Troubleshooting and Restoring the 1986 International S1600 Medium-Duty Truck

[MikePhua]

The S1600 and International’s Mid-Range Workhorse Legacy
The International S1600 was part of the S-Series lineup produced by International Harvester (later Navistar International) from the late 1970s through the early 1990s. Positioned as a medium-duty truck, the S1600 was widely used in municipal fleets, agriculture, delivery, and vocational applications. With a GVWR typically ranging from 19,000 to 26,000 lbs, it filled the gap between light-duty pickups and heavy Class 7 trucks.
The S-Series was known for its modular design, allowing a wide range of engines, transmissions, and chassis configurations. The 1986 S1600 could be equipped with either gasoline or diesel powerplants, including the International 6.9L or 7.3L IDI V8 diesels, as well as the 345 and 392 cubic inch V8 gas engines. Its mechanical simplicity and rugged frame earned it a reputation for durability, though age-related issues are now common.
Terminology Annotation

• IDI Diesel: Indirect injection diesel engine, where fuel is injected into a pre-combustion chamber before entering the cylinder.
  
• Glow Plug Relay: An electrical relay that controls power to the glow plugs, essential for cold starts in diesel engines.
  
• Bulkhead Connector: A multi-pin electrical connector mounted on the firewall, often a failure point in older trucks.
  
• Hydroboost: A brake booster system powered by hydraulic pressure from the power steering pump, used instead of vacuum boost.
  

Electrical System Challenges and Starting Issues
One of the most frequent complaints with aging S1600s is intermittent starting, especially in diesel variants. Common symptoms include:

• No crank or delayed crank when key is turned
  
• Glow plug light not illuminating
  
• Clicking from the starter solenoid but no engagement
  
• Voltage drop across bulkhead connectors
  

Root causes often include:

• Corroded or loose connections at the firewall bulkhead
  
• Failing glow plug relay or timer module
  
• Worn ignition switch contacts
  
• Ground strap degradation between engine and frame
  

Recommended diagnostic steps:

• Use a multimeter to check voltage at the glow plug relay during key-on
  
• Inspect and clean bulkhead connectors with contact cleaner and dielectric grease
  
• Verify starter solenoid voltage during crank attempt
  
• Replace ground straps with braided copper and ensure solid frame contact
  

In one municipal fleet in Ohio, a 1986 S1600 with a 6.9L diesel refused to start below 40°F. After bypassing the glow plug relay with a manual switch and replacing the bulkhead connector, the truck started reliably even in sub-freezing conditions.
Fuel Delivery and Combustion Troubles
Diesel variants of the S1600 often suffer from fuel starvation or air intrusion in the supply lines. Symptoms include:

• Extended cranking before start
  
• Engine stalling under load
  
• Fuel filter housing leaks
  
• Injector knock or misfire
  

Solutions include:

• Replace rubber fuel lines with modern ethanol-resistant hose
  
• Install a clear inline fuel filter to monitor air bubbles
  
• Upgrade to an electric lift pump for consistent fuel delivery
  
• Bleed injector lines after filter changes
  

Suggested specs:

• Fuel filter: Baldwin PF7977 or equivalent
  
• Lift pump pressure: ~4–7 psi
  
• Injector torque: ~55 ft-lbs
  
• Glow plug voltage: ~10.5V during preheat
  

In a restoration project in Alberta, a farmer added a Facet electric pump near the tank and eliminated hard starts entirely. The original mechanical pump was left in place but bypassed.
Brake System and Hydroboost Maintenance
The S1600’s braking system often uses Hydroboost, which relies on power steering pressure to assist braking. Common issues include:

• Hard pedal with poor braking force
  
• Fluid leaks at the booster or master cylinder
  
• Steering effort increases during braking
  

Recommended service:

• Flush power steering fluid every 2 years
  
• Replace booster seals and check return line for blockage
  
• Inspect master cylinder bore for scoring
  
• Use DOT 3 brake fluid and ensure reservoir is properly vented
  

In one grain hauler in Kansas, a leaking Hydroboost unit caused both brake and steering failure during a field run. Replacing the booster and flushing the system restored full control.
Cab Wiring and Accessory Reliability
The S1600’s cab wiring harness is prone to age-related degradation. Symptoms include:

• Non-functional gauges or intermittent readings
  
• Blower motor failure
  
• Lighting circuit shorts
  
• Fuse panel corrosion
  

Solutions:

• Replace fuse panel with modern blade-style unit
  
• Rewire critical circuits with marine-grade wire
  
• Add relays for high-draw accessories like lights and heater fan
  
• Install a battery disconnect to prevent parasitic drain
  

In a school bus conversion project in Oregon, rewiring the cab with a custom harness and LED lighting improved reliability and reduced current draw by 30%.
Final Thoughts
The 1986 International S1600 remains a mechanically sound platform with a loyal following among restorers, farmers, and vocational operators. While electrical and hydraulic systems show their age, most issues are solvable with basic tools and thoughtful upgrades. Its steel frame, simple drivetrain, and modular design make it a candidate for long-term service or creative repurposing.
In the clatter of IDI diesel and the hum of hydraulic brakes, the S1600 rolls forward—proof that good iron never quits, it just needs a little care and a few new wires.