Low voltage at pin 39 TVR 4.0

dogman · Post by **dogman** » Sun Apr 24, 2011 9:10 pm

Hey guys I'm new here.

I'm in the process of tracing a high RPM misfire. Quick background on this issue: everything is new - Coil, cap, rotor, leads, plugs, ign module. Dizzy also rebuilt/checked. Swapped for another known good ECU as well, nothing cured it. Also tested fuel pressure reg, pump and had The TVR Center do a live fault code test, nothing showed up, it has them puzzled! Also the battery is brand new

Problem is it will revv to 6k ONCE (either under load or at idle) then it will only go to 5k or thereabouts. If you're driving along you can even turn the car off and on again and hey presto she'll hit 6k but again, only once.

So today I followed the steps in the bible to fault check the ECU and I found only 9.5v at pin 39 which according to the bible should be giving battery voltage. Since this is where the ECU gets the RPM input from, I think I could be onto something!

So should the voltage actually be 12v at pin 39 on a 4.0 TVR Chimaera and if so what might be causing the drop?

DaveEFI · Post by **DaveEFI** » Sun Apr 24, 2011 10:05 pm

Be a help if you said what injection system is has (hotwire?) - but if it's the feed from the coil to the ECU that is a series of pulses so won't give a meaningful reading with a DVM. You'd need a scope to check it.

dogman · Post by **dogman** » Sun Apr 24, 2011 10:22 pm

Yeah, sorry I should have said what the standard 4.0 TVR uses. Hotwire I believe it is yes.

I realise an oscilloscope is needed to see properly what is going on in terms of spikes etc but according to Steve Heath's book (''the bible'') it is supposed to have 12v there at pin 39 with ignition on. Incidentally, I am reading 12v at the low tension side of the coil (from the same wire)

DEVONMAN · Post by **DEVONMAN** » Sun Apr 24, 2011 10:57 pm

dogman wrote:Yeah, sorry I should have said what the standard 4.0 TVR uses. Hotwire I believe it is yes.

I realise an oscilloscope is needed to see properly what is going on in terms of spikes etc but according to Steve Heath's book (''the bible'') it is supposed to have 12v there at pin 39 with ignition on. Incidentally, I am reading 12v at the low tension side of the coil (from the same wire)

Sorry but have to ask his question. Are you sure the engine is not reaching 6000 and it's just the rev counter reading which is at fault?
BTW there is a resistor between the coil and pin 39 which would account for the voltage drop. Check out the hotwire wire diag in the stickey on this forum.
Regards Denis

DaveEFI · Post by **DaveEFI** » Mon Apr 25, 2011 7:10 am

dogman wrote:Yeah, sorry I should have said what the standard 4.0 TVR uses. Hotwire I believe it is yes.

I realise an oscilloscope is needed to see properly what is going on in terms of spikes etc but according to Steve Heath's book (''the bible'') it is supposed to have 12v there at pin 39 with ignition on. Incidentally, I am reading 12v at the low tension side of the coil (from the same wire)

A - right. With the engine stopped? There is a current limiting resistor between the coil negative and the ECU, so it would depend on the input impedance of that ECU port what voltage you would see at it. The resistor is 6800 ohms, so you could check the resistance of that part of the loom when disconnected. IE, 39 to coil neg.

dogman · Post by **dogman** » Mon Apr 25, 2011 7:36 am

Yep, that was my next test today!

Thanks for the replies btw

So if it's down to the resistor I should see the same voltage at pin 39 even with the ECU plug disconnected right?

And if it suddenly hits 12.66v then it's the input impedance of pin 39 it's self?

Okay. Need to do some further tests

dogman · Post by **dogman** » Mon Apr 25, 2011 7:40 am

DEVONMAN wrote: Sorry but have to ask his question. Are you sure the engine is not reaching 6000 and it's just the rev counter reading which is at fault?

dogman wrote: Problem is it will revv to 6k ONCE (either under load or at idle) then it will only go to 5k or thereabouts

I did consider that but sadly it's not because you can get it to 6k as I said 'once' then it wont allow it again. Also it's clear the engine is still surging/reaching towards maximum power just when this problem kicks in.

DaveEFI · Post by **DaveEFI** » Mon Apr 25, 2011 7:56 am

dogman wrote:Yep, that was my next test today!

Thanks for the replies btw

So if it's down to the resistor I should see the same voltage at pin 39 even with the ECU plug disconnected right?

And if it suddenly hits 12.66v then it's the input impedance of pin 39 it's self?

Okay. Need to do some further tests

With a normal high impedance DVM you should see the same voltage at pin 39 as at the coil with the ECU disconnected, yes.

I've never measured this, but I'd expect the input on the ECU to pull the voltage down - too high an input impedance makes it susceptible to interference, etc.

dogman · Post by **dogman** » Mon Apr 25, 2011 8:34 am

Okay cool. I've just remembered where I think the resistor is so off to investigate now the various things.

Cheers guys will post back what I find.

spend · Post by **spend** » Mon Apr 25, 2011 9:59 am

Try disconnecting the rev counter ~ they have been known to take the whole car down by affecting the engine signal to the ECU.

HT leads (routing & composition), coils & ign amps also seem to be commonly problematic with the cars failing to rev properly. Much can be attributed to aftermarket bits &/ the very hot dustbin at the front of the engine.

dogman · Post by **dogman** » Mon Apr 25, 2011 10:46 am

Hi Spend.

I found the resistor.

Well here's what we've got: 12.66 volts on coil side all the way to the resistor. The resistor it's self seems to be reading 7140 ohms - bit high?

With ECU plug disconnected we have the same 12.66v at pin 39 and with it connected it drops to 9.4 varying slightly. Hmmmm

I know peeps have said the voltage needn't be to high at pin 39 so maybe it's another dead end for me.

Cock!

Disconnecting the revv counter sounds interesting...

dogman · Post by **dogman** » Mon Apr 25, 2011 12:05 pm

Okay I gave it 12v by removing the resistor and it didnt solve it.

Oh this is weird.

I've swapped the ECU (from another 1994 car) and again, it didn't solve it.

I read that some -1993 cars had the limit at 5,600 but if mine was somehow one of those (it's a '94) why would it revv to 6k once? And anyhow it doesnt reach 5,600 before this misfire happens.

I should film it I think, well it's exactly like a revv limiter. No backfiring

DaveEFI · Post by **DaveEFI** » Mon Apr 25, 2011 1:50 pm

dogman wrote:Okay I gave it 12v by removing the resistor and it didnt solve it.

That's not a good idea - the resistor is there to limit the considerable current from the spark.

Oh this is weird.

I've swapped the ECU (from another 1994 car) and again, it didn't solve it.

I read that some -1993 cars had the limit at 5,600 but if mine was somehow one of those (it's a '94) why would it revv to 6k once? And anyhow it doesnt reach 5,600 before this misfire happens.

I should film it I think, well it's exactly like a revv limiter. No backfiring

Think you need to get someone with proper test equipment to look at the spark at high revs. But that's not say it is the cause.

dogman · Post by **dogman** » Mon Apr 25, 2011 4:15 pm

Thanks Dave

AFAIK the resistor only absorbs/helps stop ECU seeing too many spikes, ECU is quite happy with 12v there but anyway, it didnt do anything and you were right that my initial voltages were fine. Steve Heath, I'm afraid your book led me down a dead end! I say that bearing in mind it is an excellent book!

Anyways been out all afternoon in the car getting on with driving it!

OSTY · Post by **OSTY** » Thu Jun 23, 2011 4:47 pm

Hello dogman

I've got a similar problem with my 400 Griffith (misfire at 3500 rpm though). After testing/changing many parts with no luck- I also wondered if the 9.7 volts I recorded at pin 39 was the issue (post reading Steve's bible). After reading your post- maybe not!

Anyway, while searching the net for answers- I found this which seems very similar to your issue:-

http://www.rv8r.co.uk/misfire.html

Hope this helps. Let us know how you get on