TR6 Tech Forum

If you have any suspicion the wire harness does not exactly match what the harness was like originally I suggest you start by mapping what each fuse does. No need to unwrap the harness to do that. Pull one fuse and see what does not work anymore. List those items for that fuse. Leave the 1st fuse out and then pull the next fuse. See what else gets shut off and list those items for that fuse. And the 3rd fuse.

Once all the fuses are pulled see what is still working and list those as unfused.

Now compare what you have on each fuse to what the original wiring has on each fuse. If you find any discrepancies look at the items one at a time by checking the wire color at the fuse. Find that wire color at the other end by looking at the items listed for each fuse until you locate it. If it does not appear anywhere add that info to the list. Goes to unknown. Move on to the next discrepancy.

Once you have determined as many of the wire colors associated with each electrical item as you can you will have an easier time isolating which wire is carrying the excess current to which electrical item. By process of elimination you can maybe determine where the unknowns end up. Without spending the time to unwrap anything. You can always add a different harness cover later. Whether or not the harness gets unwrapped.

The illumination bulbs in the dashboard gauges seems to be suspect so maybe concentrate on that .. which is the toughest area to work on. Start by pulling the appropriate fuse and listing everything that is on the same fuse as the gauge bulbs. then do the other two fuses.

A common short that occurs is the heater fan/blower motor switch terminal can ground against the metal dashboard panel behind the wood.

Thanks for all of the continued feedback and suggestions on my issue. I've had my head under the hood and dash these last couple days and run some experiments to narrow the list of possible suspects. I've also started mapping the circuits as suggested.

Firstly about that blue wire mentioned by Ken and Aaron - I did some tracing after unwrapping some small sections of the harness and determined that the blue wire is providing key-activated power to a thermostat that is installed on the radiator as a part of an electric fan that the PO had installed. The fan is mounted in front of the radiator. The blue wire changed color two times before it made it to the thermostat, but I was glad to finally determine that. So I disconnected the blue wire from the fuse box for now. There are several other wires and what I think is a relay that are part of the electric fan wiring, but none of it connects to the fuse box.

The fuses are installed according to Dan Masters recommendations as I understand them from other threads on this forum - 10A for red circuit, 15A for purple, and 20A for green. I'm using Bussman AGC fuses.

I located and disconnected the connector hidden behind the driver-side kick panel that disconnects the tail lights, trunk light and I think the interior trans tunnel light (mine doesn't have one). Then I turned on only the parking lights. The output side of the fuse for the red circuit still gets hot to the touch.

Then I removed the fuse for the red circuit and turned on the headlights. The output side of the fuse for the purple circuit still gets hot to the touch.

The common thing for both settings of the column-mounted headlight switch is that the dash lights come on with both parking lamps only or parking lamps w/headlights, although very dimly.

My next step will be to open up the dash and see what's lurking there. The dim dash lamps seem to say that somethings fishy there. And I'll look for the heater fan/blower motor connections pointed out by Jeff.

Thanks all.

Before you get too deep in the dash lights, consider first making a short jumper to bypass the rheostat built into the dash for dimming the dash lights. You can do this with a short length. Of wire and a couple of male spade terminals that mimic the tabs on the rheostat. Doing that means your dash lights will receive full voltage from the source, eliminating the rheostat from the equation.

Electric fan draws a lot of current so not sure why blue wire is plumbed that way.

---

-Mark

Power for the efan should go straight to the alternator and have its own fuse.

PLEASE jump the rheostat for the dash lights first. You can access the connections through the ash tray on top. Don't expect to be blinded by the light.

---

1972 Sapphire Blue TR6

1973 MGB Rover V8 LT 77

A picture is worth a thousand posts

Before you start digging too deep, try cleaning all the contact points where you feel heat. Heating only occurs when there is a resistance on the contacts or if there is excessive current draw.
Excessive current draw should cause fuses to blow, unless there is a ground before the fuse or in the fuse box. If that were the case your wire insulation would probably be melting.

I have an electric cooling fan on my TR6 also. With an upgraded aluminum radiator, the fan rarely ever runs. Usually after idling or very slow driving. It has a 20 amp fuse from an additional fuse box I added when doing my resto-mod. It has other electrical accessories with separate relays, such as air horn, heated seats, driving lights, LED headlights, etc. There are no hot connections.



.
.

With the headlight switch on does the gauge light for the new voltmeter go out when you pull the red fuse ? Purple fuse ? Any fuse ?

Have you looked to see what was done by the previous owner to change over from an ammeter to a voltmeter ?

Being somewhat paranoid, I would suggest getting a battery disconnect switch and anytime you leave the car, turn it OFF.
Secondly, for future considerations I would install a headlamp relay kit. However this issue you have described is different, so don't let this be a distraction.
And in case you haven't, I'd print out several copies of the wiring diagram from Advanced Auto wiring and use them to mark each circuit. Perhaps one print for each circuit in question. Getting extra large copies from a print shop makes it easier.

Thanks to all for all of the very helpful information in your replies. I'm putting it all to good use.

Since there were a few comments on the electric fan and it's wiring, I've done some more investigation and attached a marked-up photo of the wiring. The fan works via the toggle switch but I don't know if it would work via the thermostat. Puzzling to me why there are two in-line fuses, one in front of the relay and another behind the relay. I guess that protects the relay from frying.

I've looked into installing a jumper on the rheostat as suggested. I see that's a very tight area for access. Looks like some form of laparoscopic surgery is required. Is the correct approach to disconnect the two connectors on the back of the rheostat and connect the jumper to the disconnected leads or can I just connect to the two unused male connectors (w/a female jumper cable) also on the back of the rheostat. See photo. Haven't figured out how to tackle this tight quarters challenge yet but any advice is appreciated.

I had removed the fuse box from the fender earlier in this adventure and verified that all the connectors are clean with no corrosion. It's a fairly new box but I have considered just replacing it to eliminate the possibility that it's faulty in some way. Is there way to test the fuse box from an electrical perspective?

In answer to tirebiter's questions... With the headlight switch on (but engine not running) does the voltmeter lamp go out when the red fuse is pulled - Yes, but head lights remain on. Purple fuse pulled - voltmeter lamp goes out and so do headlights. Green fuse pulled - no effect on voltmeter lamp. Do the results of these tests tell you something? I don't know what the PO did to switch from ammeter to voltmeter but I found a product document in the papers I got from the PO that was for a Voltage Stabilizer. This could be related to your question. Not sure where it's installed if it is. See attached photo.

Paul - The car does have a battery disconnect switch installed at the negative battery post. How would a head lamp relay kit help. I've read about those but don't really understand how they work. Regarding wiring diagrams, yes, I have the Auto Wire diagrams thanks to another thread on this forum. Might be weird but I actually like studying a wiring diagram.

Sorry for this lengthy reply. Lots to discuss. And thanks for your continued interest and advice.

Tom, the headlamp switch on the original car carried all the load for the headlights. There was significant voltage drop which caused dim headlamps. There is an article where this was measured, I'll look for it. But with lots of voltage drop things tend to heat up. While many people have not installed relays, I think it should be a 'must upgrade' item. (My opinion). I do NOT think it is related to the issues you are discussing, but "While you are in there". You might consider it after the root cause problem is determined and fixed. (Don't add a fix that doesn't help as it will confuse the troubleshooting.)

This would be an incorrect wiring, unless he wanted to fuse the headlights.

The headlights are usually power directly from brown wires to the head lamp switch. (Brown wires are always hot/unfused. Purple are always hot/fused)

---

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

Yes, close quarters behind the dash. If you plan to jump the rheostat, do the jumping between the wires and take the rheostat out of the equation. If you can get your test leads on the rheostat to measure resistance, have another pair of hands operate the knob while you watch the meter. Again, with the battery disconnected. If the rheostat is original and you want to try some cleanup, you’ll want to take the dash apart to do it. Or if you want to use a replacement rheostat, same thing…I still have a replacement rheostat in the box waiting for the next “convenient “ time to pull the dash.

Your photo captions are illegible on the fan wiring photo. Too small to read…

Any gauge lights should be on the red circuit. As someone else advised, recheck the wiring done to convert the ammeter to volt meter.

Regarding the new fuse box, test it on each side for continuity with the fuses. I’ve seen the rivets loose on new boxes. If the rivets are loose, take the box off, clean the heck out of it, then solder the rivets.

Repeating what ken h said to emphasize that your headlights do not go through the purple wire fuse.

However, i believe the flash to pass feature does go through the purple fuse for power. Pulling the purple fuse would stop this feature.

With the purple fuse pulled, does the flash to pass work? If so, someone may have messed up the connections from the headlight switch to the wiring harness.

To activate flash to pass, pull the headlight switch towards you.

---

1972 Sapphire Blue TR6

1973 MGB Rover V8 LT 77

A picture is worth a thousand posts

The voltage stabilizer gives an even 10 volts to give power to the temp gauge and fuel gauge. It's on the back of your speedo. Its on the green circuit. Doubt it has anything to do with your problem.

I wear a size L glove and could jump the rheostat by going down from the ash tray.

---

1972 Sapphire Blue TR6

1973 MGB Rover V8 LT 77

A picture is worth a thousand posts

This is what should happen. The red fuse supplies power to the dash lights via the rheostat. I don't see a problem there except the bulb is apparently not the same wattage as the rest of the gauges use. That is why it is brighter than all the rest. I doubt the problem is with the red circuit wiring. To be certain can you disconnect the voltmeter light and see if the red fuse still gets hot with all the fuses in place ? Or does it have only one wire and gets it's ground through the voltmeter housing ? If so pull it out and see if the red fuse still gets hot when the lights are turned on.

What Ken H. said in post #26. The purple fuse should not be able to turn on/off the headlights unless you are holding the hi/lo beam switch back against a spring. You may not have realized the TR6 has a fash-to-pass feature. The stalk mounted dimmer switch is spring loaded like that and is supposed to turn on the hi beam even when the headlight switch is off. Is the blue hi-beam indicator on as well ?

I believe you may have come across a very important aspect of what's wrong with the wiring. I wonder what else you can find wrong if you were to finish mapping out what is working correctly on each fuse and what fuse they are powered by. As I suggested in post #16. It could save you some work by going directly to what is wrong and not spending time on things that are working correctly. I understand everyone has their way of doing things and your way is getting you closer to figuring out what needs to be fixed so carry on. Good work !

For instance does the rheostat make any or all of the dashboard lights get dimmer/brighter when you turn it ?

That will probably not help diagnose the hot fuses. It powers the fuel and temperature gauges but not the light bulbs in them. That is it's only function and as already pointed out by Aaron K. it is on the green fuse which is not getting hot. With the key on do both the fuel and the temperature gauges drop to zero if you pull the green fuse ? Do they stay at zero if you then turn on the lights ? Do the gauge lights work with the green fuse pulled ? Again more of the electrical system mapping I encourage you to finish before doing any of the difficult work behind the dashboard..