Changing the 65 Idiot Light Dash to the 66 Guage Dash

Not as complicated as you may think.....

NOTE: This web page is meant to describe what it takes to convert the wiring of an existing 65 with indicator lights over to a 66 configuration with the 5 gage cluster. It is NOT as simple as just changing out the instrument panel. If you opt not to change over the charging circuitry, but would rather leave the ALT lamp and just stick it behind the dash or something... then by all means have at it, as it is your car and your memory of the wiring status. There is probably some other web page that explains how to do that method, but it is not addressed here.  I personally believe the best approach is to accurately rewire the engine compartment completely to the 66 configuration.

Shopping list:  - Books:  First, go and buy the Jim Osborn Wiring Diagram Manuals for both the 1965 and 1966.  They are invaluable for working on these cars.  You'll basically be comparing the wiring between the two and converting from the 65's to the 66's.  Also, the "Mustang How-To, Vol.1" book's article on "How to Convert 1964 1/2-65 Mustang Idiot Lights to Gauges" is where I learned to do this.  Here are the things you'll need to make the swap.  That, and a few hours......

'66 Gauge Cluster - Swap meet 16 ga. wire - Red 
Repop Bezel 16 ga. wire - Yellow
Repop Lens Set 16 ga. wire - another color
7 New Light Bulbs Assortment of Wire lugs
Fluorescent Orange Paint Heat shrink tubing - Clear
2 - Single wire sockets (turn signals) - Swap meet Heat shrink tubing - Black
Oil Sending unit #10-32 nuts (4)
Sending Unit Extension
Water Temp Sending unit

The 'new' dash unit - '66 gauge clusters used to be a dime a dozen at the swap meets.  They were the bait that the vendor uses to tell you what kind of car he's parted out and is selling.  They can run anywhere from five to fifty bucks for the exact same unit.   They are getting increasingly harder to find.  EBAY regularly has them posted and typically go for between 20 and 40 depending on condition.  So shop around.  Look them over closely.  Forget about the front bezel and the lenses because you're going to replace them anyway.  You're after the internals and the back housing.  Make sure the units are relatively clean, straight, not been sitting out exposed to the weather.  Housings with rust on the back are typically not a good sign.  You can find one out there that's not rusty.

After you find your cluster, buy a fresh bezel, a new lens set, the paint, and 7 light bulbs.  The repop bezels will run around $30 plus another $10 or so for the lenses.  So, your initial investment should be somewhere around the $60-$80 range.  There are different bezels for different applications.  The 65's that had the gauge package as a factory option had a bezel with a half circle trim ring along the top side of the bezel.  They came in camera black and walnut trim.   The 66 bezel had a full circle trim ring along the perimeter of the bezel.  (Hold up the 65 and 66's side by side and you'll see what I mean) 

The night after you get it...go ahead and annoy your wife by taking over the kitchen table as your work table, to disassemble your new/old 66 gauge unit.  Inspect it carefully for broken wires or other damage.  Handle the gauges carefully because, for example, the Ammeter gauge is $80 to replace. (Been there, done that....) Clean the gauge face of the unit with a soft damp rag.  Put paper under the needles and touch them up with the bright orange florescent paint.  Reassemble using the new lenses and bezel.  If you're going to install white faces, now's the time.

Water Temperature - This shouldn't be much of a change, unless yours was like mine and years of previous modifications had crept in...I had the Pep Boy's Special 'Triple Gage Cluster'...installed under the dash.  So, I had to recreate the original setup.

Sending Unit - The standard FORD water temperature sending unit (electrical) needs to be installed in the intake manifold, with a Red-while lead from there to the instrument cluster. (It doesn't HAVE to be Red-white... that's just the color of the OEM wire in the harness.)  If your car hasn't been butchered...then this should be the simplest part of the gage pack swap...the setup is identical...hook up the same as on the TEMP light.  The wire will lay on the intake manifold.  I slipped two sticks of heat shrink over the wire to add additional insulation over the intake.

Oil Pressure - Not identical. The 65 has an oil pressure sending unit made for the OIL light, which is different than the one for the 66 gages.  

Sending Unit -  Unscrew the oil sending unit from the block (Drivers side, just behind the fuel pump. You'll need to install the oil sending unit extension, which looks like a piece of hex bar stock with threads on each end ('cept the female end is cocked slightly).  Then screw on the Oil gage sending unit on the end of the extension.  Position it so that it clears everything around it.  Don't forget to use some Teflon thread tape on the extension and the sending unit.   

Wiring - You'll also need to change out the connector on the sending unit end of the wire, as it needs a lug on it now to fit the screw on stud.  This white-red wire goes through the harness to the back of the gage, but has the oil light lamp socket connected to it.  Cut off the lamp socket and splice in a bit of wire with a lug on the end to reach the Oil pressure gage.  This wire connects to the radio side terminal of the gage.  The other side of the gage requires a wire that runs to the CVU.

Gas Gage - The Gas Gage on the 65 is the same hook up as on the 66.  The Yellow-white wire from the tank goes to the speedo side post, while the other side of the gage runs to the CVU. 

CVU - The constant voltage unit.  This unit drops the voltage down from 12 volts down to 5 volts for your gages to operate.  Click here to see how they work.  Take the unit off the back of your 65 dash and screw it to the hole just to the fuel gage side of the speedo.  The dash ground wire attaches here also. (I made a 4 " or so pigtail to screw down here...making it easier to attach to the ground wire on the dash brace.)  The CVU is about $25 if you feel the need to replace it (I did)..... In retrospect, the  message board guys have come to the rescue, and Bradster says..."An LM340 will also work and provide ripple free voltage.  You may not be able to find the LM340 if you don't have decent electronics supply stores in your area, but even Radio Shack will have a "LM7805".  

The gas, oil, and temp all have one side that is tied to the CVU.  The other side goes to the appropriate sensor, and the variance in ground by the sensor makes the needle move.  I made a separate little harness, running from the CVU to the three gages and hanging it on the back of the gage pack.  Connect the black-green wire from the main loom to the straight up & down post of the CVU.

Turn Signals - Slight mod required, as the 65 has one lamp for both left and right.  The 66 has separate lamps for left and right.  Take the turn signal lamp socket currently in the existing 65 harness and cut out the existing socket.  Then, splice in the single wire lamp socket you found at the swap meet to each of the cut leads.   The left turn is the Green-White wire, while the right is the White-Blue.  Where do you get these single wire light sockets? (These are the ones that have the little metal teeth around the socket that holds it in place in the metal dash housing....also grounding the lamp and completing circuit.   I got mine at the swap meet... I picked up a ragged old wiring harness out of some old GM product and the guy was trying to pay me to take it...but I insisted on paying him $1 for the harness.  I was able to salvage off a few connectors and the required lamp sockets.)

Lighting - Guess what.... the dash is going to be dim...even after you clean out the little blue green bulb lenses, put in brand new bulbs, and make perfectly soldered and fully grounded wiring connections.  But if you DON'T do all those things, they will be even DIMMER.  So polish out the little bulb lenses, put in all new bulbs, and make perfectly soldered and fully grounded wiring connections.  This is where those bright orange needles you touched up come into play. There are 4 lamp sockets.  Make sure they all work before you put the thing back together and screw in the dash.  MustangSteve offers up two additional suggestions: 1) add a couple of lights to the assembly by drilling holes in the back of the gage housing at symmetric points and wiring in two new sockets, and 2) simply removing the blue-green lenses.  Both of these increased the lighting with no additional glare.  Good advice Steve. 

High Beam Indicator - no biggie...plug the lamp with the NEW BULB into the center socket in the new dash.

Ammeter (not voltmeter) - The Ammeter us just measures the amperage flow (duh).  It's sort of a bridge between the alternator and the battery to show you if the system if charging, or drawing current.  This is not the same as a modern voltmeter, which tells you the output value of the alternator. 

Disclaimer - If all you intend to do is replace the gage cluster but leave the charging circuitry for the 65 intact, then stop here.   Leave the ALT lamp wired in and hide it somewhere (preferably some place that  you have a chance of seeing if you ever develop charging issues.  Behind the heater controls has been suggested before.)  If you decide to simply hook the Ammeter to the Alternator and the Battery and not adjust the rest of the wiring, I have no idea how that will work.

However, if you intend to actually complete the rewiring of the 65 into a 66 configuration, then read on:

Here, you're doing away with the 65's charge indicator light circuit, and running new red/yellow wires from the back of the gage pack to the + post on the starter relay (red wire) and to the large lug (BAT) of the alternator (yellow).  Note:  This yellow connection serves as more than just a pure gage lead.  It moves the 654 pigtail connection to the BAT post of the alternator.  On the gage pack, be sure that the red goes to the speedometer side term, and the yellow goes to the radio side. 

Charging System - Connection Layout

I:  Green-Red (904): This was to the "idiot light" at the dash.  "I" post from the VR is no longer used, but the Green-Red will be rerouted.
A: Yellow (152): This is connected to solenoid with battery cable
S: Was: White-Black to Alternator STA lug.  Disconnect White-Black from Alt and stow.
Now: Splice White-Black at VR into existing Green-Red (904).  Disconnect Green-Red under dash from idiot light and connect lead to center post of Ignition switch.
F:  White (35): Connected to field terminal at the alternator. (No change)
STA: No Connection
FLD: White (35) from VR
GND: Black-Red (26) from regulator (actually this and the wire (26) from the VR join and ground to chassis).  Do not join VR and ALT without grounding.
BAT: Black from solenoid
Yellow (654) from Ammeter
NEG: Red (655) to BAT post of solenoid
POS: Yellow (654) to Alternator -BAT post
SOLENOID - Incoming BAT Post:
Battery Main cable
Blk: To Alternator BAT post
B-Y: Black-Yellow (37) to main connector to fuse box
Yel: Yellow(152) To "A" terminal of VR
Red: Red (655) To POS terminal of ammeter

I had some trouble following the "How To" article on this part...basically after I got through, the alternator wasn't charging the battery...soooo...I finally figured out that the article left out one small detail about the hook up of the voltage regulator..... The 65 has a white-black wire running from the common post of the Alternator to the Voltage regulator.  This wire is not on the 66 schematic.  You have to remove it and splice in the Green Red. 

Hopefully these schematics below will also help in understanding the difference:

Other General Info/hints:  

  1. Test everything...twice.  Take a spool of wire and run a lead off the + of the battery through the passenger window, and use it to test all your light, gages, etc., before final closeout.  make sure all your lights work, ground...dim...etc.
  2. Cut yourself some slack.  There's nothing worse than trying to install the dash with only 1" of clearance to put your hand into and make the last few connections.  Too long is ok...
  3. When in doubt, pig it out... I hooked up everything I could with crimped on pigtails.  wires wrapped, twisted and covered with electrical tape are NO-NOs.
  4. Get down to the ground.  Put a 12awg wire nailed to the center dash brace with a pigtail connector to plug into.  
  5. Dress to impress.  Take your time, use wire ties to pull wires up and secure.  Use heat shrink over soldered connections to insulate rather than electrical tape.  Use crimp on connectors where ever lugs or spades are used.  Use stick on labels and mark your wires, then cover with clear heat shrink.
  6. Wires through the firewall - I ran new wires from the sending units to the gages on all except the gas gage.  When running new wires, feed them through existing holes if possible and protect them so they won't chafe, cut and ground out.  Use grommets or some other insulators around the feedthrus.

Ok...there you have it.  I wish I'd taken pics when I did this mod last year.  Maybe the next time I have the dash out, I'll take some and add them to the page.  If you have any questions, email me.

James W