TR6 Tech Forum

Ken - Thanks for that clarification about dielectric grease. So noted. I’ll go back and cleanup those connections I applied it to. You learn something new every day and there’s a lot to learn about Triumph wiring.

Did you inspect the column mounted switch? The contacts get hot and hot enough to soften the plastic they are imbedded in. This will result in intermittent symptoms.

As for testing resistance, it is testing the connectivity between two points A and B.

0 or very low resistance (Ohms) can mean that A and B are connected to one another directly or somewhere in between

Infinite or high resistance means that the two points A and B are not connected or insulated from one another.

Either can be good or bad depending on what you are looking for.

Example:

Two wires or points that are supposed to be connected to one another showing 0 or low Ohms is a good thing.
Two wires or points that are not supposed to be connected to one another showing 0 Ohms between the two is bad (short)
Two wires or points that are only supposed to connect to one another when a switch is activated might be infinite (open circuit) when the swi5vh is off and goes to 0 or low when the switch is activated.

For simple A-B connectivity checks, some meters have a beep/sound setting on the meter that will beep when two points are electrically connected.

The actual Ohms measurement matters on deeper circuit analysis, like Checking Pertronics to ground, where a few Ohms of resistance might affect its operation.

For resistance, the meters usually have a small battery in them to send some tiny voltage from A to B to measure, so just know that batteries in the meter can go flat over time and the meter cant measure resistance.

Hope this helps.

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74 TR6 aka Mr. T.
Clean stock, with a few peaks and tweaks.

Tom,

One thing to keep in mind is resistance between two switch contacts for instance can be zero ohms when there is almost no current flowing through the contacts. Such as when measuring with an ohmmeter.

When there is higher current such as when operating several incandescent light bulbs is when worn/dirty contacts will exhibit the most resistance to the flow of electrical current through them. That is why the voltage drop test was invented. It can show you how much the voltage is reduced due to the amount of resistance the switch contacts are limiting the flow of electrical current. It's normal to see the larger electrical load demonstrate a higher resistance through the switch contacts. Which results in a greater voltage drop.

I believe the deeper circuit analysis Ken mentioned could be that of using a voltage divider formula to determine resistance by measuring voltage drop. A handy voltage drop calculator can be used to see what the voltage drop translates to in terms of resistance through any switch contacts. Or any electrical connection for that matter. Such as the press fit electrical connections between the fuses and the fuse holder clips and the riveted electrical connections between the fuse holder clips and the fusebox spade terminals.

voltage divider calculator here:
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-voltage-divider
The following example can be used to measure the resistance through the headlight switch.
V! is battery voltage
Vout is what you measure between ground and the hot side of the switch contacts
R1 is be the accumulated electrical resistance of the electrical connections to the hot side of Vout. Hopefully very close to zero ohms with an electrical load but in view of the voltage drop measurements you already took at the fusebox I believe it is higher electrical resistance than desirable.
R2 is the resistance through the switch contacts when there is an electrical load on them.

Tom,
The resistance question is not complicated in the way I mentioned it. Your fuse box has tabs on both the input and output sides. Setting your meter to resistance/ohms at x1000 for example, then touching the meter’s leads together should peg the needle indicating no/little resistance. First, disconnect the battery. Really.

If you touch the input tab on the fuse box (with no fuses installed) then touch the holder for the fuse on the same side, your meter should peg indicating low resistance. That’s what you want. Repeat on each side. Then reinstall the fuses and test input-to-output. This will rule out the fuse box as the problem.
If you clip an alligator clip to the white wire at the fuse box and the other end to one of your meter’s leads, then touch the white wire at the ignition switch, it should show connectivity (pegs the meter). Use the meter and a clip (for the added length and frees up your hands) to test on the red and purple circuits. These simple exercises along with a printed copy of the wiring diagram can take a lot of mystery out of the TR6 wiring. It ain’t complicated, but perhaps unfamiliar.

HuhReally - I did remove the headlight switch from the column w/o disconnecting the wires running down the column. I was able to inspect the soldered connections which look good. My son and I did some voltage tests and measured 12.06V coming in to the switch but only 9.77V going out. Seems like the switch is questionable. Got one ordered but expect that it will be a challenge to get that sleeve of wires down through the column. Any tips on that operation would be appreciated. Thanks

Tom, if you’re seeing a voltage drop like that in the switch, I’d wager that testing the switch on ohms setting will reveal more resistance than expected- in other words, a bad switch. Whether it can be remedied, or should just be replaced, depends on if it is damaged or just dirty.

Ken, Jeff, Paul - Thanks for your explanations about resistance and testing it. I’ll run those tests on the fuse box connectors as explained. I also just received a copy of Dan Masters Electrical Maintenance Handbook which looks to be a great reference for TR6 wiring topics.

I’m planning to disconnect the front park/marker lights, already have the dash lamps and rear harness disconnected, then redo some voltage drop and resistance testing on sections of the wiring to see if something surfaces as the cause of the red circuit fuse getting hot. Since previous tests were inconsistent as to what device might be causing the problem I’m hoping that more granular testing will get me closer to the source.

Since two wires from the headlight switch were incorrectly connected to the harness causing the purple circuit fuse to get hot I’m wondering if there are other similar wiring problems also causing the red fuse to get hot.

Paul - Got it on my list to get a can of contact cleaner. I couldn’t tell from first inspection, can that headlight switch be disassembled and reassembled?

Recall a pin or two in one of my switches the could be pushed/tapped out to disassemble. There is a spring in there, so do your work in a clean, contained place so it’s easier to find the parts. Some of the contacts are accessible assembled. If you inspect and find melted plastic, get a new switch.
Some folks have installed halogen headlamps in their TRs. Those draw more current, which when running through the stock switch, can induce meltdown. Many have learned that routing higher draw headlamps through relays takes the strain off the poor old switch. But the installation of relays won’t save a bad switch.

Tom,

Did you notice if the switch is getting hot or warm at all ?

Jeff - I have not looked for heat at the switch on the column but will. The switch definitely seems suspect. I'll check ohms and voltage at the switch also. Received the new switch but haven't tackled installation yet.

Good news from testing yesterday... with rear harness and dash lamps disconnected and ONLY front park/marker lights connected on the red circuit, the fuse does not get hot. Voltage at fuse output side was 12.15V and voltage at park/marker connectors near front of car was 11.96V. Ohms testing at fuse box between rivets, clips, and spade connectors showed .8 w/scale set at 200. Same reading obtained from fuse box output spade to front park/marker connections. Testing the ground wire on one of those front park/marker connections showed 1.9. Could be grounds need some attention but not terrible.

But, I discovered that the flash-to-pass feature doesn't work w/purple fuse removed. Another head scratcher. I wonder if the contacts in the switch between blue/w and purple are corroded and grounding out.

Flash to pass is not supposed to work with the purple fuse removed.

Purple is always hot. Key on or off, it's hot. With the headlights off, the flash to pass has to get power from a circuit other than the normal headlight circuit.

Make sense now?

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1972 Sapphire Blue TR6

1973 MGB Rover V8 LT 77

A picture is worth a thousand posts

Yes, Thanks Aaron. Makes sense now that you've explained it. I was incorrectly applying the idea that the headlights should never be fused to the flash-to-pass feature since its a headlight function. That's one less wiring issue to worry about.

Really appreciate all of the good advice I'm getting on this forum.

You'll find a lot of details like that within the 34 Chapters in this manual.

It's a very easy read and well worth taking the time.
It was written with us owners in mind:
http://www.advanceautowire.com/tr2506/

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ZS carb repairs
kencorsaw@aol.com



Edited 1 time(s). Last edit at 2024-07-17 09:50 AM by poolboy.