Need Tach Advice!

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Here is how to diagnose the stock Tach from a FSM.

«1992 Mustang Service Manual Table of Contents»
«Group 13: GAUGE AND WARNING DEVICE SYSTEM»
«Section 13-05: Tachometer, Oil Pressure, Coolant Temperature Gauges/Warning Indicators»
«DIAGNOSIS AND TESTING»
«PINPOINT TEST A: TACHOMETER INOPERATIVE, ERRATIC, WRONG INDICATION (Index)»

PINPOINT TEST A: TACHOMETER INOPERATIVE, ERRATIC, WRONG INDICATION

A1 CHECK OPERATION

l Check tachometer operation.

Inoperative
GO to «A2».

Erratic or wrong indication
GO to «A3».

A2 CHECK FUSE

l Check tachometer fuse.

l Is fuse OK?

No
REPLACE fuse.

Yes
GO to «A3».


A3 CHECK WIRING

l Check for loose wiring connections in engine compartment and at instrument cluster.

l Are all connections OK?

No
SECURE loose connections.

Yes
GO to «A4».


A4 CHECK RESISTANCE AND VOLTAGE

l Disconnect battery.

l Remove instrument cluster and make resistance and voltage checks using Rotunda Digital Volt-Ohmmeter 014-00407, or equivalent at 14401 wire harness connector as follows (refer to pin locations below):

-- Check Pin 2 resistance to chassis ground--should read 1 ohm or less.

-- Check Pin 11 resistance to negative terminal of ignition coil or Pin 6 of DIS module. Should be 1 ohm or less.

-- Connect battery. Turn ignition switch ON. Check for + 12V at Pin 4. Turn ignition switch OFF. Disconnect battery.

l Is all voltage and resistance within specifications?

No
Condition is not in tachometer. SERVICE wiring.

Yes
GO to «A5».


A5 CHECK CONNECTOR CLIPS

l Check for loose tachometer connector clips on rear of instrument cluster, or damaged printed circuit.

l Are connector clips OK?

No
TIGHTEN or REPLACE clips. REPLACE printed circuit.

Yes
REPLACE tachometer.


 
The problem is usually some of the resistors for tachometer. Sometimes the solder joints crack and cause the tachometer to read really high. Depending on the cause, you can either try and resolder the joints, or install new resistors. That should fix it.
 
I have attempted to peek inside the tach circuitry with very little success. I do have the tools and chart to calibrate them, but not info on what the circuitry actually is. I don't have any diagrams and no parts list that crosses over to commercially available parts. If anyone actually has a diagram of an 87-93 Fox body tach, I sure would like to have a copy.

I do have a copy of the older Fox body tach diagram from the early 80's 4 eye Mustangs
 
I have attempted to peek inside the tach circuitry with very little success. I do have the tools and chart to calibrate them, but not info on what the circuitry actually is. I don't have any diagrams and no parts list that crosses over to commercially available parts. If anyone actually has a diagram of an 87-93 Fox body tach, I sure would like to have a copy.

I do have a copy of the older Fox body tach diagram from the early 80's 4 eye Mustangs

The tach circuit board is actually easy to get to. You must remove the overlay and gauge as a whole. Then there are 3 nuts that hold the board to the gauge. Once that is done, you can easily spot the resistors that are the problem by the discoloration around them. The solder joints tend to crack from the years of getting hot. Its a simple fix. And if you need to replace the resistors, they are easily identifiable once you have the board off.
 
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The tach circuit board is actually easy to get to. You must remove the overlay and gauge as a whole. Then there are 3 nuts that hold the board to the gauge. Once that is done, you can easily spot the resistors that are the problem by the discoloration around them. The solder joints tend to crack from the years of getting hot. Its a simple fix. And if you need to replace the resistors, they are easily identifiable once you have the board off.
I already know that much, I have a spare instrument cluster from an 89 Mustang GT. I have done the tach calibration routine several times. I know how to get excellent accuracy +/- 35 RPM over a narrow RPM range It ends up being slot about ~200-300 RPM wide. You can choose where you want that slot in the tach's RPM range.

I would like diagrams of the circuitry so that I can fix defective tachs. I also want to be able to modify working tachs to widen the accuracy slot so that it covers a much wider range. If possible, I would like to extend the width of the accuracy slot the entire range of the tach.
 
***Update***
Well after driving a few times the Tach issue appears to be random. One time it goes to 4,500 other times it works. Random on when sometimes at turn key others while driving.

NOW with that said now my temp gauge appears to only go two lines up and not in the middle where it was before. Can this be related and any ideas?

Tach and Temp gauge acting odd.
 
liljoe07's comments about re-soldering the resistors on the tach circuit board are the best place to start for a fix for your tach problem

See below for a how to solder correctly tutorial.


View: https://www.youtube.com/watch?v=f9RiXMf3z7A

Temp gauge problem: The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C

PM me your email address and I will send you a complete Ford Factory 89 Mustang electrical diagram set. The zip file is 2.5 MB and is too big to fit through Stangnet's email gateway.

89 Mustang wiring diagrams zip package–

They are in a zip file format to reduce the size of the package. If you don't have Windows 7 or Windows 8, you’ll need WinZip or other Windows archive tool to extract them from the zip file.. See MajorGeeks.Com - MajorGeeks - Download Freeware and Shareware Computer Utilities for a free download.

The diagrams show the location area and the connectors are drawn to the same shape as those in the car.

You will need the Adobe Acrobat viewer which is also a free download – Adobe Acrobat Reader DC Install for all versions - Adobe Reader download - All versions
 
So these problems are unrelated but started at the same time? Assuming all grounds are good?
Test them to make sure...

Voltage drop testing of connections and grounds.

Use a Digital Volt Meter (DVM) to measure the voltage drop across a connection or wire. Adding length to the test leads may be required, and does not affect the accuracy of the test. Use 18-20 gauge wire for the test leads if you have to lengthen them.

Voltage drop testing must be done while the usual load is on the circuit. If it is a starter, it has to be tested while cranking the starter. If it is lights, A/C or fan, they must be turned on high while testing. Fail to do this and you will not get accurate results

1.) Most grounds use the negative battery post as their starting point. Keep this in mind when checking grounds.
2.) The voltage will be small if the ground is good: less voltage drop = better connection.
3.) Be sure that the power to the circuit is on, and the circuit is being used in its normal manner. For instance, if it is a light circuit, the lights on that circuit should be powered on.
4.) To measure grounds, place one DVM lead on the battery negative post and the other on the wire or connector that goes to ground.
5.) 5.) Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections (sensors are low voltage devices and small drops can have a large effect on the devices dependent on sensor accuracy)
0.0V Connections
A voltage drop lower that spec is always acceptable.
6.)
See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .

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Where would I find the ground wires for the tach? I had this problem pop up again. Tach sits at 4,500 all day but does go up with more RPM. The temp guage has been just fine other than the couple of times it acted up.
 
@jrichker - about the ground for the instrument cluster etc., Is there a post or bolt with a ground wire on the inside of the firewall in the same location as the outside one to the engine? Or where do they ground so the engine side ground works? The gauges all topping out under certain combinations, starting with the temp gauge, like when the tank is full and the AC is on, is something I have experienced in a Fairmont, at least two Mustangs and in an 87 Cougar. I want to be ready if my 91 5.0 starts to act this way.
Someone always ends up saying refer to the wiring diagram book I do not have. I just want to know where the wires on the dash side ground to. Can you please help???

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“ Temp gauge problem: The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C”