9-Pin vs 6-Pin Truck Diagnostic Connectors — What You Need

March 8, 2026

TL;DR: The 9-pin Deutsch connector (HD10-9-1939P) carries both J1939 and J1708 signals and is found on most 2001+ heavy-duty trucks. The 6-pin Deutsch connector carries J1708 only and is found on pre-2001 trucks and some trailer applications. If you service mixed fleets, you need cables for both.

One of the first challenges any truck technician faces is figuring out which cable to grab. Unlike passenger cars — which universally use the 16-pin OBD-II DLC — heavy-duty trucks use different connector types depending on the truck's age, manufacturer, and sometimes even the specific application.

This guide covers the two primary heavy-duty diagnostic connectors and helps you identify exactly what you need for your shop.

Part of our protocol education series. See also: J1939 vs J1708 vs OBD2 — Protocols Explained

The 9-Pin Deutsch Connector (HD10-9-1939P)

The 9-pin Deutsch connector is the standard diagnostic connector for Class 4–8 heavy-duty trucks manufactured from approximately 2001 to present. Its part number designation is HD10-9-1939P (the "1939" in the part number is not a coincidence — it directly references the J1939 protocol).

Physical Description

Shape: Circular, approximately 1.5 inches in diameter
Housing: Black or dark gray thermoplastic, environmentally sealed
Mounting: Panel-mount with bayonet lock or threaded coupling
Location: Usually under the dash on the driver side, near the steering column or kick panel
Pin Labels: Letters A through J (no letter I to avoid confusion with number 1)

Complete Pin Assignment

Pin	Signal	Protocol	Description
A	Battery Ground	Power	Chassis ground reference
B	Battery +	Power	+12V or +24V (unswitched)
C	CAN High (+)	J1939	CAN bus dominant high (2.5–3.5V)
D	CAN Low (-)	J1939	CAN bus dominant low (1.5–2.5V)
E	Shield/Drain	Both	Twisted pair shield drain wire
F	J1708 (+)	J1708	RS-485 non-inverting line
G	J1708 (-)	J1708	RS-485 inverting line
H	Mfg. Specific	Varies	Manufacturer-defined (some use for secondary CAN)
J	Mfg. Specific	Varies	Manufacturer-defined

Important: Not all 9-pin connectors are fully populated. On trucks built after ~2010, pins F and G (J1708) may not be wired because those trucks only support J1939. Always verify signal presence with a multimeter before assuming both protocols are available.

The 6-Pin Deutsch Connector

The 6-pin Deutsch connector was the standard before the 9-pin took over. It carries J1708 signals only — there is no CAN bus on this connector. You will find it on:

Most pre-2001 Class 7–8 trucks
Some trailer ABS modules
Certain off-highway and agricultural equipment
Some school buses and transit buses from the 1990s

Complete Pin Assignment

Pin	Signal	Description
A	J1708 (+)	RS-485 non-inverting data line
B	J1708 (-)	RS-485 inverting data line
C	Battery Ground	Chassis ground reference
D	Battery +	+12V or +24V supply
E	Not Used	Reserved / manufacturer specific
F	Not Used	Reserved / manufacturer specific

Which Trucks Use Which Connector?

Truck Brand	6-Pin (J1708 Only)	9-Pin (J1939 + J1708)	Notes
Freightliner	1995–2000	2001+	Columbia, Century, FLD series (6-pin); Cascadia, M2 (9-pin)
Peterbilt	1995–2000	2001+	379 late models may have both; 389, 579 are 9-pin
Kenworth	1995–2000	2001+	W900 older (6-pin); T680, W990 (9-pin)
Volvo	1995–2001	2002+	VN series transitioned 2002; VNL 860 is 9-pin
Mack	1995–2001	2002+	CH, CL series (6-pin); Anthem, Pinnacle (9-pin)
International	1995–2000	2001+	9000 series (6-pin); ProStar, LT (9-pin)
Caterpillar	All on-highway	2001+ (in-chassis)	CAT engines in other OEM chassis follow chassis connector
Cummins	Pre-2001 in-chassis	2001+ in-chassis	Engine follows truck OEM connector; also has Cummins INLINE adapter

Connector Location Tips

Finding the connector is sometimes half the battle. Here are the most common locations by manufacturer:

Freightliner Cascadia: Under dash, left side, near the parking brake. Usually has a rubber dust cap.
Peterbilt 579/389: Under dash, left side kick panel area. May be behind a small cover plate.
Kenworth T680: Under dash, driver side, near the fuse panel. Often mounted on a bracket.
Volvo VNL: Under dash, center-left. Sometimes behind a removable trim panel.
International LT: Under dash, left side near steering column. Usually easy to access.
Mack Anthem: Under dash, driver side. Similar location to Volvo (shared platform).

Tip: Some trucks have multiple diagnostic connectors — one for the chassis and one for the body/PTO. Make sure you are connected to the correct one for the system you are diagnosing. The engine/chassis connector is usually closer to the steering column.

Getting Complete Cable Coverage

For a shop or mobile technician servicing mixed fleets, you need both connector types at minimum. The ThinkCar 13 HD Cable Kit includes:

9-pin Deutsch (J1939 + J1708) — for 2001+ trucks
6-pin Deutsch (J1708) — for pre-2001 trucks
16-pin OBD-II — for medium-duty crossover vehicles
Additional manufacturer-specific adapters for construction, agriculture, and specialty equipment

One cable kit, total coverage. No more borrowing adapters or turning away jobs because you do not have the right connector.