P38A 4.6 V8 won't pull, sluggish, otherwise runs fine.

Right so I've done some testing.

Nanocom reads only 9.7ish volts at the Gearbox ECU (under LHS seat), however there is a steady 11.6 volts or so at Pins 1 and 39 checked with a multimeter today.

Alternator is outputting 14 odd volts, battery is showing it's got good charge so the juice is there.... but the ECU doesn't think it is.

I did find a thread on another site with someone with the same problem, and they fixed itby taking apart the BECM and checking the connections in there.... I'll try that tomorrow.

Here is the thread.. but I never get the GEARBOX FAULT message.


Does anyone else "See" 9 volts at the ECU on their Nanocom? My BECM always says it's got 16volts when it never does. This is some firmware bug with late GEMS BECMS, and I wonder if it's just that?

So I had the chance to do some investigation today.

Trying to get to the bottom of this low 9 volts at the gearbox ECU, I took out the front seat opened up the BECM and cleaned all the connectors, and connections inside too. Did some hoovering and blasting with compressed air to clean all the dust away. There wasn't really any corrosion or even dirt that I could see, but I thought a good clean up might help. I also reseated all the +12v feeds, fuses, and earths to the BECM, and did the same to the engine fusebox.

Switched on, started up, plugged in the Nanocom, and got this....



However, when I used a multimeter direct to pin 1 and 39 of the gearbox ECU and I get 11.7v steady with the engine running and without.



I thought I'd just test the voltage at fuses 3 and 6 the ones that feed the gearbox ECU and got 11.72v. Pretty much the same then.



I've bought a new AMR 5494 tranmission controller off the 'bay and I'll swap it out and test again. I'm not sure if this 9volts at the ECU is the ECU at fault, or a bug in the Nanocom. It was cheap so I don't mind taking a punt.

I also swapped out the coil packs for another set I have, cleaned and re-seated the HT leads, and also cleaned and reseated the gearbox connector that goes down the back of the engine.

I haven't driven it yet because I want to wait until I have the new gearbox ECU, but blipping the throttle is deffo smoother with the other coil pack fitted so there is that! Maybe progress, maybe not.

you get 11.72 at the fuses/connections but 14odd at the battery?

kermit_rr said:

you get 11.72 at the fuses/connections but 14odd at the battery?

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No. I get 11.72 at the fuses, and 14 odd out the back of the alternator. 13.11 at the battery and slowly rising.

I know you guys are all over this with nanocom and complex tests, but being old school and having a 2D brain, have you checked the spark quality? Should be a nice fat purple spark. Not a thin weedy white one.
Also, surely the voltage at the battery and at the fuses should be very similar, not some volts difference?
I’ll get my coat…..

P38 Monkey said:

I know you guys are all over this with nanocom and complex tests, but being old school and having a 2D brain, have you checked the spark quality? Should be a nice fat purple spark. Not a thin weedy white one.
I’ll get my coat…..

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Good shout! I've been thinking the same..... but seeing as the Nano threw this up as something that isn't right, I just wanted to eliminate it from my enquiries!

I think the difference between battery voltage and BECM output is because the BECM provides that voltage from it's circuitry, rather than direct from the battery. I thought the same though. There is a post on RR.net about gearbox faults, and the guy gets exactly 12 volts so I think it's probably okay.

I'd love to hear what other people are getting from their BECMs though.

P38 Monkey said:

Check colour of spark plugs…?

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Took a few of the easy ones out (I know, but it was looking like rain) and they're a healthy brownish grey. Not sooty or oily or anything like that.

Surely with the engine running and 14V at the alternator, it should read the same at the battery? The +ve terminal is not fully settled on the post, but that should not matter if it’s clamped up. But if it was me I would take that terminal off, polish the post with wire wool, spread the terminal a little and refit fully down on the battery post.
And do the ground terminal while I was at it. And make sure the earth wire is properly cleaned and seated at its other end. Oh dear, very basic stuff!!

P38 Monkey said:

Surely with the engine running and 14V at the alternator, it should read the same at the battery? The +ve terminal is not fully settled on the post, but that should not matter if it’s clamped up. But if it was me I would take that terminal off, polish the post with wire wool, spread the terminal a little and refit fully down on the battery post.
And do the ground terminal while I was at it. And make sure the earth wire is properly cleaned and seated at its other end. Oh dear, very basic stuff!!

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I dunno..... but I think the battery might pull it down a bit if its charging. I can certainly adjust that post clamp though!

MrGorsky said:

I dunno..... but I think the battery might pull it down a bit if it’s charging. I can certainly adjust that post clamp though!…

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…but there is a fat copper cable between the battery +ve and the alternator, and ditto between the -ve terminal and the engine block and hence back to the alternator. So there is nothing significant to reduce the voltage. When the alternator is working hard, eg charging a flat battery, its voltage will drop and then build up as the battery voltage increases and the current into the battery reduces.

P38 Monkey said:

…but there is a fat copper cable between the battery +ve and the alternator, and ditto between the -ve terminal and the engine block and hence back to the alternator. So there is nothing significant to reduce the voltage. When the alternator is working hard, eg charging a flat battery, its voltage will drop and then build up as the battery voltage increases and the current into the battery reduces.

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My fat copper cable goes down to the starter solenoid first, and then back to the battery. I think there was a recall to change this and add in an extra cable, but my car never had it.

I'll will check my wiring though. It's under the engine so probably gets a lot of ****e thrown at it.

You have a problem twixt alternator and battery, the voltage after a few minutes should be the same at both. Whatever is causing the volt drop is also likely to be the cause of the low voltage at the fuses and ECU. The positives from the battery have been known to corrode inside the plastic sleeve but my bet would still be on a bad earth from engine to chassis.

MrGorsky said:

No. I get 11.72 at the fuses, and 14 odd out the back of the alternator. 13.11 at the battery and slowly rising.View attachment 299261

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There's something wrong there.
What voltage do you read at the battery when the engine off and let for an hour?
Either the battery is dead of you have a very bad connection somewhere or one of the cables is dead inside

Datatek said:

You have a problem twixt alternator and battery, the voltage after a few minutes should be the same at both. Whatever is causing the volt drop is also likely to be the cause of the low voltage at the fuses and ECU. The positives from the battery have been known to corrode inside the plastic sleeve but my bet would still be on a bad earth from engine to chassis.

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I would be starting at the battery<->earth cable

kermit_rr said:

There's something wrong there.
What voltage do you read at the battery when the engine off and let for an hour?
Either the battery is dead of you have a very bad connection somewhere or one of the cables is dead inside

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12.6 at the battery when it's disconnected.

I'll check my engine earths and report back....

Don't mind renewing the +ve cable either..... pretty easy to make one up.

I did read somewhere about a service bulletin to add a direct cable from the alternator to the battery +ve, if I'm making up new cables is it worth adding one of these to the circuit?

MrGorsky said:

I'll check my engine earths and report back....

Don't mind renewing the +ve cable either..... pretty easy to make one up.

I did read somewhere about a service bulletin to add a direct cable from the alternator to the battery +ve, if I'm making up new cables is it worth adding one of these to the circuit?

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A good idea IMO.

On the Thor I'm sure i recall is a Y cable between the battery/fusebox/alternator

You need a fat cable between the battery and the starter, as it is carrying up to several hundred amps (battery specs CCA figure shows this). The alternator outputs a bit less max current, but still a reasonable 120 amps max. Losses tend to occur at joints, not in the cable itself. But as said above, corrosion inside an apparently healthy connector can add resistance and limit the current flow, and show up as a voltage differential.

Thanks for all the advice and suggestions guys.

I’ve got a little bit more work to do haven't I!!

I’ll sort some new battery cabling and get it fitted and see where we are after that. The new g box ECU should have arrived too with luck, and I can see if the voltage makes any difference to that.