Member Since: 19 Jan 2015
Location: Caerphilly
Posts: 2256
Ok thanks for that ! I’ll get onto that as soon as I get home. am
I right in thinking that alternator needs to be a genuine denso one ?Jarrod
Current : D3 2007 HSE - AKA the lemon
-beanie grille
-detango with led bulbs
-club body off rebuild. TWICE.
17th Jul 2020 8:31 am
Bungle
Member Since: 07 Apr 2015
Location: Wanborough
Posts: 254
Hang on, you reckoned you were getting over 14v with the engine running, so I would do some more tests before pulling the trigger on the parts cannon for an alternator. I wouldn't expect a fault to occur at higher revs if it were related to the alternator, but easy enough to check, monitor the voltage across the battery while you hold the revs at the missfire point and see if you see anything awry.
The battery dying can give issues, but they are normally random errors and (assuming the vehicle starts) rarely affect the actual running of the engine as once it is running the voltage comes up and everything settles down.
I can't see any issues with the power supply (alternator or battery) giving you an issue with just one sensor out of the hundreds over the vehicle, if the power supply really was that bad (e.g. the alternator was putting out a really "noisy" supply that the battery could't smooth) you would be getting loads of random faults all over the vehicle.
17th Jul 2020 10:01 am
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4658
Discomadness wrote:
Alternator ?
Can I ask as I’m not the best with the electronic side but what made you say alternator ?
Hi Jarrod,
If your wiring to the rail sensor "checks out" OK the alternator would be my first port of call with that DTC.
I have seen this on a previous occasion and after a very prolonged head scratching, and lots of parts thrown at it the alternator was found to have failed internally causing a voltage spike. You could spend many hours and a lot of money trying to fix this, but given what you have already said re the wiring being OK. I would punt for a used alternator first and see if that cures it.
I never fit used parts but this would be an exception with small outlay and labour cost. If it sorts it you can always order a new alternator, if the problem persists you won't be too much out of pocket.
(This particular fault can be a head wrecker though)
Make sure the replacement is a Denso alternator only!!
Denyµ (idµ - eAµ) ψ=mψ
17th Jul 2020 12:34 pm
Bungle
Member Since: 07 Apr 2015
Location: Wanborough
Posts: 254
Quote:
I have seen this on a previous occasion and after a very prolonged head scratching, and lots of parts thrown at it the alternator was found to have failed internally causing a voltage spike. You could spend many hours and a lot of money trying to fix this, but given what you have already said re the wiring being OK. I would punt for a used alternator first and see if that cures it.
Crikey, that must have been fun to diagnose, I would never have thought to go there with just one sensor giving problems. I have come across noisy alternators before but normally they die completely before they get to the stage of putting out so much crud that the battery can't smooth it out. The definitive answer would be to put a scope on the output from the alternator but given the OP said electronics was not their thing I am guessing that isn't an option.
17th Jul 2020 11:58 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4658
Although I didn't have a "hands on" participation the last time I came across this problem, if I recall correctly it took weeks to nail what was causing an intermittent spike.
It wasn't helped by the owner using non dedicated LR diagnostics , but it was one of the most challenging diagnostic problems I'd come across in a long time. The scope didn't show anything abnormal, but just when you weren't looking at it limp mode would suddenly return. yµ (idµ - eAµ) ψ=mψ
18th Jul 2020 1:21 am
Discomadness
Member Since: 19 Jan 2015
Location: Caerphilly
Posts: 2256
Thanks for all the replies ! In the end I had to abandon the disco and take the mrs beetle (the shame) I’ll pick up a denso alternator, the aux belt could do with a swap anyway also and attack the job when I get back.
In relation to some random faults, I have had a few odd codes come up randomly. Mostly U series codes. Lost coms etc so fingers crossed !
I’ll report back !
Jarrod
Current : D3 2007 HSE - AKA the lemon
-beanie grille
-detango with led bulbs
-club body off rebuild. TWICE.
19th Jul 2020 8:26 am
Pete K
Member Since: 15 Jan 2016
Location: GL
Posts: 10367
I think we all get U codes
19th Jul 2020 9:55 am
Discomadness
Member Since: 19 Jan 2015
Location: Caerphilly
Posts: 2256
Replaced the alternator with a brand new denso. Car still the same. Still reading N/A for fuel rail pressure sensor also.
Don’t know where to look next.Jarrod
Current : D3 2007 HSE - AKA the lemon
-beanie grille
-detango with led bulbs
-club body off rebuild. TWICE.
25th Jul 2020 5:48 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4658
Ouch! That's why I suggested a used one first
You're going to have to get the DMM outyµ (idµ - eAµ) ψ=mψ
25th Jul 2020 6:36 pm
Discomadness
Member Since: 19 Jan 2015
Location: Caerphilly
Posts: 2256
DMM ? Jarrod
Current : D3 2007 HSE - AKA the lemon
-beanie grille
-detango with led bulbs
-club body off rebuild. TWICE.
25th Jul 2020 7:19 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4658
Sorry, digital multi meter.
Edit: If you pull off the plug on the fuel rail sensor and test the signals present with the ign key in the on position. Post the figures back here including the earth resistance value, and we can go from there.
(Do a test of same with key off as well!)yµ (idµ - eAµ) ψ=mψ
25th Jul 2020 7:20 pm
Discomadness
Member Since: 19 Jan 2015
Location: Caerphilly
Posts: 2256
Ok buddy will do. Jarrod
Current : D3 2007 HSE - AKA the lemon
-beanie grille
-detango with led bulbs
-club body off rebuild. TWICE.
25th Jul 2020 7:41 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4658
Sorry there's no earth so skip that. Three cables, yellow with a green trace, white with a green trace, and I think the other is brown with a green trace. Measure and record the values present for each with the key "On", then do the same with key "Off"
Are you familiar with ohms / resistance tests?yµ (idµ - eAµ) ψ=mψ
25th Jul 2020 7:59 pm
Discomadness
Member Since: 19 Jan 2015
Location: Caerphilly
Posts: 2256
I’m not amazing with it, could you give me a brief run down ?Jarrod
Current : D3 2007 HSE - AKA the lemon
-beanie grille
-detango with led bulbs
-club body off rebuild. TWICE.
25th Jul 2020 8:06 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4658
Yeah no problem but just do the other tests for now and we can deal with the resistance and shorts later. I'll be awol for an hour or so but will check in later. yµ (idµ - eAµ) ψ=mψ
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
, but it was one of the most challenging diagnostic problems I'd come across in a long time. The scope didn't show anything abnormal, but just when you weren't looking at it limp mode would suddenly return. 
