threaded end of the sender:  "60" for 60 PSI, "80" for 80 PSI.  Unless you've installed a high-pressure oil pump or spring, the 60-PSI gauge is more than adequate.  Actually, availability of the 80-PSI sending unit may be the determining factor - it's becoming increasingly rare.

[driberif19]

Got that oil and alt gauges working.  Thank you very much.

[bd]

Cool!  You're welcome.