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  • 1965 - 1973 Classic Mustangs -General/Talk-
  • Classic Mustang Specific Tech

bouncing needles on new gauges

  • Thread starter Thread starter mach1one
  • Start date Start date Dec 29, 2007

mach1one

Boulevard Bruiser
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Living in the past
Dec 29, 2007
#1
  • Dec 29, 2007
  • #1
The coolant temp. guage never worked in the 73 mach, so I decided to update it with new guages (along with the oil pressure/volt. guage) while troubleshooting the problem

With a new sending unit the guage doesn't read, it just bounces below 100; even with the car warm. To test the wiring I ran a new line directly to the sending unit, didn't fix it.

The oil pressure guage is now acting the same way. It bounces around way too much to work right. Before the guage swap it too was working.

The alternator guage is working right and has always worked.


Is this the symptoms of the IVR being bad? I only have a schematic out of a haynes manual and I'm having problems understanding it. Wiring is not something I understand very well.
 

BullittStangV8

Founding Member
Oct 5, 2002
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NJ
Dec 30, 2007
#2
  • Dec 30, 2007
  • #2
Do they have a good ground or share the same ground?
 

Rusty67

20+ Year Stangneter
Dec 3, 2002
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LA, CA
Dec 30, 2007
#3
  • Dec 30, 2007
  • #3
The wire that goes to the sending unit is actually the ground wire. The sending units provide a certain amount of resistence which keeps the needle from pegging one side or the other. Make sure you have your wires setup correctly. Try running a wire directly from a regular 12v source to the gauge and then another to the sending unit. Actually, don't try that unless the guage can take 12v continuous. I'm not sure if your aftermarket gauges need an IVR. Check the instructions for the gauges and see if they need one, otherwise by pass it.
 

wicked93gs

15 Year Member
Sep 30, 2006
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Nashville TN
Dec 30, 2007
#4
  • Dec 30, 2007
  • #4
bouncing needles(or pegged) on the autometers I used in my neon always had bad grounds...my oil pressure didnt bounce since the Isolator acted as the ground through a stainess steel line....however if it isnt getting a good ground through the engine, you can always run a ground line to the frame as well(for the isolator...I had it setup like that once) anyway for the other gauges doublecheck all wiring and grounds...I assume you have all your splices soldered right?
 

Edster

Founding Member
Aug 13, 2000
500
1
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League City, Texas
Dec 30, 2007
#5
  • Dec 30, 2007
  • #5
If the gauges are after market and the IVR is hooked up that could be your problem. try it w/o the IVR and see what happens.
 

mach1one

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#6
  • Dec 30, 2007
  • #6
The IVR wasn't hooked up; originally I wasn't sure if it was suppose to be.

So here's an interesting conclusion. I went to check the engine/chasis ground and there was none! So I bought one, installed it, problem solved.

The previous owners had also wired the guages such that they were on. So we swapped that over to a switchable power source too. Now if I can just get the gauges mounted I'll be all set
 

wicked93gs

15 Year Member
Sep 30, 2006
1,198
228
93
Nashville TN
Dec 30, 2007
#7
  • Dec 30, 2007
  • #7
wow...with no grounds I'm kinda surprised your engine even ran, I've always had to have 2 or more grounds with most of my engines(even my old 67 had two grounds if I remember correctly)
 

mach1one

Boulevard Bruiser
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Oct 30, 2001
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#8
  • Dec 30, 2007
  • #8
Yeah, what's even crazier is that its been running fine. No charging or electrical problems aside from the one non-working guage.
 

Rusty67

20+ Year Stangneter
Dec 3, 2002
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109
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Dec 31, 2007
#9
  • Dec 31, 2007
  • #9
After hearing that, I'd recomend you rewire the entire car just to be safe. Sounds like these guys were the type of people that make gremlins.
 

mach1one

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#10
  • Dec 31, 2007
  • #10
It looks like the engine will need to be rebuilt anyway so I'll have time to take that on. That is probably a good idea since I wasn't very fond of some of the things the previous owners did.
 
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