ASV PT100 Tachometer And Hour Meter Electrical Diagnosis

[MikePhua]

ASV PT100 Machine Background
The ASV PT100 is a high-horsepower compact track loader designed for demanding forestry, construction and land-clearing work. In the mid-2000s, when the PT100 was introduced, ASV (All Season Vehicles) was already known for its suspended rubber track undercarriage and strong auxiliary hydraulics aimed at mulchers, brush cutters and other heavy front attachments. Across the industry, machines in the 90–110 hp compact track loader class have been selling in the many thousands of units per year globally, and the PT100 has secured a solid place in land-management fleets thanks to its power-to-weight ratio and flotation.
With this level of performance, monitoring engine speed and hours accurately is more than a convenience. The tachometer and hour meter are central to:

• Setting correct operating RPM for hydraulic attachments
  
• Tracking service intervals and total machine life
  
• Diagnosing charging and electrical issues that often show up first in the dash instruments
  

When those two instruments stop working together, it usually points to a shared wiring or alternator-signal problem rather than two separate failures.
Symptom Overview Tachometer Dead And Hour Meter Uncertain
On a 2008 PT100, the owner began with a machine that had what they described as a “colorful” repair history. After chasing multiple issues, they focused on the instrument cluster because:

• The alternator was not charging correctly.
  
• The tachometer was not registering RPM at all.
  
• The hour meter behavior depended on how the alternator terminals were wired.
  

The wiring diagram obtained for the machine showed:

• A gray alternator exciter wire
  
• A white wire feeding the tachometer and then the hour meter
  
• Both gray and white wires supposedly connected to the D+ terminal on the alternator
  

However, the actual machine wiring had been altered by a previous repair attempt. The gray and white wires were swapped around the alternator terminals, stopping the alternator from charging and confusing the tach.
This kind of mis-wiring is common on older equipment: terminals get mislabeled, diagrams contain typos, and owners or mechanics make their best guess under time pressure. In this case, the fix required carefully reconciling the diagram with the real alternator hardware.
Restoring Alternator Charge And Warning Lamp Function
The first step back to normal operation was restoring proper charging:

• The main B terminal on the alternator (larger stud) was correctly tied to the battery.
  
• The gray exciter wire needed to be on the correct D terminal so that the charge indicator lamp and regulator would function properly.
  
• Once rewired, the amp light on the dash began working, and measured voltage at the battery stabilized around 13.8 V with the engine running, confirming that the alternator was charging correctly.
  

This confirmed the alternator itself was capable of working and that at least one part of the wiring diagram was reliable enough to follow. It also demonstrated a useful diagnostic rule:

• If the alternator warning lamp and charging voltage behave properly, the B and D terminals are likely wired correctly, and attention can turn to the tach signal and its dedicated terminal.
  

Tachometer Replacement And Initial Failure
The original tachometer on this PT100 had essentially failed mechanically. The face was full of dust and the gauge did not move at all. The owner chose to replace it with a factory tachometer rather than an aftermarket unit, assuming this would minimize compatibility issues.
After installation:

• The new tachometer still did not read any RPM when wired strictly according to the existing diagram.
  
• The hour meter had previously worked when wired as shown but now seemed tied to the alternator signal behavior.
  

This led to a closer look at how diesel tachs typically get their signal and to questions about the alternator’s W terminal.
Understanding The W Terminal And Tach Signal
Many diesel engines use a tach signal generated by the alternator’s W terminal:

• The W terminal outputs an AC signal whose frequency rises with alternator (and therefore engine) speed.
  
• The tachometer counts the pulses and, based on a fixed calibration (pulses per engine revolution), converts them to RPM.
  
• Because this is an AC pulse train, it is not a simple steady DC voltage that a multimeter reads easily.
  

With this in mind, the owner tried:

• Moving the white wire that feeds the tach to the W terminal.
  
• Leaving the gray exciter wire on its proper D terminal.
  

When this was done:

• The tach needle moved off zero to around 800 RPM but then stopped, regardless of engine revs.
  
• When the engine shut down, the needle dropped back to zero.
  

This behavior suggested that the gauge was seeing some form of signal, but either:

• The frequency was not changing as expected (faulty W output), or
  
• The tachometer calibration or input configuration did not match the alternator’s W terminal characteristics.
  

At the same time, the owner worried about how the hour meter, designed for 12 V DC, could function properly if fed from a pulsing or AC source.
Discovery Of Errors In The Wiring Diagram
After further testing and rechecking, the owner concluded that the published wiring diagram contained significant mistakes. Their practical findings:

• The alternator has three relevant connections:
  • B terminal (main battery output)
    
  • D terminal (exciter/charge indicator, gray wire)
    
  • W terminal (tach and hour meter signal, white wire)
    
• The diagram had mislabeled the B and D terminals.
  
• The diagram also indicated that both white and gray wires landed on D+, which does not match the actual alternator layout or operation.
  

Once they corrected the wiring:

• B terminal → battery
  
• D terminal → gray exciter wire
  
• W terminal → white wire feeding the tachometer and then the hour meter
  

the system behaved as intended:

• The alternator charged correctly.
  
• The tachometer received a proper RPM-proportional signal.
  
• The hour meter received the power it needed through the same circuit.
  

This illustrates an important lesson for owners of older machines: factory diagrams can contain typos or be drawn for a different alternator variant. When in doubt, the physical labels on the alternator itself and the known behaviors (charging voltage, indicator lamp function, tach response) are often more reliable than a single schematic.
How The Tachometer And Hour Meter Share A Signal
In many diesel machines of this era, the tachometer and hour meter are tied together:

• The W terminal sends a frequency-based signal into the tach.
  
• The tach electronics translate this frequency into an RPM needle position.
  
• The hour meter is often powered or driven internally from the same circuit, counting “engine hours” only when the tach sees a running signal.
  

Depending on the design:

• Some hour meters simply run whenever the engine is above a threshold RPM (e.g., anything above idle).
  
• Others scale hours with RPM, so one hour at full rated speed counts as one hour, while one hour at half speed might register less than a full hour.
  

In the PT100’s case, the hour meter is effectively slaved to the tach signal provided through the white wire from the W terminal. This explains why:

• Both tach and hour meter misbehave together when the W wiring is wrong.
  
• Restoring the correct W terminal connection fixes both, as long as the instruments themselves are healthy.
  

Temperature Gauge And Cold-Start Sender Side Discussion
In the same troubleshooting journey, the owner raised another issue: the factory temperature gauge was not working because the incorrect sending unit had been installed. Another PT100 owner offered details and reasoning that highlight the complexity of the engine’s sensor network:

• The PT100 uses a Perkins 1104 diesel engine, which typically has multiple temperature-related senders:
  • A sender dedicated to the dash gauge
    
  • A temperature-dependent switch or sender for cold start or timing advance functions
    
• The cold-start sender is located on the passenger side of the cylinder head above the injection pump.
  
• A white wire tied into the fuel pump and injection pump power feed likely passes through this sender to a cold-start solenoid on the injection pump when the engine is cold.
  

The proposed logic:

• When the engine is cold, the sender acts as a near-short to ground, allowing current to flow to the cold-start solenoid and enriching fuel delivery or advancing timing.
  
• As the engine warms, the sender opens or changes resistance, cutting off the cold-start function.
  

Because of that behavior:

• It is unsafe to simply hook that cold-start sender into the dash gauge circuit, as its on/off characteristics are very different from a smooth, resistive gauge sender.
  
• The correct location and part number for the gauge sender must match the cluster’s calibration, otherwise the gauge will behave unpredictably or read wrong.
  

This side story reinforces the broader theme: on modern diesel equipment, a single engine may have several temperature senders with different roles, and mixing them up can create confusing electrical symptoms.
Practical Diagnostic Approach For Similar PT100 Issues
For owners facing dead tachometers and erratic hour meters on ASV PT100 machines, a structured approach helps:

• Verify charging system first
  • Check battery voltage at rest (around 12.5–12.8 V).
    
  • Start the engine and confirm charging voltage of roughly 13.5–14.5 V.
    
  • Ensure the charge warning lamp on the dash behaves correctly: lit with key on/engine off, off when running.
    
• Confirm alternator terminal layout
  • Identify B, D and W terminals physically on the alternator case.
    
  • Do not rely solely on one diagram if the labels or shapes do not match.
    
  • Ensure:
    • B → battery
      
    • D → gray exciter/charge light wire
      
    • W → white wire feeding tach/hour meter
      
• Check the white tach signal wire
  • Ensure continuity from W terminal to the tachometer harness connector.
    
  • Inspect for breaks, corrosion and poor crimp joints, especially near the alternator and the back of the dash cluster.
    
• Inspect the instrument cluster
  • Confirm ground and ignition-switched power to the cluster.
    
  • If the original tach is physically contaminated or seized, replacement with a known-good unit is reasonable.
    
  • After replacement, re-test with the W terminal correctly wired.
    
• Evaluate related senders such as temperature sensors
  • Identify each sensor by location and function using a service manual, not just color codes.
    
  • Match replacement senders by part number and resistance curve.
    

Common Mistakes And Preventive Tips
From these experiences and similar cases on other equipment, several recurring pitfalls appear:

• Over-trusting a single wiring diagram
  Different alternator brands and updates can make older diagrams partially obsolete. Physical inspection and labeling on the component take priority.
  
• Swapping D and W terminals
  It is easy to misunderstand D+, D and W markings, especially when alternators are replaced with aftermarket units whose letters differ slightly. A mis-wired W terminal often leads to:
  • No tach reading
    
  • Hour meter stopping
    
  • Sometimes no charging, if the exciter circuit is also affected
    
• Ignoring ground paths
  Poor instrument cluster grounds can cause “ghost” readings or intermittent operation. Cleaning and tightening grounds is cheap insurance on any older machine.
  
• Assuming digital hour meters are independent
  Many hour meters are internally tied to tach signals, alternator outputs or oil-pressure senders. If the gauge that usually sits next to the hour meter fails, the hour meter often fails with it.
  

To prevent trouble:

• Label alternator wires with tags or heat-shrink markers when removing them.
  
• Take clear photos before disassembly.
  
• Verify new alternators have the same terminal functions as the original, not just similar shapes.
  

Conclusion Reliable Gauges From Correct Alternator Wiring
The ASV PT100 is a serious work machine, and its tachometer and hour meter are key tools for safe and efficient operation. In the case described, the root cause of the non-working tach and confusing hour meter behavior was not a mysterious electronic failure but rather incorrect alternator wiring and errors in the reference diagram.
By:

• Restoring proper B, D and W terminal connections
  
• Feeding the tach and hour meter from the W terminal as intended
  
• Treating the wiring diagram as a guide rather than infallible truth
  

the owner brought the instrument system back to life and confirmed that both RPM and hours could again be trusted.
For anyone maintaining an older PT100 or similar machine, this story is a reminder to combine service literature with hands-on observation, and to remember that a single mis-placed wire can silence both tach and hour meter even when the rest of the machine still runs strong.