B

BrynT

Member
Drove my V8 3.9 EFi Defender less than one mile last weekend to pick up animal feed, parked up, left it for a day then it wouldn’t start. I get no spark. This seems to be a common problem.

I’m trying to diagnose the source of no spark and have tested resistance of the HT leads, it’s got brand new plugs about 750-1000 miles ago so I’m hoping / doubting that’s the issue (I’m open to hear more wisdom though). I’ve tested for spark from the coil to distributor and get nothing, so the coil is the next thing to check.

After tracing from plugs to coil I discover it's got recommended kit (Magnecor leads, Bosch coil etc.), and approximate battery voltage drop at the coil, which means it's got power, so I'm hoping it's just a loose wire.

Does anyone know the primary and secondary resistance of a Bosch PRC6574 ignition coil? Haynes Discovery 1 1989-1998 (I think the engine came from one) states range as below. I measured my primary at 0.8 ohms, secondary 9K ohms. Given what Haynes states it sounds high but I though Bosch range was higher, but I do not have anything definitive stating that fact. A dumb question, would a higher resistance indicate it has failed?
fullsizeoutput_ba3.jpeg


I cannot find an ignition amplifier. I thought it was supposed to be installed near the coil but maybe I'm being optimistic about the wiring organisation.

Any other suggestions (tbh the wiring is a bit of a shambles so it could easily have been me knocking a wire or something when I was rummaging in the engine bay for the engine serial number).
 
The ignition amp is a small black box "want of a better word" mounted on the side of the Distributor, it has a three pin plug..

Crook coils are rare, dead modules are 50x more common. Also test the dissy pickup by disconnecting the red and blue wires from the plug near the coil. It should show some thousands of ohms and react to the rotor moving past the pickup by fluctuating the resistance reading

Also start with a fully charged battery you should have the same voltage at the coil as at the battery ;)

Make sure your multi meter has a secure negative side earth at the battery negative or the engine block. :)

Test the voltage at the battery (+). :)

Test the voltage at the white wire out of the ignition switch. ;)

Test the voltage at the (+) wire to the coil with it disconnected from the coil.

Test the voltage with the same wire reconnected to the coil.

A coil should have a very high resistance in other words no connection to earth when not firing.
 
Sounds like ignition amp or the wires connected to it. Check and trace the wires to the ignition amp and test. Also, depending on age of the engine, ignition amp may have only 2 pins/wires but it does the same.

I note the OP is in Chester, not a million miles away so if it still doesn't work after a bit of investigation I could swing by and take a gander but no guarantees of a fix!
 
BrynT said:
Drove my V8 3.9 EFi Defender less than one mile last weekend to pick up animal feed, parked up, left it for a day then it wouldn’t start. I get no spark. This seems to be a common problem.

I’m trying to diagnose the source of no spark and have tested resistance of the HT leads, it’s got brand new plugs about 750-1000 miles ago so I’m hoping / doubting that’s the issue (I’m open to hear more wisdom though). I’ve tested for spark from the coil to distributor and get nothing, so the coil is the next thing to check.

After tracing from plugs to coil I discover it's got recommended kit (Magnecor leads, Bosch coil etc.), and approximate battery voltage drop at the coil, which means it's got power, so I'm hoping it's just a loose wire.

Does anyone know the primary and secondary resistance of a Bosch PRC6574 ignition coil? Haynes Discovery 1 1989-1998 (I think the engine came from one) states range as below. I measured my primary at 0.8 ohms, secondary 9K ohms. Given what Haynes states it sounds high but I though Bosch range was higher, but I do not have anything definitive stating that fact. A dumb question, would a higher resistance indicate it has failed?
View attachment 212368

I cannot find an ignition amplifier. I thought it was supposed to be installed near the coil but maybe I'm being optimistic about the wiring organisation.

Any other suggestions (tbh the wiring is a bit of a shambles so it could easily have been me knocking a wire or something when I was rummaging in the engine bay for the engine serial number).
Click to expand...
if as you say it’s a D1 3.9 then the ign amp will be within inches of the coil linked with a patch lead, with a D1 the coil is over the near side near the headlamp, to trace its position just follow the cable from the dizzy.

A simple thing like iffy connections at the coil top terminals could give you intermittent firing or non at all.
 
Last edited:
Thanks for the great advice. I shall be busy this weekend.

Thanks for the offer Classic Kev. I may take you up on that to hear what you've got to say about it. Every day's a school day.
 
In line with what Discool said, I have found issues with ignition because old wires become brittle and break inside the insulation or are just corroded so the ends need cutting off and new spades crimped on. Not sure how your ignition is wired up but I would expect the circuit to be the same so open the loom and trace the wires to/from the ignition amplifier; there is a splice in there (this is the ignition +12v feed from the ignition switch IIRC). I found mine corroded and needed cleaning so worth a look.

Just have a good look around, check all the connections. If you do a search for ignition tests or similar you should find lots of useful info, I know I have posted on that sort of thing in the past as have others on here.
 
It’s funny how threads go dead without any conclusions from the OPs, but this one had only posted twice. :mad:
I’ll stay clear of newbe’s :D
 
Apologies, thought I had closed this out.

Conclusion is, after replacing a number of parts, the problem was a loose connection. It's worrying because of the lack of a definitive 'aha, that's the problem'. Anyway it got running again, it is running again. I am contemplating when work permits to replace a number if not most of the electrical wires - it looks like someone has thrown a bucket of spaghetti into the engine bay at the moment.

However there's always something new. Now I'm getting loss of power as of last night which I think is due to too much air. I'm convinced the butterfly in the plenum is sticking so that's another job to test when work allows.
 
BTW - I have not discounted the possibility it could still be yet more wiring issues. Whilst a dirty butterfly is still on the list of things to sort out (I get over revving at idle), I have seen a similar problem created by wiring faults. Like I said, it's a mess under the bonnet.

Many thanks for all the advice, looks like I have a long way to go to sort things and make any fault diagnosis an easier affair.
 
Latest update. Got RoverGauge, plugged in and am getting a repeat fault code on the throttle pot. Removed, cleaned and noticed some wear on the insulation but no severed wires, put back on and it seems to behave (I was getting a high idle - 2000RPM) a bit. It's intermittent, it occasionally reverts to high idle. Also still loss of power at high revs / load (uphill for example). Nextstop, replace the throttle pot. Clean the butterfly. Any suggestions?
 
You might change your mind when you see the price of a throttle pot. That said, I have a vague recollection of seeing a more reasonable replacement based on a put together adaptation of a different pot and mounting. Anyway, whatever.

As for the butterfly - yes, absolutely. Take off the plenum and ram housing and clean all of it, check/set throttle butterfly, check throttle spindle for signs of wear. You can also check for loose trumpets at the same time.

Good luck with it.
 
classic kev said:
You might change your mind when you see the price of a throttle pot. That said, I have a vague recollection of seeing a more reasonable replacement based on a put together adaptation of a different pot and mounting. Anyway, whatever.

As for the butterfly - yes, absolutely. Take off the plenum and ram housing and clean all of it, check/set throttle butterfly, check throttle spindle for signs of wear. You can also check for loose trumpets at the same time.

Good luck with it.
Click to expand...
 
Thanks for that Kev. I gave the butterfly another really thorough clean and the idle is now consistently coming down to the right level after about 15 seconds after start. Looks like that was the problem. I did buy a throttle pot off eBay which will go in my growing bag of spares for the moment.

Next job, check for vacuum leak. Suddenly started losing power and misfiring at high revs. Going to look for at the obvious culprit. I think I can hear a hiss but not knowing what it should sound like I’m not certain.
 
A misfire at high revs could be a number of things. I think I might start with the ignition amplifier. Also check the battery and the alternator output. Vac leak is a possibility and unfortunately so is HGF, so maybe do a full (wet and dry) compression test - you'll need a strong battery for that. Hope that helps.
 
classic kev said:
A misfire at high revs could be a number of things. I think I might start with the ignition amplifier. Also check the battery and the alternator output. Vac leak is a possibility and unfortunately so is HGF, so maybe do a full (wet and dry) compression test - you'll need a strong battery for that. Hope that helps.
Click to expand...
It’s more often that I get hesitation than a misfire. Sorry for my ignorance, what is HGF?
 
Thought I’d close this off with the happy ending. Inspection of the plenum showed a split in the hose from the fuel regulator, fixed it, but still no success. Took it to the local 4x4 garage and a test showed a faulty fuel regulator. New one in and it’s running better than ever. Success!
 
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