Ignition live not dropping when engine off - resistor needed
Discussion
Guys,
I'm not sure where to put this thread; Classics and Yesterdays Heros, Home Mechanics or here. So I've gone for here. Mods, please feel free to move if it's more appropriate / likely to gain more traction elsewhere.
A friend has a TR4a. It misfires when it gets hot. We suspect it's an ignition problem (rather than fuel.) We've tried various things (new coil/ballasted coil/new plugs/new plug leads) to no avail.
I wonder if it's the low-tension side wiring which is causing problems. The wiring is 60 years old, the switches are the same age and anno domini is not kind (we all know that). The low-tension side is simple; a +12v feed from the ignition switch to the +ve terminal of the coil when the ignition is on, and the engine is stopped when the ignition switch is turned to 'off' and this feed is cut.
As a diagnostic for whether this wiring is causing the problems I have put a relay in. The idea is that the current ignition live turns on the relay, which in turn connects the +ve terminal of the coil straight to the battery via some nice new cable. Here's a before-and-after picture.
All well and good, but when the ignition switch is turned off the engine carries on running because the relay is not turning off. The reason for this is that the ignition feed only drops to +8v when the ignition is turned off, which is enough to keep the relay on and the engine running. When the engine is stopped (by unplugging any of the connections from the relay) the ignition feed drops from +8v to 0v.
This could be solved by putting a resistor across the ignition feed and earth terminals of the relay, as pictured in the 'after' picture above. The question is what value of resistor should I choose? It needs to be small enough to not interfere with the operation of the relay under normal use, but big enough to bleed away the +8v when the ignition is off and make the engine stop.
What value of resistor would you choose? All answers on a postcard please .... thanks!
I'm not sure where to put this thread; Classics and Yesterdays Heros, Home Mechanics or here. So I've gone for here. Mods, please feel free to move if it's more appropriate / likely to gain more traction elsewhere.
A friend has a TR4a. It misfires when it gets hot. We suspect it's an ignition problem (rather than fuel.) We've tried various things (new coil/ballasted coil/new plugs/new plug leads) to no avail.
I wonder if it's the low-tension side wiring which is causing problems. The wiring is 60 years old, the switches are the same age and anno domini is not kind (we all know that). The low-tension side is simple; a +12v feed from the ignition switch to the +ve terminal of the coil when the ignition is on, and the engine is stopped when the ignition switch is turned to 'off' and this feed is cut.
As a diagnostic for whether this wiring is causing the problems I have put a relay in. The idea is that the current ignition live turns on the relay, which in turn connects the +ve terminal of the coil straight to the battery via some nice new cable. Here's a before-and-after picture.
All well and good, but when the ignition switch is turned off the engine carries on running because the relay is not turning off. The reason for this is that the ignition feed only drops to +8v when the ignition is turned off, which is enough to keep the relay on and the engine running. When the engine is stopped (by unplugging any of the connections from the relay) the ignition feed drops from +8v to 0v.
This could be solved by putting a resistor across the ignition feed and earth terminals of the relay, as pictured in the 'after' picture above. The question is what value of resistor should I choose? It needs to be small enough to not interfere with the operation of the relay under normal use, but big enough to bleed away the +8v when the ignition is off and make the engine stop.
What value of resistor would you choose? All answers on a postcard please .... thanks!
You need to be looking at why there’s 8v on the ignition circuit with the ignition off rather than putting resistors across the relay coil. The resistance of the coil itself should be enough to pull it to 0v.
There’s something back feeding that ignition circuit - does it have a dynamo? Or aftermarket radio?
There’s something back feeding that ignition circuit - does it have a dynamo? Or aftermarket radio?
I expect you'll find the switched ignition feed is being back-fed from somewhere - the most obvious potential sources are the no-charge warning light or the ballast resistor. Anything that was electrically connected to the coil +ve could cause this, since your relay would then be self-energizing.
Why don't you fix the wiring / connector problem instead of adding this bodge?
Why don't you fix the wiring / connector problem instead of adding this bodge?
How repeatable when hot ? Only when driving ? At idle ? other ?
Have you tested spark during these times ? When does it recover ? Presumably this is a new problem that just started ? It ran fine before and no changes have been made ?
Your post seems to deviate from a misfire, to a non shut off after changes ?
So does that mean the misfire is now gone ? Or you now have two problems ?
Have you tested spark during these times ? When does it recover ? Presumably this is a new problem that just started ? It ran fine before and no changes have been made ?
Your post seems to deviate from a misfire, to a non shut off after changes ?
So does that mean the misfire is now gone ? Or you now have two problems ?
Strange posting. Fault finding requires a structed and methodical approach. Inserting a relay into a simple on/off circuit is certainly not that.
So the relay is picking up some back feed from the charge regulator, poor earthing whatever. Does that have anything to do with the misfire? Very hard to tell. Complicating a simple circuit is not the normal route for fault finding.
The classic problems are worn/abraded wiring, poor switches - easy to check by putting a load on the ignition feed. Similarly the engine earthing.
If you have already changed coil, plugs, leads that does not leave a lot. Condenser if the dizzy still has one might be a good idea, equally excess fuel once the carbs have warmed up is a possibiity.
So the relay is picking up some back feed from the charge regulator, poor earthing whatever. Does that have anything to do with the misfire? Very hard to tell. Complicating a simple circuit is not the normal route for fault finding.
The classic problems are worn/abraded wiring, poor switches - easy to check by putting a load on the ignition feed. Similarly the engine earthing.
If you have already changed coil, plugs, leads that does not leave a lot. Condenser if the dizzy still has one might be a good idea, equally excess fuel once the carbs have warmed up is a possibiity.
Guys,
Thanks for the answer. Yes, it was a slightly quick-and-dirty attempt at diagnosis, with a view to implementing a more permanent solution if it did indeed solve the problem. However it has thrown up interesting questions of it's own.
The car does have an aftermarket radio and an alternator conversion. How would/could those produce an ongoing low voltage on the ignition live after the ignition has been shut off?
I know that the answer is to fault-find it properly so I probably ought to arrange a time with the friend to do just this!
Thanks again for your input.
Thanks for the answer. Yes, it was a slightly quick-and-dirty attempt at diagnosis, with a view to implementing a more permanent solution if it did indeed solve the problem. However it has thrown up interesting questions of it's own.
The car does have an aftermarket radio and an alternator conversion. How would/could those produce an ongoing low voltage on the ignition live after the ignition has been shut off?
I know that the answer is to fault-find it properly so I probably ought to arrange a time with the friend to do just this!
Thanks again for your input.
Are you sure its a switch + and not a switched - ?
Also I believe 8v is the correct voltage when the engine is running, it will supply a full 12v when the starter motor is engaged. This is to overcome the drain the starter motor can have on the battery, to make sure there is a big enough spark to start the engine.
Also I believe 8v is the correct voltage when the engine is running, it will supply a full 12v when the starter motor is engaged. This is to overcome the drain the starter motor can have on the battery, to make sure there is a big enough spark to start the engine.
2Btoo said:
Guys,
Thanks for the answer. Yes, it was a slightly quick-and-dirty attempt at diagnosis, with a view to implementing a more permanent solution if it did indeed solve the problem. However it has thrown up interesting questions of it's own.
The car does have an aftermarket radio and an alternator conversion. How would/could those produce an ongoing low voltage on the ignition live after the ignition has been shut off?
I know that the answer is to fault-find it properly so I probably ought to arrange a time with the friend to do just this!
Thanks again for your input.
If you have an alternator conversion the voltage regulator is now effectively on the alternator. I would get a meter and measure the drain. Disconnect the alternator and check it again. Thanks for the answer. Yes, it was a slightly quick-and-dirty attempt at diagnosis, with a view to implementing a more permanent solution if it did indeed solve the problem. However it has thrown up interesting questions of it's own.
The car does have an aftermarket radio and an alternator conversion. How would/could those produce an ongoing low voltage on the ignition live after the ignition has been shut off?
I know that the answer is to fault-find it properly so I probably ought to arrange a time with the friend to do just this!
Thanks again for your input.
I'm clearly no expert but I would give that a try.
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