There is a very large class of people that own 65/66 Mustangs that, as far as I can tell, anyway, have been, for the most part, ignored entirely. They don't really want that 100 pt. show car that is so nice and was soooo expensive that they're afraid to drive it, they also don't want to make their car capable of achieving warp factor three. They just want this car that they dearly love to be able to cruise around smoothly and reliably, without having it dump them out on the side of the road or have it start making weird noises or belching out big clouds of funky-smelling smoke. And I think, truth be told, that this is by far the largest class of Mustang owners. They take their car to some technician when what they actually need is a mechanic, and this, frequently, does not work out very well at all for the owner. They don't want to re-engineer the entire car, they just want someone to fix what broke. These are the people that I am trying help out with this blog. Some problems require a little bit of back and forth, as in, "Try this." "I tried that and it didn't change anything."
" Oh. well, you probably need to try that." " I tried that and it helped, but it still isn't quite right." "Now you need to try this...." If you go to http://www.allfordmustangs.com/ and then go to the classics forums, you will be able to do that with a pretty hefty gathering of some very knowledgeable people that also happen to be very friendly. None of that ridiculous one-upmanship, no flaming or abuse, none of that stuff. Just good, solid advice from people that know what they are talking about.

Monday, March 31, 2008

Mustang gauges 1965 and 1966

This is the wiring diagram for the gauges on a 66 or a 65 that has either the GT (Performance/ Image Option) package or the Interior Decor Group, a.k.a. Pony Interior. If you click on the diagram, you will be able to see all of it, instead of just the left side. What I'm dealing with here will just be the water temp, fuel and oil pressure gauges. The ammeter gauge is a free-standing, independent system that has absolutely nothing to do with the other three gauges. The gauges on your car make up a system that supplies you with some very useful information when it is working correctly, and is a very simple system to understand and repair. What you have is a black wire with a green stripe on it coming from the back of the ignition switch going to the instrument cluster voltage regulator. The ICVR takes the 12V from the switch and, by means of thermally actuated contact points, knocks the voltage down to about 6 volts. The reason that Ford did it like this was because they had all of these perfectly good gauges of a proven level of reliability that were originally intended for a car with a 6V electrical system, and, rather than redesign and manufacture all new stuff, they just used a voltage regulator in the system. Ok, now that we have gotten that out of the way, you still have that black wire with the green stripe going from the ignition switch to the ICVR. On the other end of the ICVR you have a wire, still black/green that splits off to the three gauges. That wire plugs onto the what is the driver's side of the gauges when they are mounted in the dash. This wire brings power to the gauges. On the fuel gauge there will be a yellow wire with a white stripe that goes to the fuel sending unit in the gas tank. On the water temp gauge there will be a red wire with a white stripe that goes to the water temp sending unit, located towards the front of the intake manifold on a V8 and towards the rear of the cylinder head on a 6cyl. There will be a white wire with a red stripe going to the oil pressure sending unit, down near the oil filter. The wires going to the sending units work as a ground, with the sending unit itself determining how 'good' of a ground the gauge is getting.
If all three gauges are acting stupid on you, the problem is either the ICVR or the wire bringing the power to it. There is a dash to chassis ground wire that is hooked to the mounting screw of the ICVR, but that is what makes your dash lights and stuff go goofy on you. It doesn't have anything to do with the gauges.
If one or two of the gauges seem to be functioning normally and only one or two of them is having trouble, then the problem is almost always either the sending unit, the wire going to the sending unit, or the wire going from the ICVR to the gauge. Once in a blue moon it will actually be the gauge itself, but that happens so seldom that it really isn't even worth mentioning.
If the gas gauge is the one acting goofy the first thing that you want to do is to unplug the wire from the sending unit and, with the key in the 'On' position, ground that wire out to a suitable place on the car that isn't insulated from the rest of the car. The leaf spring shackles, for example, are in a handy location, but are completely insulated by the bushings. If the gauge pegs to full, the problem is inside the tank. The problem inside the tank is, far more often than not, the float isn't floating anymore. The brass floats cost like 5 bucks, whereas, a Ford sending unit costs nearly two hundred, and the repops cost about fifty and normally don't work. If the gauge does not peg to full, then the problem is either the wire going to the sending unit or the wire coming from the ICVR to the gauge.
If the water temp gauge is the one acting funny, again, ground the wire out somewhere and see if the gauge pegs. If yes, replace the sending unit. They also are cheap. If no, it's probably the wire going to the sending unit. People have the annoying tendency to cut that wire and then strip back both ends and tie the two ends back together. Don't do that. A brand new engine gauge feed only costs about twenty bucks, and you'll have brand new stuff that will work reliably for many years to come. If money's a bit tight, use some proper connectors of the male/female plug-in variety and wrap that to insulate it. All of what I just said about the water temp gauge is also true about the oil pressure gauge.
This isn't complicated. You can do this, and you can do it right.


billa said...

This is a great discussion of the gauges and troubleshooting all 3 of them.

Can you discuss your opinion of the solid state voltage regulator that is available?

Finally, why are they called "sending units"? They don't send anything. Really, in fact they are receiving current by dropping voltage through a resistor.

Thanks for the great tips throughout this blog, especially the wiring schematics!


Veronica said...

Thank you Bill. That is very kind of you. I've never actually used a solid state instrument cluster voltage regulator, so what I would have to say about them wouldn't mean much, but, I would assume that they were of a similar level of reliability to the solid state alternator regulators, which is, from my experience, quite good. I would also imagine that, if there are different manufacturers, some would be better than others, but, as far as particulars of the different manufacturers go, I really couldn't say. I know that Borg/Warner has always put a quality product on the shelves of auto parts stores, so, if they make a solid state instrument cluster voltage regulator, it's probably the best one out there.

That 'sending unit' title always did strike me as being backwards, also. I guess it comes from the way that the old positive ground systems worked, or possibly because, ultimately, it is responsible for sending you some useful information. But, it always did strike me as more of a receiver than a sender.

David said...


I have a weird problem with my gauges I hope you can help with. My gauges seem to read too high. Gas gauge needle is way past the 'F' when the tank is full. Also the temp gauge is pointing more towards the 'H' than it should be (alomost all the way) even thought the water temp is within range. I can live with the gas gauge, but I'd like a reliable temp gauge so I know if its really running hot.



Veronica said...

Hi David.
It sounds like it might be time to check the instrument cluster voltage regulator. It should be allowing about 6V with the key on at the wire that splits off to the gauges. If the contacts are starting to get a little sticky in there, it would make the average voltage be a little higher than it should be, and, consequently, the gauges would read a little high. It could also be that the alternator regulator is going bad on you. If it's allowing the full output of the alternator into the system, that could also have the gauges read high. I would check the instrument cluster regulator first, though.

James said...

Any thoughts on a gauge cluster (1966) that seems to be operating as the car is idling in the garage/driveway but after driving a while they all just go dead? I changed the VR thinking that may be the issue but obviously not.



Veronica said...

Is the car still doing exactly the same thing now that you've changed the voltage regulator, or is it still messed up, but acting a little bit different? If it's still the same, I would trace that black wire with the green stripe from the back of the ignition switch that supplies power to the voltage regulator to see if it was bouncing around and shorting itself out on something while your going down the road. You could simulate that by pulling the gauge cluster out enough to get your hand on the back of the ignition switch, but leave every thing hooked up, and wiggle the back of the ignition switch around some, then wiggle the wire itself around, and see if you can duplicate that failure sitting in your driveway. Also check and see if the turn signals no longer work when the gauges die.If no, it's probably a problem with the ignition switch itself.

If the gauges still act goofy, but in a different way than before you swapped out the voltage regulator, it could be that you have a defective voltage regulator.

Mauri Banycky Norman said...


I just purchased a 66 Mustang and found your column. It's excellent. I am having a similar problem. Occasionally my gauges all die along with the turn signals. The problem is intermittent. I put on new ICVR in and still have the same problem.


Veronica said...

Thanks. I try to help out as much as I can.
If they work sometimes, then don't work, and they are all three working and failing at the same time, the problem would have to be the power source to the instrument cluster voltage regulator, since that and the regulator itself are all that they have in common. The problem could be the black/green wire, or the connector that attaches it to the regulator could have a bunch of corrosion, it could possibly be the connector on the back of the ignition switch, but, it will almost certainly be something pertaining to that wire.

Shane Fellows said...

You blog is great! This post just solved my problem... your recommendation of looking at the black/green wire from the back of the ignition was spot on. My gauges kept on going out intermittently, and it turned out that the little nut that tightens down that wire had just come loose. So happy that is fixed. Thanks!

Veronica said...

Thank you, Shane. That is very nice of you to take the time to say that. It's nice to know that I'm not just in here talking to myself. (insert smiley face)

Unknown said...

Is it correct that the ICVR does not ground through its body? I have added aftermarket gauges and used a plastic pod. Will this be a problem mounting the ICVR to the pod? The ICVR was originally mounted to the back of the original gauges, but was that for grounding purposes?

Veronica said...

The question of grounding the ICVR is actually a bit more complicated than just a simple yes/no. For the original design ICVRs to cycle properly, they have to have a ground, and all of the drawings show that they do indeed seem to ground themselves on the back of the instrument cluster, which has a ground wire going to bracing behind the dash. And, I've had engineers that were also very smart people explain this to me in intricate detail. They were also unsure of whether or not this ground was actually needed, because the wires going to the sending units are also a ground. With the key on the ICVR has a ground if there is any gas in the tank. With the motor running, it also has a ground if you have any oil pressure. If the motor is off, the temperature of the motor is dead cold,and the car is out of gas, but the key is on, the question could be asked about whether or not the ICVR is cycling correctly, but, in that state, it really doesn't matter and there isn't any good way of answering the question because any conceivable way of checking would involve a ground of some sort in whatever you are checking with. So, the answer would be maybe yes, maybe no, and nobody can really make a definitive statement saying either one for certain. If somebody does make a definitive statement of certainty one way or the other, they probably don't know what they are talking about.

Sorry to bore you with all of that, but, there's a little more to it than just yes or no. As far as your reason for needing to know goes, most aftermarket gauges that I've seen work on 12V, not 6V, and draw power straight from some keyed source, have a connection of some sort, either a wire or a conduit of some sort, with the appropriate sending unit, and have a provision of some sort for a ground that allows the gauge back-light to work. The ICVR isn't even used. I would say look at the manufacturer's instructions that came with the gauges and they will tell you if they need some sort of independent ground for the gauge itself, but, I don't recall ever having seen one that did. Hope that helps.

Kyle Brown said...

I've been troubleshooting my gauges and this is my current problem:
The fuel and temp read low, and the oil pres reads slightly below normal. I traced and tested all the sender wires and they have proper continuity and voltage from the cluster. The fuel tank is new and is grounded well, the engine is grounded well to. All of my sending units have been replaced within the last 2 years and I tested them all for proper resistance, all check good. I grounded all of the sender wires and the gauges will read full as they're supposed to, I did this with each gauge individually and all at once. Except for the fuel gauge which goes to about half then clicks and drops slowly back to around 'E'. The top of the needle appears to be burned and I think it is the only gauge that may need to be replaced. The ICVR was also replaced within a few years. I have 5-6V coming to and leaving every gauge and all have no resistance, checked with my volt meter. All grounds appear to be good, no loose connections, and every other electrical system works as advertised on the car. My gauges are my only issue. I'm stumped. I've scoured the internet and your blog and trouble-tested every method I can think of. The alt VR has been replaced with an electronic unit as well. I installed mechanical oil pressure and temp gauges (factory gauges and senders are still installed and hooked up) to monitor those systems and everything is working correctly. Any suggestions???

66 coupe, 200 inline.


Fastback said...

I just replaced the 65 gauges with idiot lights with a gauge cluster from a 66. When I turn on the headlights the blinker light bulbs come on and the temp gauge pegs at hot. I'm not sure yet if the fuel gauge is effected because the tank is full. Is this related to the dash voltage regulator? Thanks!

Veronica said...

The symptoms that you are describing are frequently the result of not having the ground wire that goes from the instrument cluster to the dash brace connected. There is supposed to be a wire that is attached to the instrument cluster by the same screw that holds the voltage regulator onto the back of the instrument cluster, and it goes to the brace of the metal part of the dash, over to the left of where the speedo would be.


Great post! I have a 65 gt and none of the gauges work so I suspect it is the ICVR (they all show zero). Except the amp meter - it sort of moves and stays near the center. I can get a multimeter to the feed wire on the back of the ICVR and see 12V with the key on. On the wires that lead to the gauges from the ICVR, I don't see power. How does the ICVR attach to the dash? I can't quite see up there to figure out how it mounts.

Veronica said...

On your car the amp gauge works independent of the other three gauges. On the 65 model Mustangs that came with either pony interior or the GT package, the amp gauge doesn't have the two prongs sticking out with wires connected to them. What you have is a metal loop with the main power wire passing through that loop. The instrument cluster voltage regulator has nothing to do with the amp gauge. Your problem will be either a dead ICVR or an interruption of some sort in black/green wire that goes from the back of the ignition switch to the ICVR.

Bo Edgar said...

Veronica - on my '65, the gas gauge and the reverse lights aren't working. Can these be related? I was reading somewhere that there is a correlation, but the person did not elaborate. Any help would be greatly appreciated.

Veronica said...

That is going to be two different problems. The gas gauge has absolutely nothing to do with the back-up lights. If the water temp gauge seems to be working normally, then I would turn the key on and then unplug the wire from the sending unit at the gas tank, and ground that wire out on the car somewhere that it can make good contact. If the gauge pegs, the problem is inside the gas tank.

If you look at the March of 2009 section here, you will see a post titled Mustang back-up lights. That you should help with the back up lights.

deaksify said...

HI Veronica
First of all, i'd just like to say what a great blog this is.
I have 2 66 coupes and the wiring has been a mess on both.
I've just sorted a lot of issues on one but i'm still getting a problem with the dash lights, when i pull the headlight switch on both the ind lights illuminate, along with the lights and the turn signals are a little erratic. However, when the lights are off, the turn signals dont work at all. Could it be the dash ground causing an issue ?

Veronica said...

Assuming that the outside front turn signals work like they are supposed to, the ground from the instrument cluster to the dash could be part of the problem. It sounds like there might also be some other issues, though.I would start by pulling the instrument cluster out, cleaning everything up as far as connections, corrosion in or around the bulb sockets, and see if that accomplished anything.

deaksify said...

I've got all the dash out now, thanks to your blog, my service manual and stuff i've gleaned off the net, i've fixed the heater, wipers, washers, dash lights, fuel, temp and oil gauges and some of the lights. The turn signals worked before i started repairing everything, so i would assume it's something i've done wrong.
Could the wrong bulbs cause an issue ? when i put my meter on the battery cables i'm getting a parasitic draw of 11 volts

Veronica said...

When you say that the turn signals don't work now, do mean that, when you hit the turn signal, nothing happens, some work and some don't, or the appropriate lights come on but don't flash? That draw could be something else entirely, since the key has to be on for the turn signals to work. I did a post here in November of 09 on finding a draw which should help you out with that.