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MG TD TF 1500 - Electrical gremlins TD-4834

I’m pulling snippets from a thread I started on the MG exp. bear with me a moment

Okay I’m now needing to be on the receiving end of some good advice

Re TD-4834 The car was converted to neg earth but that shouldn’t be the issue. The car has a Moss harness with embedded turn signal wiring, transitorized relay box and LEDs in the rear only, new electronic flasher and switch. The turn signals have never worked. They flash as four way hazard lights in all positions on the switch including neutral. I disabled them but then didn’t have brake lights. I bypassed the wiring temporarily at the brake switch so I at least had brake lights. Now I’m trying to sort the turn signals out again. I traced the wires and all seem to be correctly located on the brake switch, flasher, relay box and switch. Still four ways flash all positions. I then changed the relay box. No change. I changed the flasher. No change. I have a grounding issue in a front wing which I will attend to next but this has me perplexed. That’s the first issue

Next, I wired my fog lights and the lights only work with the headlights off. May be a wiring issue to the key switch. I haven’t checked into it yet

Next the horns don’t work. They were restored and worked when hotwired directly to a battery. But not in chassis. Both horns have good grounds. My late father had restored all my switches but he took them apart without photo documentation and I didn’t watch his work. The switch may need to come back apart. Does anyone have pics of the switch laid out in pieces to verify how it goes back together?

Trying to get the gremlins worked out once and for all.

I really need help with this. Electrical is my weakest suit. Trying to get ready for GOFWest


Bill Chasser
TD-4834
W A Chasser

This is what was found. The Moss transistorized relay box had a 6v backfeed within box. As tested out of chassis. A second new in box Moss relay box was tested with the same issue. This spare was going to be put in my dad’s car.

I contacted Lawrie Alexander to see if he had any original breaker DB10 relay boxes. He did and I made the trip up the hill to his place and picked up two boxes. After installing an original Lucas box into the harness the issue is now resolved. I have brake lights and turn signals albeit with one burnt 1156 bulb in one of the 1130 wing lights. Off to get some spares. So two repop boxes of junk quality to attempt a return to Moss. Doubt they will do anything since they’ve been on my shelves awhile. Does any one offer Neg earth LEDs for the 1130 housings yet?

So now, we moved on to the horn malfunction.
Problem No.1-Upon removal of the left horn cover the driver was burnt to a crisp.
Problem No.2- Both the brown/green and the brown/black wires are hot without the button being pressed and not hooked up to the horns. I will need to investigate this further. I may have erred when extending my harness to the dash. I had extended them with two 16 pin connectors and 18” pigtails and may have crossed a wire within the plug.

Bill Chasser
TD-4834
W A Chasser

1) The turn signal issue was resolved with a change to the original style Lucas breaker relay box.

2) The fog light and dash light issue was corrected at the ignition switch. I had wired the wrong switch terminal

3) Dynamo light was a burnt bulb which has failed again. Don’t know why

4) The low fuel bulb had failed as well. The bulb was replaced and burned dimly with six gallons in the tank. Filled the tank and the bulb still burns dimly. Don’t know why.

Re 2-3
The bulbs supplied by Moss are the same screw in bulbs as use in the dashguage backlights but use a spring wrapped around the bulb. Is this correct? With the jewel removed from both the dynamo and fuel lights I can see that the housing has notches for bulbs with tabs. What is the correct bulb setup? It would appear that the spring wrapped screw bulbs are not what belongs in these positions?

4) The horn issue became a horn failure. See new post “Horn Failure”

Bill Chasser
TD-4834
W A Chasser

Bill

If electrical is your weakest suit it is hard to assist for a solution from far. All electricians need a wiring diagram under most circumstances. In your case, the electronic turn signal system in conjunction with the wiring and negative earth seems to have a problem. Do you have a wiring diagram with your extensions?
W_Mueller

W Müeller thanks for your input.
The turn signal issue has been resolved. Using an original Lucas DB10 breaker relay box cured the problem. The reproduction transistorized relay boxes are faulty. Both boxes were checked on and off the car and were backfeeding 6V within the boxes.

When deciphering the current harness I had to flip back and forth between the schematics in the WSM on page N.21 for all wiring to the original chassis layout and the schematic on page N.23 that related to the turn signal wiring.

Why the dynamo bulb burns out in a short time I haven’t figured out yet. The dynamo system is not over charging.

The low fuel light doesn’t go out but is very dim. The sending unit was replaced two years ago so I doubt the sender is the issue. Idk without pulling it down.

BIll Chasser
TD-4834


W A Chasser

Bill - If you haven't already done so, take the schematic into FedX and have them enlarge it as large as it will go without distortion, then have it laminated. and hang it on a wall where you can trace the wires with erasable color pens. This way you can work in one color for each circuit.

When you start working on a given circuit, get some color tape and follow the wire you are chasing and every foot or so, put some tape on the wire from one end to the other.

The other alternative is to get a new harness with nice clean, color wires to chase around - much easier on the eyes.
Cheers,
DW DuBois

Quote; 4) The low fuel bulb had failed as well. The bulb was replaced and burned dimly with six gallons in the tank. Filled the tank and the bulb still burns dimly. Don’t know why.
Unquote.

The fuel tank sensor is simply an on-off switch to ground.
If the low level light is on, dimly, you have either;

1) a bad sensor with a poor ground,

2) an additional or secondary, poor, ground path.

I would suggest;
(First disconnecting the battery ground)
Remove the wire from the sensor.
(Reconnecting the battery ground)
If the light still glows dimly you have an additional (poor) ground path.

If the light is out, Then check with an Ohmmeter on the sensor terminal to ground. (Wire still removed)
With a full tank you should read very high Ohms. If you read almost anything, your sensor needs to be looked after.

Jim B.
JA Benjamin

Dave thank you for your suggestions. The schematics are difficult to read, especially in my advancing age. I do have an enlarged laminated schematic that is color coded. Thank you for reminding me. Now to figure out we’re I stores LOL.

Jim I will investigate further using your suggestion in the next couple of days.

Cheers

Bill Chasser
TD-4834
W A Chasser

"The sending unit was replaced two years ago so I doubt the sender is the issue." Virtually all of the replacement senders with plastic floats fail from ethanol- hope yours was not plastic. Jim right on- take the wire off the sender- if bulb off, sender unit is the problem. The wiring diagrams are really hard to read. Try these diagrams:
http://www.ttalk.info/TDWiringDiagrams.htm
There are some even better ones around somewhere. George
George Butz III

Check that the sender is correct. If a rheostat type has been fitted the bulb will glow and change brightness as the fuel level changes. This type is for a fuel gauge.
Our type is a pure on/off switch but I have seen few of the other type bought at autojumbles because they looked the same.
Easy to check with an empty tank, just take off the cover plate and if it has a coil or a carbon rod it is wrong. You will need a mirror.
Ray TF 2884
Ray Lee

Good point Ray, I ran in to that very thing some years ago. George
George Butz III

One other point.
The warning lights, even when operating properly, glow dimly. They have a large resistor built into the housing. They glow dimly enough so you can actually see the filament.

Another possibility is that the glow you see is proper but the lug on the sender is touching ground.

Jim B.
JA Benjamin

Bill,

I had a "new" fuel sensor with plastic float, and it cracked and failed within a year. Float sinks, light stays on.

I refurbished the original and after a while, it seemed to also not work - light stays on.

Order a new back plate gasket from Moss and take off the back plate to access the float mechanism. My refurbished unit had become stiff and no longer floated freely.

Jim
J Barry

Why did you convert to negative ground in the first place? I suspect that has been the cause of many of your problems. Admittedly I am not an electrical genius. Just MHO.
Cheers, Byron W.
JB Warwick

Changing to Negative Earth seems sensible to me and I have done it on more than just the TF. If anything there should be less gremlins once you start fitting modern upgrades as the Negative Earth compatible parts are easier to find and usually cheaper. Still, we all have our own preferences, and that's all to the good.
Dave H
Dave Hill

My reason for changing to negative earth was twofold. First so some idiot mechanic didn’t fry my car not being aware of the Positive earthing system should it need to be worked on away from home. Second so a mentioned in the previous post that modern conveniences could readily used ie USB jacks for cell phones gps music when parked etc. conversion is easy to do and doesn’t affect anything except battery terminal location, initial flashing of the dynamo and and switching of the amp meter wires from pos to Neg so it reads correctly. I may have missed something as it’s been three years since I’ve done the change but I followed the directions given by a trusted member on one of the MG forums. Everything else stays the same.

I’m going to put off the horn and indicator light issues until after GOFWest as there are more pressing issues to get done. Finished with installing new brakes and hub seals and led upgrades to my trailer. Fluid changes, lube, valve adjustments, and misc nitty details to the TD. Safety checks to the truck and trailer. All vehicles washed and the truck detailed. All I have left is to wash and wax the TD and detail it. Bathe the Aussies and pack for the trip.

For those attending hope to meet up with you there. See old friends and meet new ones.

Bill Chasser
TD-4834
W A Chasser

This thread was discussed between 01/09/2018 and 06/09/2018

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