Fix a Td5 throttle pedal

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Martin gets to the bottom of the problem : credit: © Martin Domoney
Has your accelerator stopped accelerating? Martin explains the cure with an overlay wiring harness

Need to know

Time: Two hours 
Cost: £34
Difficulty: 3 out of 5  
Models: Defender Td5 (1999-2006)
Tools needed: Screwdrivers, picks, side cutters, electrical tape, multimeter or code reader, brake pipe or metal coat hanger, cable ties.
Parts and costs: Td5 throttle pedal repair harness, £34, tinyurl.com/352ujbvd (eBay).
Work safely: Always disconnect the battery when working on electrical systems. Wear safety glasses when cutting material.

 

Electrical problems can be incredibly frustrating. One minute you’re driving along without a care in the world, and the next you’re stranded in the lashing rain with no way of even trying to find or fix the cause – at least, that’s what happened to LRM’s Associate Publisher, Steve Miller, a couple of months ago. He was so frustrated by his Defender’s lack of forward motion that he wrote about it here.

The problem that he experienced is one that’s not uncommon on Td5 Defenders. The accelerator pedal, which is fed with a supply voltage from the engine control module (ECU) feeds back a signal voltage telling the ECU how much fuel to inject, in simple terms, depending on how far you’ve got the pedal pushed down. If any one of the wires between the ECU and pedal fail, the engine will still start and idle fine, but the engine won’t rev as the ECU won’t see an input from the throttle, which is the problem Steve experienced.

Happily, because the problem is quite common now, there are repair looms on the market that don’t necessitate cutting into the vehicle’s original – albeit faulty – wiring to fit. You simply replace the throttle pedal plug, ECU connector pins, and do away with the brittle, problematic old wire with a length of fresh wire in between the two, which gets routed along the same path as the OE harness. It’s one of those jobs that sounds more complicated than it actually is when it’s there in front of you, but well within the remit of most DIY-ers.

The diagnosis: Chances are if you’re doing this fix, it’s because you’ve experienced failure of the throttle pedal while driving – the engine won’t rev when the pedal is pressed. This can either be down to the engine ECU (rare), the throttle pedal itself, or the wiring between the two.

Telltale code: We can confirm the issue with diagnostics, but not necessarily the culprit. The codes stored here can mostly be ignored (this Defender doesn’t have a/c or ABS) but the ‘Driver Demand Faulty’ one relates to an issue with the accelerator pedal, when you demand power.

 

The backstory

You might be wondering how we’re so sure that the problem lies with the wiring, rather than the pedal itself. The short answer is that this Defender, belonging to LRM Associate Publisher Steve Miller, had its throttle pedal replaced relatively recently with a genuine part. While it’s not unheard of for a new part to fail this quickly, the fact that the fault initially occurred in heavy rain and the pedal went from working fine to not working at all (rather than the fault starting out intermittently and getting gradually worse) also points more squarely at an issue with the wiring. As Td5s get older, the degradation of this particular piece of wiring is becoming more common, hence the looms are available as an overlay repair.

If you are unsure whether your pedal or wiring is the problem, you need to get the vehicle into a faulting state, then use a multimeter to check for high resistance on each of the wires that runs from the pedal plug to the ECU – we’ll explain which ones those are shortly.

Fly-by-wire: Unlike a good old cable throttle, electronically-controlled engines such as the Td5 rely on an electrical signal from a potentiometer inside the throttle pedal to tell the engine control unit (ECU) how much power to give. The potentiometer feeds back a signal voltage to the ECU.

Red or black? At the other end of the throttle pedal wiring is the engine ECU, which is housed in the seat box compartment beneath the driver’s seat. Lift out the seat base, and remove the metal cover from beneath. The pedal’s wiring terminates in the black connector.

A handy solution: Because failure of the wiring between the pedal and ECU is becoming more common, a few suppliers have started to offer pre-made overlay looms, which are easier and simpler to install than reusing both original connectors. This one was from eBay and cost £34.

Start the stripdown: We need access to the inside of the bulkhead in order to route the new wiring, so start by undoing the series of screws that hold the instrument cluster in place. Take note of which one goes where, and be careful not to force anything, as the brittle plastic damages easily.

Dashboard access: With the cluster loose, it can be positioned to the left of the steering wheel, ensuring the wires aren’t stressed. We’ve removed the steering wheel here as it’s on a quick-release boss, but it isn’t really necessary. Undo the four screws and remove the under-dash speaker.

Protect the pins: Before trying to thread the replacement loom into its new home, use electrical tape to secure the ends of all the wires together. This will make the loom easier to position, and prevent the metal pins from getting damaged or bent as they pass to the ECU.

Unplug old loom: Reach behind the metal guard to the side of the accelerator pedal and lift up the latching tab on the connector, then unplug it from the pedal and push it up into the dash. A quick inspection shows the wires are quite brittle and showing signs of strain at the plug.

Route the new one: Starting in the footwell, post the taped end of the new loom up through the kick panel and inside the dash, following the route of the original as closely as possible. Tape the end to a piece of stiff wire and pull it through the bulkhead grommet, so it emerges in the engine bay.

Down and across: Pull the rest of the loom through the grommet, then route it down the side of the footwell, along the chassis and up to the driver’s-side seat box compartment, following the original loom and avoiding chafe points. Secure it neatly in place with cable ties as you go.

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A new entry: Choose a point in the large seat box grommet for the loom to pass into the ECU compartment. This Defender has an extra unused grommet hole, so we’re snipping the end off for the loom to pass through. Alternatively, cut a small hole with a knife and use that.

Into the seat box: Head back underneath and carefully post the pins through the hole in the grommet. It can help to have an assistant carefully pulling them through from inside the seat box, as the rubber grommet is quite thick. Once in, pull the loom through to remove any slack.

Black plug out: The black plug is the one we’re removing the old throttle harness pins from and fitting new ones to, so press down on the latching tab and pull the connector off the ECU. Also check the red plug for oil contamination – a sign of a faulty injector harness.

Easy does it: All the wiring pins are locked into the main connector body by the white plastic piece that you can see inside the plug. Use a small flat-blade screwdriver to ease it upwards, unclipping it all the way around. Once released, lift it off and keep it somewhere safe.

Before going further… Give the old plug a wipe over, and look closely at the plastic connector. You’ll notice small letters moulded in to the connector body. Either note down the wire colours and each corresponding letter, or snap a picture on your phone to remind you of their location.

Compare new plug: You’ll see the new plug has the pins in the same places, but in this case, the wires are different colours. You’ll also see there are seven pins at the pedal plug, but only five at the other end – two splice down into one along the way. Note which new colour matches which old one.

ECU pinout

The number of wires going into the black ECU plug may look scary, but remember, we’re only dealing with five out of the whole lot.

Look closely at the black ECU plug and you’ll see the pin rows are numbered – they’re small, but they’re there. The ones we’re concentrating on are: white/green (pin 12), white/slate (pin 36), black/yellow (pin 26) red/green (pin 20) and white/purple (pin 14). You can work out which you need to remove by both the colour of the wire and the location of each one in relation to the moulded numbers, and the other pins.

On our new harness, the wires are different colours, but it’s simple enough to convert them.

White/green on the old is solid green on the new – white/slate is yellow, black/yellow is black, red/green is white and white/purple is now red.

Release the pins: Use a precision screwdriver or a pick tool to depress the small tang that holds each of the five original pins into their locations in the black plastic plug. Pull lightly from behind while pressing the tang down, and slide the pin free. Work methodically; do one at a time.

Slide them in: Using the photos you took or by comparing the new and old plugs, slide the new pins into the back of the connector until they clip into place. Lightly pull on the wire to ensure they are properly latched; do one at a time until all five new wires have replaced the old.

A better view: Here’s a better look at the inside of the black ECU plug, with all five new wires in place. You can just about see that the new pins are silver in colour rather than gold, but all are fully home and latched in place. Refit the white plastic locking cover to the connector.

Tidy old wires: There’s no need to cut the vehicle’s original wiring and it’s now not connected to anything at either end, so simply wrap the ends of the wires in electrical tape and cable tie them neatly out of the way. If there’s extra length in the new loom, loop and secure it.

Stash in the dash: If you haven’t already pushed the old plug up and out of the footwell and into the dash (it’s a tight fit!) do that next, then coil the old pedal loom up and cable tie it neatly out of the way. Screw the under-dash speaker back in and refit the instrument cluster and seat base.

Plug in the pedal: Finally, the new throttle pedal wiring harness plug can be pushed onto the pedal. Ensure the locking tab clicks, securing it in place. All that’s left to do now is reconnect the battery, clear the fault codes and take yourself out for a test drive – problem solved.

 

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