TVR Tasmin, Coil and Ballast Resistor Replacement

I’m a little behind on posting this, as I did this work a couple of weeks ago, but since I’m now stuck at home in a COVID-19 Lockdown, I thought I would give a quick update.

When I first got the car there was a brand new Bosch GT40 coil fitted. This concerned me for two reasons, first, this isn’t a coil designed to work with an ignition system that is fitted with a ballast resistor (which the TVR has. Bosch makes the GT40R for this use), and secondly, although they are great coils they are known to have issues with electronic ignition and prefer to be triggered by points (the TVR has an electronic ignition). Fail.

At first, I tried to replace it with the 40 year old original Ford coil that came with the car

Although this worked, when this didn’t fix the issue I was having (knowing what I know now, it never would have anyway) I tried swapping it for a slightly newer known good coil from my SD1 spares (SD1 has both electronic ignition and a resistor, like the TVR)(update, it turns out my memory failed me and SD1s don’t run a ballast resistor, so the SD1 coil probably wasn’t happy on the lower voltage). This worked fine and has been in the car until now, but being that it’s from about 1983 its no spring chicken, so I looked for an alternate option.

I’ve been chasing a slight misfire/stumble when at cruise at operating temp, which I’m hoping is ignition and not fuelling, so the three things in the firing line were plugs, coil and ballast resistor.

The plugs that came in the car were a bit old and had seen some shit during my initial attempts to get the car running, so replacing them wasn’t a bad idea. A new set of NGK plugs went in.

I went back and forth regarding the ballast resistor and coil. Was it best to keep the system as is and just replace the coil with another resistor coil, or bypass the resistor and fit a 12v coil instead?

In the end, it was the resistor it’s self that made the decision for me. In the TVR/Ford use of it, its a section of resistance wire, not a traditional resistor. I believe in the Ford applications its run in a single length and kept in the air stream, but in the instance of my TVR, it was just a coil of wire floating around on the fibreglass wing behind the air box, with little to no airflow. This had obviously taken its toll over the 40 years of its life, as the coil of wire was looking worse for wear with sections where it had clearly melted the insulation (and zip ties). Keep in mind that a resistor generates heat by nature; This wire would get too hot to touch when the engine was running.

A plan was hatched. I would get a 12v coil and bypass the resistor. The resistor wire is joined into the loom via two bullet terminals, so that was easy to do.

Just get a short section of decent gauge wire, crimp two terminals on it, and plug it in.

Now that there is a solid 12v heading for the coil it was time to fit a replacement. I went with a Lucas Gold Sports coil, DLB105. I know not everyone is a fan of them, but it was a reasonable price, and would do what I needed.

This was basically plug and play. Remove the old coil, Check the terminals are around the right way on the new coil, bolt it to the car and wire it up with the existing wiring.

The car starts, runs and drives OK, so I guess that’s a win. I haven’t had a chance to get the car up to operating temp and see if it still stumbles, and won’t for about a month now thanks to this pesky virus forcing the whole country into lockdown. Oh well, that gives me time to do a few more jobs on the TVR that I have been putting off, like front brakes and carpet. No excuses now.