rev counter/low revs problem

[grahambell]

So I've finally bought a TVR (1999 4.0 Chim) and it seems to have developed a wee problem.

Get the rev counter to about 4,200 and the engine essentially cuts out, just like a hard limiter kicking in, until revs drop and it's OK again. Tried with the car stood in neutral, and the engine certainly seems to be revving higher than 4,200 before it cuts out.

Having had a similar problem with my Quantum years ago (fixed by replacing the rev counter) I suspect this is also down to a duff rev counter.

Is this a common problem? Or do any of you TVR experts out there have any other suggestions?

[QBee]

Do you know what speed in what gear you were doing when it cut out? Early or late 4 litre?

[Steve_D]

Under the inlet air pipe that runs from the MAF to the throttle body there will be a loom (going to the MAF). Attached to that loom you should find a single wire and a connector. This is the signal wire to the tacho. Unplug it and see if your problem goes away.

Steve

[grahambell]

Hi chaps,

Thanks for the replies. 1999, so late model. First happened coming off roundabout with tight switchback exit onto what was first straight, clear, dry road since I bought it (flaming June my arse!) so in either first or second when I floored it.

Steve - by MAF I assume you mean the air flow meter on the left hand inner wing? The wiring running from main engine loom to that has no single wire attached - just the sheathed wiring for the multi-plug.

Oh - and rev counter is reading 500rpm when engine not running. Don't remember it doing that when I got it.

[Steve_D]

Sounds like you need to go hunting for it.
It will run from either the coil negative or the ignition amplifier. It then runs along beside the injectors on the near side (UK) to then connect into the loom from the MAF (Mass Air Flow sensor).
As I said before this is normally on the run under the pipe but could be closer to the engine.

Steve

[Belle427]

You should see 2 wires at the coil both with a resistor connected to them. One of these is for the tacho, the other goes back to the ECU.
I believe mine were white/black in colour.

[blitzracing]

If you have a failing HT coil it will do this, the reason being it cant keep the spark up under high RPM and high compression (ie under load) unlike a simple rev test in neutral where the compression pressure is low so the required spark arc voltage is much lower.

[QBee]

^^^^^^^ he knows about stuff like this.

Whereabouts in the midlands are you? I have a couple of spare coils you could try, but they are presently with a friend near Coventry.......

[grahambell]

Thanks for the extra replies chaps. Was at local car meeting yesterday evening so bit late seeing this.

OK, I'll hunt for the wiring for the rev counter feed and check wiring for coil.

No idea how old coil and plug leads are, so might be worth renewing all usual ignition parts anyway.

QBee - I'm in Leamington, so not too far from Coventry...

[QBee]

grahambell said:

Thanks for the extra replies chaps. Was at local car meeting yesterday evening so bit late seeing this.

OK, I'll hunt for the wiring for the rev counter feed and check wiring for coil.

No idea how old coil and plug leads are, so might be worth renewing all usual ignition parts anyway.

QBee - I'm in Leamington, so not too far from Coventry...

I would do the suggested wiring checks first. If that doesn’t solve it, the guys that have my box of bits have more than just the coils. There are plug leads, ignition amp, dizzy cap, dizzy etc. Helps to work out what isn’t working without having to buy loads of stuff you don’t need. Would you like me to put you in touch with them?

[grahambell]

Hi QBee, Thanks for the offer, As you say, best to do the suggested checks first and go from there.

Car's certainly running well enough to use, and I have a couple of other cars requiring more pressing work to deal with, so might take me a while to get round to it.

[grahambell]

Finally managed to get round to looking at this. The coil has four wires going to it. Positive side has a black going to main engine loom and white going to ignition module. Negative side has a couple of white/black wires, one to ignition module, and one to main engine loom (tacho?).

Have tried disconnecting and then reconnecting wires with ignition off and it's made no difference. Tacho still reads 500 rpm with engine not running, and engine still cuts at indicated 4,200 rpm (car driven, engine under load). Engine seems to run fine up until that point.

Checking TVR coils on internet, some have a ring round the top (ballast resistor?) which mine doesn't have. Unfortunately coil casing has gone manky, so can't read serial number. Doesn't seem to be leaking any oil though, which apparently can be a problem. Can anybody confirm whether or not 1999 4.0 Chim needs coil with ring/ballast resistor?