Factory-verified wiring reference

2019 Chevrolet Express 2500 · 2.8L Duramax (LWN)

ECM/PCM, TCM, and the high-speed GMLAN “CAN high loop.” Every pin below was read directly from GM’s 2019 Express/Savana Electrical Body Builder Manual (model-year exact, free from GM) and cross-checked against the 2022 Colorado/Canyon manual, which shares the E98 ECM. Factory schematic pages are embedded below — click any page to zoom.

ECM: E98 · GM “K20” TCM: GM “K71” · 8L90 (RPO M5U) Primary bus: ckt 2500/2501 DLC: pins 6 / 14

Quick checks

Start here if you’re chasing a no-comm / U-code problem.

Bus health, 30 seconds

≈60 Ω

Key off, battery negative disconnected: measure across DLC pins 6 ↔ 14. Two 120 Ω terminators in parallel → ~60 Ω. Reading ~120 Ω means one end of the loop is lost; open or shorted means a broken or grounded pair.

The two terminators

K20 + R6A

One 120 Ω resistor lives inside the ECM. The other is R6A, a separate 2-wire resistor plug in the engine compartment, wired after the TCM (pins 53/54). If you read ~120 Ω and the ECM talks, go find R6A and its pigtail first.

Best probe points

DLC pins 6/14 · JX250 splice pack behind the brake pedal at the carpet edge · X100 IP-to-engine harness connector, left-rear engine bay by the underhood fuse block · X116 engine-to-chassis connector (diesel only), engine compartment. Unplugging these splits the bus into halves for isolation.

Live case: bus collapses only when the TCM is plugged in

Symptoms: scan tool communicates only with the TCM unplugged; plug it in and both CAN lines sit ≈1.3 V instead of ≈2.6/2.4 V; blue (CAN High) reads near nothing; DLC normal with the TCM out.

What the topology already rules out: the cross-cab / cowl run (JX250 → X100 → X116 → TCM pins 37/38) stays connected to the bus even with the TCM unplugged — an unplugged module doesn’t disconnect its harness stub. A passive short anywhere in that run would still kill the DLC with the TCM out. Since the DLC is clean, the fault is in what the 66-way connector adds: the 53/54 → R6A pigtail, the TCM’s power/ground feeds, or the TCM itself. Both lines pinned at the same ≈1.3 V is the classic signature of a transceiver clamping through its protection diodes (bad ground/supply) or a hard fault dragging the tied pair down.

#	Where	Measure	Good	If bad →
1	DLC 6↔14, TCM unplugged, key off	Resistance	≈120 Ω	~120 Ω here is normal — R6A leaves with the TCM. If you see ~60 Ω with the TCM out, stop: something else is terminating and this page’s assumptions need a second look.
2	TCM harness connector, pins 53↔54	Resistance	≈120 Ω	Open = broken pigtail or R6A unplugged/missing. (This pair is dead-ended here with the TCM out — safe to measure.)
3	TCM harness connector, 53→gnd and 54→gnd	Resistance	open / MΩ	Low ohms = the chafed spot. The short run from the TCM connector to R6A in the engine bay is the prime suspect — it only joins the bus through the TCM, which is exactly your symptom.
4	Pin 66→B+ (check fuse F41UA “TCM BATT” 20 A) and pin 35 key on	Voltage under load (test light)	B+	Dead feed = transceiver clamps the bus. Check the fuse before anything else.
5	Pin 65→battery negative	Resistance / voltage-drop cranking a load	<0.5 Ω	Bad ground = same clamping signature. Load-test it; a DMM alone will lie about a corroded ground.
6	On the TCM module pins: 37→65 and 38→65	Resistance	high (kΩ+)	Near-zero = shorted transceiver, TCM is bad. Note: 37↔38 across the module should not read 120 Ω — this TCM has no internal terminator (R6A is external on this van).
7	The 66-way itself + R6A 2-way	Eyes	clean & tight	Green crust, backed-out or spread terminals, water in the sealed shell — all reproduce “bad TCM” symptoms.

If steps 2–5 and 7 all pass, the module itself is condemned by elimination — step 6 just confirms it on the bench. If step 3 finds the short, follow the 53/54 pigtail from the connector to R6A before buying anything.

The CAN high loop (primary high-speed GMLAN)

Linear backbone, ECM at one end, R6A resistor at the other. CAN High is circuit 2500 (blue wire), CAN Low is circuit 2501 (white wire) everywhere on the diesel van. The DLC is a stub off the JX250 splice pack — it is not in the middle of the backbone.

ckt 2500 · CAN High (+) ckt 2501 · CAN Low (−) Dashed boxes = only present with that RPO; without it the pair passes straight through a splice.

K20 ECM (E98)120 Ω inside

right side of engine

X1-55 (+) · X1-40 (−)

X100 pins 36/40
left-rear engine bay

JX250 Splice PackDLC stub: pins 6/14

behind brake pedal, at carpet edge (next to JX200)

A·B·C = 2500 · K·L·M = 2501

IP harness

K9 BCM

instrument panel

X3-8/16 · X3-9/17

—

K73 TelematicsRPO UE1 only

OnStar module — bus loops through it

X2-3/4 · X1-10/12

X100 33/37
X116 5/6

K17 EBCM (ABS)

frame rail, center of vehicle

pins 9/8 · 21/20

—

E19 Coolant HeaterRPO K08 only

fuel-fired aux heater — bus loops through it

C/E · B/D

X116 39/40
engine bay

K71 TCM (8L90)

on transmission assembly

37 (+) / 38 (−) in · 53/54 out

2-wire pigtail

R6A Terminating Resistor120 Ω

engine compartment · 2-way GT-150 connector

A (+) / B (−)

Not on this bus: instrument cluster, airbag SDM, radio, and theft module are on the low-speed single-wire GMLAN on this van (DLC pin 1, gatewayed by the BCM) — different from most GM trucks. Don’t chase a cluster complaint on the high-speed pair.

GM factory schematic: High Speed GMLAN (Diesel), 2019 Express/Savana — **Factory schematic — “Data Communication Schematics (High Speed GMLAN (Diesel))”**, 2019 Express/Savana Electrical Body Builder Manual p. 6-6 (PDF page 100). This is the CAN high loop, as built.

ECM (E98 / GM K20) — key pins

Three connectors, all female terminals, all on the engine harness: X1 66-way (OEM 35059716 / service 19370822), X2 96-way, X3 56-way. All CAN + power basics live on X1; glow plug drives on X2. Highlighted rows are the serial-data circuits.

Pin	Wire	Ckt	Function
X1-1	VT/GY	1439	Run/Crank Ignition 1 voltage
X1-2	VT/D-BU	5291	Powertrain main relay fused supply 2
X1-3	VT/D-BU	5292	Powertrain main relay fused supply 3
X1-4	RD/BN	440	Battery positive voltage
X1-5	RD/YE	2	Battery positive voltage (6 mm²)
X1-6	BK/WH	451	Signal ground (6 mm²)
X1-7	VT/YE	5985	Accessory wakeup serial data
X1-11	BU/YE	6861	Water-in-fuel sensor signal
X1-39	WH	7494	HS GMLAN serial data (−) 3 — powertrain expansion bus
X1-40	WH	2501	HS GMLAN serial data (−) 1 — CAN Low, primary bus
X1-50	L-GN/GY	465	Fuel pump primary relay control (direct-wired — no CAN fuel module on LWN)
X1-51	YE	5991	Powertrain relay coil control
X1-54	D-BU/BK	7493	HS GMLAN serial data (+) 3 — powertrain expansion bus
X1-55	BU	2500	HS GMLAN serial data (+) 1 — CAN High, primary bus
X2-13	GY/BN	1582	Glow plug control 2 (direct drive, 2 mm²)
X2-14	GY/YE	1584	Glow plug control 4
X2-15	GY/L-GN	1583	Glow plug control 3
X2-16	GY/D-BU	1581	Glow plug control 1

Watch out: X1-54/X1-39 (ckt 7493/7494) look like a second CAN pair at the ECM — they are the powertrain expansion bus to the NOx/DEF sensors, gatewayed inside the ECM. It is a separate 120 Ω-terminated network (see other buses); a resistance reading there tells you nothing about the primary loop.

K20 Engine Control Module X1 (LWN) connector face and pin table — **K20 X1 (LWN) connector face + start of factory pin table** — manual p. 6-368 (PDF page 462). Full X1/X2/X3 tables continue on the following manual pages.

TCM (GM K71) — 8L90, key pins

The 2019 diesel van runs the 8L90 8-speed, RPO M5U (confirmed in the 2019 manual; the 6.0L gas kept the 6L90 “MYD”). The TCM is a standalone external module on the transmission assembly with one 66-way connector (OEM 33168232 / service 19329822). The aftermarket calls this hardware family “T87/T87A” — GM literature only ever says K71. Battery feed is underhood fuse F41UA “TCM BATT” 20 A.

Pin	Wire	Ckt	Function
28	BK/BN	586	Trans oil temperature sensor low reference
33	L-GN/GY	6387	Transmission high-side driver 1
35	VT/BK	2139	Run/Crank Ignition 1 voltage
37	BU	2500	HS GMLAN serial data (+) 1 — CAN High in
38	WH	2501	HS GMLAN serial data (−) 1 — CAN Low in
47	GY/YE	4169	PRNDL S signal
49	GY/BN	6388	Transmission high-side driver 2
51	VT/YE	5985	Accessory wakeup serial data
53	BU	2500	HS GMLAN (+) 1 — CAN High out to R6A terminator
54	WH	2501	HS GMLAN (−) 1 — CAN Low out to R6A terminator
59	GY/WH	4168	PRNDL P signal
60	WH/BK	5983	PRNDL C signal
61	GY/BN	5982	PRNDL B signal
62	VT/WH	5981	PRNDL A signal
63	BN/WH	585	Trans oil temperature sensor signal
65	BK/WH	1551	Signal ground
66	RD/L-GN	1840	Battery positive voltage (fuse F41UA “TCM BATT” 20 A, underhood block)

Two CAN pairs, one bus: 37/38 bring the backbone in; 53/54 send it out to the R6A terminator. Unplug the TCM and the R6A terminator drops off with it — a ~120 Ω DLC reading with the TCM unplugged is normal, not a second fault.

Factory pages: K71 connector face + full (LWN) pin table

K71 66-way connector face drawing — K71 66-way connector face and part info — manual p. 6-382 (PDF page 476). Same 66-way housing for gas and diesel.

K71 Transmission Control Module (LWN) pin table — K71 (LWN) factory pin table with the CAN pins — manual p. 6-385 (PDF page 479).

Junctions, splices & the DLC

DLC (X84) — OBD-II port

Pin	Wire	Ckt	Function
1	—	—	Low-speed GMLAN serial data (single wire)
4	—	—	Scan tool power ground
5	—	—	Common signal ground
6	BU	2500	HS GMLAN serial data (+) — CAN High
14	WH	2501	HS GMLAN serial data (−) — CAN Low
16	—	—	Battery positive (scan tool power)

JX250 splice pack — where the bus branches

12-way Metri-Pack 280 (OEM 15305288), instrument panel harness, left front floor where the carpet ends behind the brake pedal, next to splice pack JX200. Pins A·B·C are all ckt 2500 (CAN High), K·L·M all ckt 2501 (CAN Low): one leg to the DLC, one leg toward the ECM via X100, one leg into the body/BCM side. Pull this pack and you can isolate all three directions from the driver’s seat.

Factory page: JX250 splice pack connector + pin table

JX250 Splice Pack connector drawing and pin table — JX250 splice pack — manual p. 6-556 (PDF page 650).

R6A terminating resistor

Discrete 120 Ω resistor plug, engine compartment, 2-way F 150 GT-series sealed connector (OEM 13510085 / service 13580114), pin A = ckt 2500, pin B = ckt 2501. This is the terminator you can actually get your hands on — check it before condemning a module.

Factory page: R6A connector detail

R6A Terminating Resistor connector drawing — R6A Terminating Resistor — High Speed Bus — manual p. 6-488 (PDF page 582).

In-line harness connectors on the CAN pair

Conn	Joins	Location	CAN pins
X100	IP harness ↔ engine harness	Left rear of engine compartment, near underhood fuse block and horn	36/40 (ECM leg) · 33/37 (EBCM leg)
X116	Engine harness ↔ chassis harness (LWN only)	Engine compartment	5/6 · 39/40 (+ 32/33 expansion bus)
JX250	3-way splice: DLC / ECM leg / body leg	Behind brake pedal at carpet edge	A–C = 2500 · K–M = 2501

The other three buses (don’t mix them up)

Bus	Circuits	Who’s on it	Terminators
Primary HS GMLAN	2500 / 2501	ECM, TCM, BCM, EBCM, DLC, (+OnStar K73, coolant heater E19)	ECM internal + R6A
Powertrain expansion (diesel only)	7493 / 7494	ECM (gateway, X1-54/39), NOx sensor 1, reductant/DEF module K115, NOx sensor 2, PM sensor B136	ECM internal + NOx sensor 2
Chassis HS GMLAN	6105 / 6106	Chassis expansion modules (K38 chassis control, trailering) — see schematic	see schematic
Low-speed GMLAN	single wire, DLC pin 1	IPC cluster, airbag SDM, radio, theft, chime, door locks — gatewayed by BCM	n/a (single wire)

Factory schematic: Powertrain expansion bus (LWN) — NOx / DEF / PM sensors

Powertrain High Speed GMLAN (LWN) schematic — Manual p. 6-7 (PDF page 101). Note: the 2022 Colorado/Canyon manual labels these same E98 pins ckt 4498/4499 (“serial data 7”) — same pins X1-54/X1-39, different circuit label. On the van it’s 7493/7494.

Factory schematic: Chassis High Speed GMLAN

Chassis High Speed GMLAN schematic — Manual p. 6-9 (PDF page 103).

Factory schematic: Low Speed GMLAN (cluster, SDM, radio…)

Low Speed GMLAN schematic — Manual p. 6-4 (PDF page 98).

Factory schematic: Accessory wakeup line (ckt 5985 — ECM/TCM wake signal)

Accessory Wakeup schematic — Manual p. 6-8 (PDF page 102). A dead 5985 line makes modules look asleep, not failed — worth knowing before condemning a TCM. Also the page that proves the diesel pairs with M5U.

Getting the complete factory diagrams

Free — official GM PDFs (source of everything on this page)

2019 Express/Savana Electrical Body Builder Manual (~35 MB, 823 pages) — data-comm schematics, module pin tables, connector faces, component locations, fuse maps. Model-year exact.
2019 Express/Savana chassis section — body/chassis supplement.
2022 Colorado/Canyon Electrical BBM — same E98 ECM; deeper LWN engine-harness detail (all 218 ECM pins).
gmupfitter.com 2019 index — airbag supplement, spec sheets.

Paid — when you need every circuit in the van

Source	Price	Notes
ACDelco TDS (GM SI)	~$20–30 / 3 days	The actual dealer service info — complete VIN-exact schematics. Pricing changed July 2026; confirm at checkout.
ALLDATAdiy	$19.99 / mo · $59.99 / yr	Interactive color diagrams, single vehicle. Easiest UI; pricing verified live.
Mitchell1 DIY	$19.99 / mo · $39.99 / yr	Equivalent alternative; confirm Express is in their vehicle selector before paying.

Dead ends (so you don’t waste time)

charm.li (Operation CHARM) stops around MY2013 — no 2019 Express.
BBB Industries free diagrams: discontinued.
duramaxforum / dieselplace / coloradofans now sit behind a bot-wall for automated access; threads like “Anyone need wiring diagrams for free?” are still worth browsing by hand.

Verification log

How each claim was confirmed, and what’s still soft.

Claim	Status	Evidence
ECM CAN High/Low = X1-55 / X1-40	Confirmed ×2	2019 Express BBM pin table + schematic; identical in 2022 Colorado BBM (shared E98).
TCM CAN High/Low = 37 / 38 (out: 53 / 54)	Confirmed ×2	2019 Express BBM K71 (LWN) pin table + diesel schematic. Matches the previously unverified forum claim.
Terminators = ECM internal + discrete R6A	Confirmed	120 Ω symbols on the 2019 diesel schematic at K20 and R6A; R6A located “in the engine compartment.” (Corrects the common assumption that the second resistor is inside a module.)
2019 diesel trans = 8L90, RPO M5U	Confirmed	2019 BBM RPO usage throughout (“K71 (M5U)”, fuse table, wakeup schematic branch tagged LV1/LWN). The “MQD” diesel RPO only appears in 2022+ documents.
No glow-plug or fuel-pump module on the CAN bus	Confirmed	Glow plugs driven directly from ECM X2-13…16; fuel pump via relay from X1-50. The K34 glow module some sources mention belongs to the old 6.6L LGH V8 vans, not the LWN.
“T87A” naming	Unsettled	GM literature only says K71. Aftermarket disagrees on whether the 8L90 unit is “T87” or “T87A” — irrelevant for wiring (pinout above is factory), matters only when ordering/flashing a replacement.
Body Builder Manual vs full SI	Scope note	The free BBM covers data-comm schematics, module pin tables and locations — everything on this page. For every last body/interior circuit diagram, use TDS or ALLDATA above.