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fastback351

Change dash circuit board now dead

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I had a radio issue and decided to fix everything. One of the things I did was replace dash circuit board due to mine was pretty raggedy and now nothing seems to be working except voltage gage. turn signal lights are fine but thats about it. none of the dash lights come on. no power to the fuel, oil or temp gauge. I was in the rear poking around cleaning up old wiring and i did pull the harness to a dana corp speedostat aftermarket kit for cruise control I don't want anymore that I'm trying to get the diagram to, but looks like constant power is also out however engine starts and headlights come on. Where do I start?

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There are two versions of the flex circuit: one is for a non-tach car, and another is or a car with a tachometer. Can I assume  that since you mention "voltmeter" you really mean ammeter? Or perhaps your ammeter has been converted to a voltmeter? If not done properly that could be a source of trouble also.

I suggest you see my post in the "How tos" section on this forum: "Fixing the Instrument Panel"

Of special interest to you would be the picture on page 2- read notes #6 and #7. Also see Page 3 "Gauge posts shorting to metal housing". Here is an excerpt from that:

 GAUGE POSTS SHORTING TO METAL HOUSING

This is a VERY frequent problem. The gauge posts must not come in contact with the metal housings. Any time the gauges have been moved or the flex circuit reattached, or in any way taken apart, you definitely need to check for a short circuit. If even one of the gauge posts touches the metal housing, it will create a short, the voltage will drop, nothing will work, and the gauge voltage regulator could be damaged. It is fortunate that the 69/70s have a 10 ohm resistor wire in front of the regulator, so it may survive the short. Clamp one ohmmeter lead to the center metal housing, and clamp the other, in turn, to each temp and fuel gauge post. You must have infinite ohms (no conductivity), if not you need to reposition the gauge(s) so they don’t touch the edge of the holes. Do the same for the oil gauge, connecting one lead to the left housing and the other to each gauge post in turn.

 

This should get you started, but it sounds like you may have a couple of problems. Please let us know what you find- its nice to have closure on the problem. 

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Mach 1 driver your an awesome fellow to have in the mustang corner. I saw the answer you gave me in email and looking at the key harness i realize i probably changed that 25 yrs ago. Turns out there is a very strong violet color wire to the circuit plug. The pinkish wire that I said solder is leaving the key red/blue strip and seems to be going to the engine bay. I didn't use a traditional solder method rather one using a heat gun and melting it. I'm attaching a picture. I don't know why the green wire is also paired but it goes to the main harness. I was really wanted to solve that to illuminate the dash light and gauge issues. I checked fuses and they seem fine but also the dome lights are off and really got me thinking ground but the harness ground is fine also.  I will start looking at what you posted...

wiring.jpg

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Maybe Midlife will wander in- he has much more experience with the harnesses and can often tell just by looking at a picture like that what wire it is. That is an odd color. My wife would tell you that I'm color blind, and I am suppose I am in some shades. For instance I have a car whose color is called Pewter (which should be a silver-gray), but I bought the car because it looks like a light gold to me in most light conditions.

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These are wires going to the ignition switch?  OK...strong violet but thin wire is one branch of the proportioning brake valve signal line.

Pink resistor wire is paired with green/red wire for non-tach applications.  You CANNOT solder resistor wires unless you are experienced with silver solder which jewelers use.  The wire won't accept normal solder and must be butt-spliced. 

These have nothing to do with dash lamps nor does the shorted circuit card described by Mach1Driver.  The problem probably lies in the fuse box clip contacts or the headlight rheostat within the headlight switch.

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Ok reading Mach 1 article I tested a few things and here's what I have. On the printed board side nothing is shorting. On the plug power side with key in the on position I have power to dash lights with head light switch on. I have ammeter power but 12vt to the VR and nothing to gauges sensors. thanks guys you've gotten further then I've been.

Had to update there is power to IVR sorry I'm old!!!    

Ok so getting turn signal power, power to dash light, AMP, and power to IVR. dome light are still out but things are looking better if i get gauges working.  Light switch is loose as all get out and maybe issue. Testing ohm's on gauges seems fine. 

 

 

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Sounds definitely like one of your gauges (not ammeter) is shorted to case.  Measure resistance from gauge post to dash cluster housing and if you see 1 or 14 ohms, then one of the gauges is shorted.  You'll need to loosen up the gauge nuts, move the gauge around, and then tighten the nut back up and try again.  The problem is that any one of the non-ammeter gauges is shorted, and since they are all connected via the IVR output, you won't be able to identify which gauge is shorted.  Another solution is to wrap the small rectangular hole that the gauge posts stick out from the back of the dash cluster with black electrical tape.

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This is the entire gauge circuit shown schematically. The area in red is all that resides on the instrument panel. If the IVR pulses then your problem is probably a gauge shorted to the housing. Caution, when testing don't connect 12v directly to the IVR, you need a 10 ohm resistor in front of it.

image.thumb.png.3efade8e2863cbb26571ef67bcdfa40d.png

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By the way, be sure the IVR has a good ground, or it can't pulse. If you have an aftermarket electronic IVR, it won't work either without a ground, but when working it will give you a steady 5v.

This is on page 8 of 69-70 Instrument Gauge Cluster:

image.thumb.png.57d806f87600743f0f61881661dad957.png

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6 minutes ago, fastback351 said:

OK great news I worked the cluster and got the gauges to work but now I got to tap the unit to get the needles to move on gas and temp then when i shut it off gotta tap on dash for the needles to drop. what did i do

It sounds like the needles are contacting something and it is impeding their movement, maybe the front face that has the scales is miss-positioned.

The front face is removed here. Note the first line where you can use D batteries to operate the meters for testing:

 image.thumb.png.803245c238ef6a75407ecacfd2b8e590.png

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13 hours ago, fastback351 said:

Had to update there is power to IVR sorry I'm old!!!    

 

Old? You're a year younger than me, you ain't old. Now the other two guys replying to this thread...

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OK Mach 1 driver.....you are a pretty awesome contact. You seem to have a long history helping people. GRACIAS Amigo. Well here's an update got tired of dis-assemble re-assemble so just did a hillbilly bench tester. I must have ever so slightly bent the needles so everything looks good. What a job!! now I have to start another thread some reason dome lights died so have to trouble shoot those. i decided to clean wiring up on the car and still have a few things but this was a nightmare. For sure you have to watch post touching body when doing new flex board. 

20220909_115516.jpg

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9 hours ago, RPM said:

Old? You're a year younger than me, you ain't old. Now the other two guys replying to this thread...

You talking to me?  Watch it bub...both Mach1Driver and i control the smoke Jeannie in your Mustang...so be nice to us!

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