Start by isolating the main fuse panel–positioned beneath the dashboard on the driver’s side. The upper row services primary circuits: ignition, fuel pump, and cooling fans. Label each terminal before disconnecting. Note the rheostatic dimmer switch wiring (brown/violet leads) branches directly from the instrument cluster harness; corrosion here mimics dashboard illumination failures.
Locate the Bosch relay sockets behind the glovebox. Relay III (fuel injection) and Relay IV (headlight washer) share a 30-amp feed from the battery junction. Swap relays temporarily to diagnose intermittent power drops. Verify ground J7 (rear left chassis) resistance–exceeding 0.1 ohms indicates frame oxidation, often causing erratic sensor readings.
Critical connections:
- ECU pins A6 and B12 (gray/white and green/red traces) demand shielded replacements–aftermarket splices degrade signal integrity within 3 years.
- Headlight motor wires (yellow/black) chafe near the front crossmember; reroute using PTFE tubing to prevent grounding.
- Tail light sockets require silver-plated crimp connectors–standard copper corrodes at solder points under thermal cycling.
For ABS conversion models, the module harness (black 32-pin) retains factory wiring but adds a dedicated power relay. Terminal 15 (red/green) supplies ignition-switched voltage–do not substitute with fused ignition leads, as this risks ABS pump burnout during braking.
Document each step. Original schematics omit later emissions refinements: the evaporative charcoal canister solenoid (purple/blue) taps into the vacuum line, yet errors here prompt false EVAP trouble codes. Cross-reference with ETM 3.7 (Revised 1981) for revised pin assignments if retrofitting later climate controls.
Electrical Schematic Guide for the 1979 V8 Grand Tourer
Begin by locating fuse panel A beneath the dashboard on the driver’s side–this cluster handles lighting, gauges, and auxiliary systems. Label each fuse with its amperage (10A, 16A) using a multimeter to verify continuity before testing circuits. If readings fluctuate, inspect the aluminum-body relays for corrosion, particularly near the firewall where moisture accumulates.
Trace the thick black cable (battery positive) from the rear-mounted accumulator to the starter solenoid–this 8 AWG wire should register
Critical Relay and Sensor Pathways
The dual-map sensor harness (two green/white leads) connects to the ECU via a 12-pin rectangular plug–clean contacts with DeoxIT, not sandpaper, to avoid removing gold plating. A faulty sensor triggers erratic fuel trim; log voltage swings using a scope (normal range: 0.5–4.5V) while cycling ignition.
For the climate control resistor block, detach the passenger-side kick panel to access the three-speed blower motor wiring. Test each coil with 12V directly–if the motor spins only at max speed, replace the pack (OEM part #991 684 406 00). Avoid bypassing; this disrupts current draw calculations in the body computer.
Inspect the headlight delay module (left of the steering column) by probing terminal 6 with a test light while turning the ignition off. The delay should last 45–60 seconds; failure indicates a faulty transistor (common in early models). Swap the entire module rather than attempting component-level repair–solder joints on these boards fatigue easily.
Ground straps under the rear seats often corrode; scrape paint from attachment points and apply dielectric grease before reattaching. The tan/black wire from the instrument cluster to the rear bulb harness carries turn signal impulses–intermittent operation suggests a broken splice inside the driver-side C-pillar. Strip 10mm of insulation and solder with rosin core, then heat-shrink to prevent shorts.
For fuel pump diagnosis, back-probe the inertia switch (mounted above the pedal cluster) with a DVOM set to 20V DC. Expect 12V on pin 3 when cranking; no voltage means either a failed relay or broken yellow/red wire near the charcoal canister. Jump the relay socket (terminals 30–87) temporarily–if the pump primes, focus on the relay circuit.
Finding Authentic Electrical Schematics for the 1979 Classic GT
Begin with the Stuttgart factory archives–Porsche’s official restoration department retains the original engineering blueprints for all early models. Contact Porsche Classic Customer Center via their website or phone (+49 711 911-0) to request schematic copies. Expect a fee (€50–€150) and a 4–6 week wait, but this guarantees factory-accurate materials with proper color codes (e.g., “BN” for brown, “GN” for green) and terminal designations. Avoid third-party reprints unless verified by a Porsche-approved vendor like Pelican Parts or Autobarn Classics.
Join marque-specific forums where owners share verified scans of original manuals. The 928 Registry (928Registry.org) hosts a private database with member-uploaded resources, including wiring overlays for the 4.5L V8 LH-Jetronic system. Rennlist (rennlist.com), though focused on later variants, has archived threads from the early 2000s with attached schematics–search for “‘79 harness layout” or “fuse panel detail.” Always cross-reference these with the factory microfiche index, available through Porsche Club of America chapters for regional meetups.
| Source Type | Access Method | Verification Tip | Cost |
|---|---|---|---|
| Porsche Classic | Official request form | Color-coded tapes match OEM | €50–€150 |
| 928 Registry | Forum uploads | Check member post count (>100) | Free/$25 membership |
| eBay seller | “Complete shop manual” listings | Inspect sample page for “DIN 72551” stamp | $40–$80 |
| SW USA swap meets | Physical binders from restorers | Look for “PF 8.1” revision date | $20–$50 |
For physical copies, target estate sales and dismantlers specializing in European grand tourers. The Southwest U.S. hosts annual swap meets (e.g., PorscheSwap.com, CarPartsUSA.com) where restorers liquidate personal archives–binders labeled “PF 8.1” or “Production Folder 1978–1980” often contain folded schematics. Inspect pages for DIN 72551 standard markings (German industrial wire sizing) and avoid photocopies with handwritten notes, which may transpose pinouts incorrectly. Garage sales in California, Nevada, and Arizona occasionally surface private collections, as these states had higher original importer volumes.
Resources to Avoid
Steer clear of “universal” harness guides or aftermarket repair manuals (e.g., Chilton, Haynes) for this model–they omit critical details like the dual-relay ignition logic or K-Jetronic fuel pump circuitry. Ebay listings titled “full workshop manual” often resell Porsche 944 schematics mislabeled as 928; verify by checking the presence of the “E-box” label (early 928 only) or the absence of the “Motronic” header (introduced in 1983). Online PDF aggregators typically compress files, losing resolution on crimp points and splice markers–request raw scans from the original uploader instead.
Decoding Electrical Schematics: Color Standards and Conductor Sizes in Classic GT Blueprints
Start repairs by isolating the main harness connector near the fuse block–most late-70s European grand tourers use a standardized color key where:
- Red: Primary power feeds (battery or ignition-switched)
- Black: Ground return paths, often bolted to chassis points
- Yellow: Lighting circuits (headlamp, taillamp, instrument illumination)
- Green: Sensor or switch inputs (temp sender, oil pressure)
- Blue: Auxiliary loads (horn, wipers, heated rear screen)
- Brown: Secondary grounds or low-current returns
- White/Stripe combinations: Signal or data lines (tachometer, fuel level)
Verify each color with a multimeter set to continuity mode before cutting or splicing; aftermarket suppliers frequently substitute darker shades to avoid trademark conflicts.
Conductor gauge flows predictably from source to load–track thickness changes along the run:
- 10–12 AWG: High-current paths–starter solenoid, electric fan motors
- 14–16 AWG: General lighting and ignition circuits
- 18–20 AWG: Dash instruments, low-power sensors
- 22 AWG and thinner: Signal wires, oxygen sensor heaters, early ECU links
Jumper repair wires only if the original gauge is unavailable; undersizing risks overheating and voltage drop. Use heat-shrink tubing over crimps unless the schematic specifies otherwise.
Trace dotted or dashed lines separately; these denote shielded cables–often ignition noise suppressors wrapping spark plug wires or radio antenna coaxial feeds. Preserve original shielding foil during repairs; bypassing it invites EMI that disrupts fuel injection pulse timing.
Common stripe conventions repeat across legacy schematics:
- White/Black stripe: Ground-side switch output
- Green/Yellow stripe: Injector control or idle air bypass
- Black/White stripe: Door ajar or courtesy lamp triggers
- Red/White stripe: Fused accessory leads (radio memory keep-alive)
Mark splices with corresponding stripe tags before disconnecting connectors–factory crimps use color-matched sleeves absent in generic repair kits.
Avoid solder on stranded conductors thicker than 12 AWG; vibration fractures joints over time. Instead, apply high-pressure crimps with a ratcheting tool rated for automotive copper. Verify each connection by load-testing with a 10 A circuit probe before insulating.
Cross-reference color swatches against the legend printed on the original blueprint’s margin–German suppliers sometimes labeled Brown as “BR” and Yellow as “GE” on vehicles exported outside Europe. Photograph both sides of each connector before disassembly; terminal housing shapes and lock-tab positions pinpoint correct orientation.
Substitute wires only when the specified gauge exceeds available stock; 18 AWG can replace 20 AWG for short instrument runs without risk, but never downgrade primary feed cables–voltage drop becomes noticeable under load.
Label every detached wire with masking tape and note its routing clip position; hidden channels behind trim panels and under carpet often secure single wires separately from main harness bundles. Reassemble clips in reverse order–factory clips press-fit into body grommets that degrade if forced.