Triumph tr7

[Gareth jones]

Can anyone help me please ? My tr7 starts up but 2 seconds later cuts out! Changed plugs still the same but plugs were wet with fuel ! Any ideas please ?

[RobPearce]

Is it actually two seconds or is it faster? There's a problem I had on a Stag once whereby it would start beautifully but die as soon as I released the key from the starter position. It was caused by a bad connection on the ignition switch but a failed ignition ballast resistor will do the same.

[Howard]

FYI on a 7 the ballast resistor is inside the wiring loom.

[Gareth jones]

Thanks guys yes it stalls as soon as I release the key ! What can I do ? But a new ballast resistor ? Do you know where it is in the wiring loom ? Thanks 

[poppyman]

Not sure where it is on the TR7 but it is a ballast resistor problem for sure. You could bypass it for good, but you will need a non ballast coil.

Tony.  

[Howard]

1 hour ago, Gareth jones said:

Thanks guys yes it stalls as soon as I release the key ! What can I do ? But a new ballast resistor ? Do you know where it is in the wiring loom ? Thanks 

Assuming its a later car fitted with the 12 way fuse holder in the glovebox, the ballast resistor is in the feed to the starter relay which can be disconnected and replaced by a new feed and resistor fed from any of the white wires - see sheet 1 of the attached drawings for an 81 DHC. I cannot confirm this, but the resistor appears to be 16 Ohm / 2 Watt which is available from a number of the usual suppliers ( Ballast Resistor - 134176) for a under 10 quid.

Cheers

Howard

 

UK DHC 1980-81 TR7 (2020-09) with pektron fuel gauge & fan details.pdf

[RobPearce]

3 hours ago, Howard said:

the resistor appears to be 16 Ohm / 2 Watt

I'd expect it to be 1.6ohm to match the 1.5ohm coil.

Do check the wiring on the column and ignition switch because the feed to the ballast wire comes from there and it might be a cheaper and easier fix! I'd expect that to affect other things too, but you may not have noticed. Does the fuel gauge read correctly (ignition on, engine stopped) and the fasten belts warning light come on? If not then its the ignition switch (or its wiring) and not the ballast wire.

[Howard]

Rob,

Thanks for the clarification since it detailed on the ANG classic cars web shop as 16 Ohm. Seeing that the available replacement is a ceramic encased resistor & the heat dissipation is a not insignificant 20ish watts based on 1.6 Ohms it puzzles me that it is part of the wrapped wiring loom. Any idea what the original looks like?

Garath,

I suggest you check the heater fan and wipers first since these are connected to the white wire before the feed to the ballast resistor/ignition, so if they work the ignition switch is not the problem. However as Rob says, connecting a new feed from the ignition switch is an easy fix since a white wire can easily be found behind the steering wheel, just remember to disconnect and insulate the old white/yellow wire to the coil to prevent any possible short circuits in future. If you want to obtain a white/yellow wire for the replacement, autoelectricsupplies.co.uk can provide a few meters of whatever you need.

Cheers

Howard

 

[A TR7 16V]

6 hours ago, poppyman said:

Not sure where it is on the TR7 but it is a ballast resistor problem for sure. You could bypass it for good, but you will need a non ballast coil.

Tony.  

I take it all the supply leads to the coil are all in place?

Since the boost connection back up from the solenoid is separate, if that's there but the feed from the ballast resistor is broke, or a connector has come off, it will start and then stop like that.

[RobPearce]

1 hour ago, Howard said:

Seeing that the available replacement is a ceramic encased resistor & the heat dissipation is a not insignificant 20ish watts based on 1.6 Ohms it puzzles me that it is part of the wrapped wiring loom.

I take it you did the simple P=VI=VV/R calculation with a 6V drop? That's not the correct formula - it gives the worst case "left the ignition on with the points closed" condition. Normal operation will be about a quarter of that - half because of ~50% dwell and half again because the coil spends most of that time "charging" so the current is ramping up from zero. I could argue another halving because the voltage also ramps, but since it's likely to saturate at low speeds we'll call that close enough.

The core of the ceramic resistor does get quite warm but the "in the loom" version is quite a long piece of wire, so the 5-10W of heat is dissipated along a reasonable area.

[Beans]

12 hours ago, Gareth jones said:

it stalls as soon as I release the key ! What can I do ?

As it is a fairly easy job, check your ignition switch first!

[Gareth jones]

Checked that all ok

[Gareth jones]

I’m located north of Cardiff 

[A TR7 16V]

9 hours ago, Gareth jones said:

Checked that all ok

So, with the ignition in the run not the start position, you've checked there's battery voltage at the top of the coil and across the points with them open; and something less, but rather more than zero, at the top of the coil and 0v across the points when closed? 

If that's as it should be, and you get one of them separate, ceramic ballast resistors, you could wire it up direct from the battery to the coil top and see if that fixes the problem. If it does, then wire the resistor to the ignition switch output, which I assume you've also checked is battery voltage at the input to one of the fuses that supply something switched on the ignition. If it runs then, that's isolated the problem to the ballast resistor or the wiring to or from it. 

Graham