From surveying the original loom to the build board: schematic, 1:1 Layout, Deutsch, Metri-Pack or Superseal connectors pulled from Digi-Key with their cavities, BOM in Excel and PDF drawings with lengths and a connection table. In the browser, documented the way a serious workshop expects.
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A car packs an oven, a salt shower and a cocktail shaker into two metres. The wiring has to survive all three without anyone remembering it exists.
Oil, petrol, 125 °C beside the manifold, vibration and pressure washing. Cabin-grade PVC cracks within two winters; an unsealed connector corrodes the terminal from the inside. Choosing temperature class, connector family and protection by zone is the design, not a detail.
In a restomod or a conversion the factory wiring gets cut, spliced and patched until nobody knows what each wire does. The alternative is to survey it, define a new architecture (fuses or PDM, standalone ECU, CAN to the dash) and build from scratch, measured on the actual car.
The car will be back in five years with a fault and a different technician in front of it. A from-to table, a harness drawing with lengths, the BOM and the schematic turn a hand-built job into a repairable product. Without them, every fault is a post-mortem.
The criteria a tier-one harness maker applies, translated into what an auto-electrical workshop, a restorer or a small vehicle manufacturer needs. They do not replace the standard or the connector datasheet, but they avoid the mistakes that show up in almost every harness that arrives to be redone.
Vehicle cable is not building wire under another name. ISO 6722 sets how the insulation behaves against temperature, fluids (oil, petrol, diesel, brake fluid), abrasion and ageing, and sorts it into classes: A (85 °C), B (100 °C), C (125 °C), D (150 °C), E (175 °C) and F (200 °C). The class is the temperature the insulation withstands continuously for thousands of hours. In the cabin, class B is enough; near manifolds and turbo, C or D is specified.
Europe uses the DIN 72551 designations. FLRY-A and FLRY-B are thin-wall PVC over fine-stranded copper, class B (100 °C, often sold as T105); the A has the thinner wall. It is the cabin cable and most of a production loom. FLR2X, cross-linked polyethylene (XLPE), moves up to class C with 150 °C variants: engine, gearbox, exhaust area. FLR9Y, a thermoplastic elastomer (TPE), covers the same range with better cold flexibility and abrasion resistance. The American world works to SAE J1128: TXL, GXL and SXL are 125 °C XLPE that differ only in wall thickness (TXL thin, SXL thick), and GPT is the basic 80 °C PVC that should never enter an engine bay.
AWG and mm² do not line up (0.5 mm² is close to 20 AWG; 1.5 mm² sits between 16 and 15; 2.5 between 14 and 13; 6 mm² between 10 and 9): if a terminal datasheet says "16-20 AWG", 1.5 mm² just fits and 2.5 does not.
| Type | Insulation | Class / temperature | Where it goes |
|---|---|---|---|
| FLRY-A / FLRY-B | Thin-wall PVC | B · 100 °C (T105) | Cabin, boot, doors, dashboard |
| FLR2X | XLPE | C · 125 °C (150 °C variants) | Engine bay, transmission, near the exhaust |
| FLR9Y | TPE | C-D · 125-150 °C | Engine with movement, abrasion, extreme cold |
| TXL / GXL / SXL (SAE J1128) | XLPE, three wall thicknesses | 125 °C | American equivalent; TXL for weight, SXL in harsh zones |
| Shielded pair | PVC or XLPE + braid or foil | As base cable | CAN, crank and cam sensors, knock, lambda |
For colours, IEC 60757 defines the international abbreviations (BK, BN, RD, YE, GN, BU, VT, GY, WH) and DIN 47100 the sequence for multicore cables, but European schematics use the German codes: SW black, BR brown, RT red, GE yellow, GN green, BL blue, VI violet, GR grey, WS white. The stripe follows the slash (SW/WS: black with a white stripe). Two traps: GR is grey, not green, and SW (schwarz) is not WS (weiß).
Wire size is decided by three things at once: continuous current, real run length and acceptable voltage drop. The formula is ΔU = ρ × L × I / S, with ρ for copper around 0.0175 Ω·mm²/m at 20 °C and about 0.021 when hot, and L equal to feed plus return (if the return is through the chassis, count the ground lead up to the star point). At 12 V a sensible limit is 0.3 to 0.5 V for power loads and less for ECU and sensors. Example: a 15 A fuel pump with a 4 m feed and 1 m of ground, for a 0.4 V drop, needs S = 0.021 × 5 × 15 / 0.4 ≈ 3.9, that is, 4 mm²; by current alone 2.5 would have done, and the pump would have run below its calculated pressure.
The fuse protects the wire, not the load: choose it above the service current, with margin for motor start-up, and below what the wire can carry inside the bundle, which is less than in free air. The run between battery and first protection is unfused, so it must be short, heavy and mechanically protected. And the size has to fit the connector terminal; often it is the connector that fixes the final wire size.
Outside the cabin every connector is sealed, and each family has its niche: Deutsch (now TE) DT, DTM and DTP in special vehicles, off-road and club motorsport; Aptiv (formerly Delphi) Weather-Pack and Metri-Pack 150, 280 and 480 (the number is the terminal width in hundredths of an inch, with GT 150 and GT 280 as OEM successors) in the American world; Bosch Kompakt (BDK), TE AMP Superseal 1.5 and the Junior Power Timer of the MCP family in the German groups; Sumitomo and Yazaki in Japanese vehicles and motorcycles; Molex MX150 in machinery and heavy vehicles. Inside the cabin, unsealed connectors with 0.64 mm MQS terminals fill the control units.
| Family | Sealing | Current per way (indicative) | Cable | Typical use |
|---|---|---|---|---|
| Deutsch DTM | IP67 / IP68 | 7.5 A | 0.2-1 mm² | Sensors, signals, standalone ECU |
| Deutsch DT | IP67 / IP68 | 13 A | 0.5-2 mm² | Actuators, lighting, medium power |
| Deutsch DTP | IP67 / IP68 | 25 A | 2-6 mm² | Pumps, fans, power feeds |
| Aptiv Weather-Pack | Per-wire seals | Up to 20 A | 0.5-3 mm² | Classic GM and American aftermarket |
| Aptiv Metri-Pack 150 / 280 / 480 | Sealed or unsealed | 14 / 30 / 42 A | 0.35-1 / 1-3 / 3-6 mm² | Sensors / pumps and alternator / heavy loads |
| TE AMP Superseal 1.5 | Sealed | Around 14 A | 0.5-2.5 mm² | Tuning, sensors, small engine-bay loads |
| TE JPT / MCP 2.8 · Bosch BDK | Sealed and unsealed versions | Low to medium | 0.35-2.5 mm² | Relays, injectors, sensors, German OEM |
| TE MQS 0.64 | Unsealed | Signal | 0.22-0.5 mm² | ECU and module connectors in the cabin |
The terminal matters as much as the housing and only works with its cable and its tool. Stamped open-barrel terminals are F- or B-crimped with the die for their geometry and wire size; solid Deutsch contacts, with a calibrated indent crimper. The insulation crimp must not bite the conductor, and the wire seal must match the insulation outside diameter, which differs between FLRY-B and FLR2X of the same size. Secondary locks close the system: the TPA locks terminals in their cavities and the CPA stops the halves separating under vibration. A sealed connector is only sealed with every cavity populated or plugged, and the IP rating (IPX7 immersion, IPX9K hot pressure water) refers to the closed connector. Bulkhead pass-throughs use rubber grommets or, where disconnection is needed, bulkhead connectors, the norm in motorsport.
In the cabin, PET fleece tape (Tesa 51608 and equivalents) damps vibration, prevents rattles and resists abrasion at minimal weight; spiral-wrapped with overlap, it is enough for most branches. In the engine bay the reference is PA6 polyamide convoluted conduit, split or closed; high-temperature PET cloth tapes (Tesa 51036 and equivalents, 150 °C and flame-retardant) protect without the bulk of the tube, and PVC or TPE sleeving handles short branches with tight bends. At ends and transitions goes adhesive-lined heat shrink, which seals, holds the conduit and stops its edge cutting the insulation.
Cable should not pass near the manifold, turbo or catalyst; where there is no alternative, keep it as far away as possible and protect it with aluminised fibreglass sleeving. In the splash zone every connector sits with its wire exit pointing down, a drip loop and plugs in unused cavities. Fixings go every 20 to 30 cm with lined P-clips, never cable ties on brake or fuel lines, and every bulkhead pass-through gets its grommet. The bend radius should not drop below four or five times the harness diameter, and the connector exit must run straight for a few centimetres before the first bend.
The first drawing of a new harness is the ground plan. Battery, block and body are joined with heavy cables onto bare metal; the power ground (injectors, coils, pump) comes off the block, and module and dash grounds come off a single star point. Sensor grounds do not go to the body: they return to the ECU on their own wire. Power is distributed from a fuse and relay box (mini or micro, MIDI and MEGA for the alternator and heavy loads) or from a power distribution module (PDM) with solid-state outputs; each output is sized to the wire it protects.
The CAN bus follows ISO 11898-2: twisted pair, linear topology, 120 Ω at each end (60 Ω measured between CAN-H and CAN-L with the system off) and short stubs, of the order of tens of centimetres. With a digital dash, standalone ECU and PDM on the same bus, only two may have termination enabled. LIN is a single-wire, slow, cheap bus for switches and climate control, with no termination. A standalone-ECU harness is designed from the manufacturer's pinout: crank and cam in shielded pair with the shield grounded at the ECU end only, power through a fused main relay, separate grounds and coil wiring kept away from sensor wiring. Inductive loads generate transients when switched off (ISO 7637-2 describes the pulses; ISO 16750-2, the tests), and the harness answer is a diode or suppressor at the load and separate routing. In electric conversions, high voltage runs in orange shielded cable (LV 216), with an interlock loop (HVIL) that cuts the voltage if a connector opens, physically separated from the 12 V harness; it requires specific training and only that separation is noted here.
The most widespread acceptance standard is IPC/WHMA-A-620, whose class 2 is the usual reference in automotive and special vehicles. Connector system performance is specified by SAE/USCAR-2 and crimp performance by SAE/USCAR-21; ISO 8092 covers on-board wiring connections and LV 214 is the German OEM connector specification. For a workshop these translate into four checks: crimp pull test (with minimums that rise with wire size), cross-section micrograph on the important series (uniform compression, no voids or stray strands), inspection of the insulation crimp, and 100 % continuity and short-circuit testing against the from-to table. The minimum documentation is the schematic, the from-to table with connector, cavity, colour and size, the harness drawing with lengths, and the BOM with part numbers.
The process for a restomod or a kit car is always similar. Survey the original loom: photos, labels at every end, continuity and a reconstruction of the schematic. Define the new architecture (fuses or PDM, ECU, dash, what is kept) and draw it complete before cutting anything. Measure lengths on the assembled car, with string following the real route, or on a 1:1 drawing. With those measurements build the formboard with forks at every breakout, label every end with printed heat shrink and test the harness on the bench before fitting. The same ideas, with more demanding materials, apply in motorsport, aerospace and military and industrial work.
Every row is a real function of the application. What harnessPRO does not do (simulate, calculate derating, manufacture) you will not find here either.
With the car surveyed, draw the new schematic: fuses or PDM, ECU with its imported pinout, sensors, actuators, grounds and CAN to the dash. Every wire carries colour, size and cable type from the start.
Load the engine-bay photo or chassis drawing as background, place connectors and branch points at real scale and orient boots and branches. The lengths you measure are documented in the harness.
Pull housings and terminals from Digi-Key, add seals, plugs, cable, conduit and tape as accessories and export the BOM to Excel to order everything in one go.
Export the PDF with schematic, harness drawing, lengths and from-to table. Build on the board from it, test continuity and leave the customer the car's folder.
ISO 6722 class C or D cable (125 or 150 °C): FLR2X (XLPE) or FLR9Y (TPE) in European designations; TXL, GXL or SXL to SAE J1128 in American ones. PVC FLRY-B (100 °C) is cabin cable, not manifold, turbo or gearbox cable. Near direct heat sources, also keep the harness away and add aluminised fibreglass sleeving.
Current alone does not decide it. Work out the voltage drop over the real run: ΔU = ρ × L × I / S, with ρ for hot copper around 0.021 Ω·mm²/m and L the sum of feed and return. Set a limit (0.3 to 0.5 V at 12 V for power loads, less for the ECU), solve for S, then check that the fuse protects the wire and that the size fits the connector terminal.
DTM uses size 20 contacts for 0.2 to 1 mm² and about 7.5 A per way: signals, sensors and ECU wiring. DT uses size 16 contacts for 0.5 to 2 mm² and about 13 A: actuators, small pumps, lighting. For more current, DTP (size 12) reaches 25 A with 2 to 6 mm². All three are sealed, but contacts and crimp tooling are not interchangeable between series.
By zone. In the cabin, PET fleece tape against noise and abrasion. In the engine bay, PA6 convoluted conduit or high-temperature PET cloth tape, with aluminised sleeving near the exhaust. At transitions and connector exits, adhesive-lined heat shrink. Fixings every 20 to 30 cm with lined clips, a grommet at every bulkhead pass-through and generous bend radii.
Conduit protects better against abrasion and radiated heat, but it is heavier, bulkier and transmits vibration to the connector if it runs right up to it with no relief. PET tape is light and follows bends and breakouts. A good harness combines both: tape in the cabin and on thin branches, conduit on exposed engine-bay and underbody runs, and heat shrink at the ends so the tube cannot bite the wire.
Start from the manufacturer's pinout. Separate grounds: sensor grounds go only to the ECU, power grounds (injectors, coils, pump) to a solid engine ground. Crank and cam sensors run in shielded pair with the shield grounded at the ECU end only. Power comes in through a main relay with its own fuse, and CAN is terminated with 120 Ω at each end. Draw everything before cutting, and measure lengths on the installed engine.
Yes, it is one of its most common uses. You draw the new schematic with connectors, splices, relays and diodes; bring the engine-bay photo or chassis drawing into Layout as a background image and route the harness at 1:1; pull Deutsch, Metri-Pack or Superseal connectors from Digi-Key with their cavities; and export the BOM to Excel and PDF drawings with lengths and a connection table to build on the board. The free Garage plan lets you start with nothing to install.
Free Garage plan with 3 harnesses and 50 connections per harness. Pro at €39/month or €372/year and Team at €29 per user per month (minimum 3), taxes included. The full manual is in the documentation; the features, on the home page.