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My tachometer occasionally jumps all the way CW and stays there.

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No other instruments are effected...
My tachometer occasionally jumps all the way CW and stays there. No other instruments are effected. 2001 ford windstar. I can blow the needle back into a normal position and the tach works but randomly pegs again and again.
Submitted: 2 years ago.Category: Ford
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Answered in 1 hour by:
6/3/2015
Ford Mechanic: Pete, Auto Service Technician replied 2 years ago
Pete
Pete, Auto Service Technician
Category: Ford
Satisfied Customers: 30,552
Experience: Over 17 years diagnosing and repairing automobiles various makes and models with some training and certifications.
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Hi,my name is ***** ***** I will be happy to try to assist you with this.I may need more information in some cases to help with answering your question.I apologize for any delay.
If no other gauges are acting up and if the actual engine speed is not pegging like the gauge is this indicates a fault with the tach. Gauge itself.I have seen the electronic stepper motor fail in the instrument cluster and cause this here.To correct this the instrument cluster will need removed and repaired or removed and replaced.
Thanks Pete
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Customer reply replied 2 years ago
How would a stepper motor intermittently fail? And then work properly 95% of the time? The tach is the only gauge that is fed from the PCM. Since this issue started when we started using our A/C this year i thought it might be a bad A/C clutch diode.
Ford Mechanic: Pete, Auto Service Technician replied 2 years ago
Does this only act up when the ac is switched on?
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Customer reply replied 2 years ago
Not every time, this only happens when my wife drives the car. I cannot repeat the problem and witness it happening based on any specific cause. I thought a voltage spike may be the culprit, the alternator is producing only 0.05Vac noise, no spikes noticed. A bad A/C diode could cause such an intermittent spike. The A/C relay is driven by the PCM, also the tach is the only gauge that is driven by the PCM. I can't find the A/C diode to test/replace it, i assume it is buried somewhere in the wiring harness, do you have a general idea of its location?
Ford Mechanic: Pete, Auto Service Technician replied 2 years ago
Looking at the diagram info on your model here your system doesn't use an AC diode for the AC system.The PCM does send the ground signal to the AC control relay to energize the relay to then engage the AC clutch but there isn't a diode in line on your model here.
I have seen the instrument cluster gauge itself cause this issue here before.One way to check this is with a scan tool that displays data display to see if the PCM reads the tach. signal normal when the tach. itself is acting up.If the PCM does read it as normal from here this indicates a fault with the tach. gauge stepper motor itself acting up.
If there are any engine running issues when the tach. acts up this can indicate a different issue like the PCM at fault or crankshaft position sensor at fault (most common).
Thanks Pete
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Customer reply replied 2 years ago
The schematic i have, for 1998 and later, does show an A/C diode (see schematic below). Previous models used a module to handle the A/C clutch control. The PCM does control the A/C relay ground (that wire is isolated from the clutch coil field collapse) but there is also a wire that connects directly to the A/C coil, the PCM and the A/C control switch (driver call for A/C). That wire is not isolated from the coil collapse reverse voltage. The engine runs fine at all times. I already replaced the A/C Clutch control relay and i placed an additional diode in parallel to the A/C clutch coil and the problem seems to have gone away (not too many miles yet , so i'll see). I just wanted to FIND the factory diode, replace it and do the job right. Do you have any idea where it is?
Ford Mechanic: Pete, Auto Service Technician replied 2 years ago
The diagram below is the diagram I show for your year and model.
https://www.dropbox.com/s/znhbalqrtsd94qp/2015-06-05%2011.45.04.gif?dl=0
The one you have here is good to model year 2000.In 2001 the diode was eliminated.
Thanks Pete
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