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This one I did in Copilot using the Think Deeper setting. I'm doing diagnostics on a non-sparking 3.5V8Originally Posted by loanrangie
Trying to trouble shoot an ignition system on a LAnd Rover Discovery 3.5 V8. There is no spark from a spark plug attached directly to the ignition coil. The coil is getting power and it is getting power while cranking the engine. I have fitted a new ignition module and checked this is getting returned power to the coil. I have tried new leads and new spark plugs. What else can I check to get spark?
Got it, Simon—no spark right at the coil tower narrows things down nicely. On the Rover 3.5 V8 (Disco 1–era), the coil only throws a spark if it’s getting a solid supply and its negative terminal is being switched by the ignition amplifier/distributor pickup. Here’s a practical, step‑by‑step flow you can run through in the driveway.
1) Basics: power & grounds while cranking
- Battery voltage under crank: Make sure the battery stays healthy (>~10 V while cranking). Low voltage can kill ignition energy.
- Grounds: Do a quick voltage‑drop test while cranking—measure between battery negative and the coil bracket/distributor body; aim for <0.2 V. Clean the engine‑to‑chassis earth strap.
(General good practice; no external source required.)
2) Verify coil type & resistance (quick meter checks)
The 3.5 V8 commonly uses a Lucas electronic‑ignition coil (e.g., DLB198 / RTC5628). Check yours with a multimeter:
- Primary resistance (between + and −): Expect roughly 0.7–1.0 Ω on DLB198‑type coils. [holden.co.uk], [britishautousa.com], [partsinmotion.co.uk]
- Secondary resistance (centre HT to −): Typically ~7–9 kΩ on DLB198‑type coils. [holden.co.uk], [britishautousa.com]
If your readings are way outside those ranges, the coil may be faulty even if it has power.Also confirm your coil model suits electronic ignition (DLB198 family) used on these engines. [island-4x4.co.uk]
3) Check for a ballast/resistance feed & crank bypass
Many Rover V8 installations use a ballast resistor or resistance wire feeding the coil with a bypass during cranking from the starter solenoid. If the ballast feed is open (or bypass missing), you can have 12 V present but still get a weak/no spark in certain key positions. Inspect for a ballast resistor/resistance wire in the loom and confirm the bypass feed is live during CRANK. [johnrichar…plus.co.uk], [lrparts.net]
4) Is the coil negative actually being switched?
You can test this with a test light:
- Clip a test light between coil − and battery +.
- Crank the engine. The light should flash if the ignition amplifier/distributor pickup is toggling the coil. If it doesn’t flash, the fault is upstream (pickup, amplifier, wiring). [tomorrowst…nician.com]
A classic bench check of the coil itself:
- Fit a plug to the coil HT tower and earth the plug body.
- Apply 12 V to coil +. Make & break the connection to coil − to simulate switching—each break should produce a spark. If no spark, suspect the coil. [landyzone.co.uk]
Tip from that thread: don’t hard‑earth the coil negative—if it’s tied to ground permanently, you’ll never get spark. [landyzone.co.uk]
5) Distributor pickup & air gap (very common culprit)
On the Lucas electronic distributors (e.g., 35DLM8), the magnetic pickup inside must be healthy and gapped correctly:
- Pickup resistance test: Unplug the two pickup wires and measure across them—you should see “thousands of ohms” (kΩ range) and the reading will fluctuate as the reluctor passes the pickup. [landyzone.co.uk]
- Air gap: Set the reluctor/pickup air gap to 0.20–0.35 mm (0.008–0.014"). Use a non‑metallic feeler if possible. [lr4x4.com]
If the rotor doesn’t turn while cranking, the drive gear/oil pump drive engagement is suspect—but if you’ve got good mechanical rotation, focus on that pickup gap and signal.
6) Ignition amplifier/module & its mounting
Early setups mounted the ignition amplifier on the distributor body; heat soak and dry joints made these notorious for intermittent/no‑start failures. Many folks relocate the module to a cooler spot near the coil. Even with a new module, confirm solid ground and good connectors; relocating can improve reliability. [v8forum.co.uk]
For additional context and testing hints on these Rover V8 amplifiers and distributors, this “Ignition Masterclass” video shows bench‑testing the amplifier and generating spark with a simple rig. [youtube.com]
7) Cap/rotor & HT path sanity checks
- Inspect the distributor cap’s centre carbon brush and spring—if missing/broken, the coil’s HV never reaches the rotor.
- Try a known‑good HT lead from coil tower to a plug with a 8–10 mm gap spark tester.
(General good practice; no external source required.)
8) Wiring logic (keep it simple)
- Coil + should get a clean switched 12 V (RUN) plus a bypass feed (CRANK) in ballasted systems. [johnrichar…plus.co.uk], [lrparts.net]
- Coil − is controlled by the ignition amplifier (and typically also feeds the tachometer). Make sure it’s only connected through the amplifier—not bolted straight to ground. [landyzone.co.uk]
9) If you had the distributor out (timing sanity check)
- Set No. 1 cylinder to TDC on compression and align the rotor to the No. 1 post.
- Fit leads clockwise in 1‑8‑4‑3‑6‑5‑7‑2. [v8forum.co.uk]
Quick decision tree
- Test light on coil − flashes while cranking?
→ Yes: Coil or HT path (cap/rotor/lead) likely at fault. Re‑try the bench “make & break” coil test and inspect cap/rotor.
→ No: Focus on pickup air gap/resistance and amplifier wiring/ground. Consider relocating/replacing the module if mounted on the distributor. [landyzone.co.uk] [landyzone.co.uk], [lr4x4.com], [v8forum.co.uk]- Ballast/bypass present and working during CRANK?
→ A missing/open ballast feed or dead bypass can produce a no‑spark symptom; verify both. [johnrichar…plus.co.uk], [lrparts.net]- Coil resistance out of spec?
→ Replace the coil with the correct Lucas electronic‑ignition spec (DLB198/RTC5628 family). [island-4x4.co.uk], [holden.co.uk], [britishautousa.com]
What I’d like to know to tailor this further
- Is yours carb or EFI? (Both use the distributor pickup + amplifier, but wiring/ballast details can differ.)
- Where is the ignition module mounted—on the distributor or remotely near the coil? [v8forum.co.uk]
- What coil part number is fitted (e.g., RTC5628, DLB198, PRC6574, etc.)? [lrdirect.com], [advancedfa…tors.co.uk]
Share those, and I’ll map your exact wires/voltages and the next tests to run. If you want, I can also pull the relevant pages from the V8 overhaul/service docs to cross‑check specs. [ovlr.ca], [tmbbooks.com]
If you’re stuck at any step, grab your meter and I’ll walk you through the reading you should see (RUN vs CRANK) and what it means.
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