TR2/3/3A TR3A Blown Fuse

[SteveBones]

Hello,

I was driving my TR3A this weekend and am now blowing fuses. It happened initially during my drive. I am not sure if I post frequently enough so my TR3A is the car that is back on the road after 2 years of work and 10 years of sitting in the PO's garage. The blown fuse is for the fuel gauge, turn signals, wiper, and I believe heater.

I wanted to ask for guidance on the following:

1) I am uncomfortable needing to ask, but what is the best way of testing for shorts? I do have a voltage meter and test light. I feel like I should know this but......

2) Any suggestions on where to start ? I was planning on starting with the fuel gauge and then wiper switch.

3) Would it make sense to isolate areas ......if I start with the fuel gauge, should I test the wires that connect to the gauge? If so, what should I test - measure?

4) Also, the heater switch is not hooked up. I have the ground wire connected to the chassis - dash support, and the black wire that goes from the heater to the switch is not connected and the end is taped.

6) How should I check - test the wiper and wiper wire connections to determine if this is causing the short?

7) Would the next area to look at be the turn signal control head? The horn is working so not sure if this is an issue or not.

I will have limited assess to the forum (and my computer) starting Friday afternoon but will be back on Sunday.

Thanks,

Steve

 

[TR3driver]

Not a stupid question and unfortunately there isn't a single method for finding shorts. Here are a few suggestions, though:

Rather than wasting a fuse every time you want to test, try temporarily connecting a headlight bulb across the fuse terminals. I save old headlight bulbs just for this sort of thing, since there is always one filament that burns out first, leaving the other intact for testing. The bulb will limit the current to about 5 amps or so, safe for the wiring; while also giving you an indication of whether the short is currently present or not. Of course it also won't pass enough current to run everything at the same time, but you won't be running everything at the same time while testing. This is easier IMO than trying to interpret readings from a meter.

There are 3 or 4 separate green wires at the fuse block, I would start by disconnecting and reconnecting them one at a time to help isolate which wire leads to the short. The one that makes the lamp light brightly (with the ignition on but other controls off) leads to the short. Once you find that, leave it disconnected and connect the others to find out which devices still work.

Does the control head turn with the steering wheel? If so, your short is very likely behind the control head. It's a likely place anyway.

My second suspect would be the wiper motor. The motor gets power all the time, not just when the switch is on; and having the wires reversed on the terminals will cause a short only when the motor is not at it's parked position.

My third suspect would be the brake lights or the wires leading to them.

PS, note that there are several minor variations in wiring, and the diagrams in the workshop manuals are wrong for your car. The Advance Auto Wire diagram is pretty close, or the ones found in the back of "Practical Hints" (preferably the edition that matches your car). Here is one for an early TR3A, from PH 4th ed which I've doctored a bit to show the connections for LHD instead of RHD.

Click on "Options" near the top and select "Download" to get a full size copy. (For some inexplicable reason, PhotoBucket no longer allows zooming to full size on their web page.)

Here's a later version (without the doctoring)

 

[SteveBones]

Hi Randall,

Very good suggestions on how to proceed. Very much appreciated. Here is an update:

1) Based on your suggestions, I narrowed it down to the green wire for the fuel gauge. I verified by inserting back the fuse and verified the wipers, and turn signals are working with this one green wire removed.

2) I now need to narrow down the cause of the short in the fuel gauge electrical "system". I will let you know what I find out.

3) Also, the control head does move when turning the steering wheel. I would like to fix this. Any recommendations on the best way to fix is much appreciated. I have not done any work on the control head. The previous owner replaced the wiring and the steering wheel, and looks to have done a very nice job. I have the adjustable steering wheel. The control head is original.

Thanks,

Steve

 

[Geo Hahn]

1) Based on your suggestions, I narrowed it down to the green wire for the fuel gauge. I verified by inserting back the fuse and verified the wipers, and turn signals are working with this one green wire removed.

2) I now need to narrow down the cause of the short in the fuel gauge electrical "system". I will let you know what I find out...

In case you find it useful, here is a website that describes the MGA fuel gauge in great detail. That MGA gauge is essentially the same as the TR3A gauge.

 

[TR3driver]

1) Based on your suggestions, I narrowed it down to the green wire for the fuel gauge. I verified by inserting back the fuse and verified the wipers, and turn signals are working with this one green wire removed.

Which would also leave the heater switch, no? I believe you'll find that it is connected to the fuel gauge, so that one green wire feeds both of them.

For the control head, start by having someone turn the wheel back and forth while you watch the end of the stator tube sticking out of the face of the steering box. If the tube moves, then that joint is loose for some reason. More likely, though, the joint is fine and the stator tube is broken inside the steering column. They tend to break at the weakest point, the base of the slot that engages with the control head. Macy's Garage has new stator tubes.

Probably you should also disassemble, clean and lubricate the control head while you've got it off. I suspect that binding in the control head is what breaks the stator tubes. There are some photos at https://s258.beta.photobucket.com/user/TR3driver/library/Control Head which might help.

 

[Geo Hahn]

...Rather than wasting a fuse every time you want to test, try temporarily connecting a headlight bulb across the fuse terminals. I save old headlight bulbs just for this sort of thing...

Randall -- I've meant to ask before: Is there a reason you use a headlamp bulb rather than a smaller bulb like a tail lamp or turn signal?

 

[TR3driver]

Randall -- I've meant to ask before: Is there a reason you use a headlamp bulb rather than a smaller bulb like a tail lamp or turn signal?

With a smaller bulb, it can be difficult to tell the difference between an actual short and a normal load. For example, if you try the brake lights with a tail light bulb in place of the fuse; the tail light bulb will light at nearly full brightness while the brake lights won't visibly glow at all. A headlight bulb might glow a bit, but will obviously be much less than full brightness and you will be able to see the brake lights come on (if there is no short).

The smaller bulbs are also more of a pain to hook up; you need to either have a socket with pig tails or try to solder wires to the bulb. With a headlight bulb, you can just clip onto the terminals.

 

[SteveBones]

Thanks. I believe at this time the issue is with the actual gauge itself. I will explain more in a following message.

Steve

In case you find it useful, here is a website that describes the MGA fuel gauge in great detail. That MGA gauge is essentially the same as the TR3A gauge.

 

[SteveBones]

Randall,

Your message is extremely helpful with determining and isolating the control head issue - I also had no idea where to go to replace the stator tube so thanks again for this info. This is something I will want to fix.

I also need to address the heater switch. I did not realize until after I recovered the dash that the section and whole for the heater switch was cut out and a new sheet metal section was welded over. I have the heater switch and will need to drill a new hole at the original location. I think this will be the 2nd to-do item on my list after resolving the fuel wiring short issue........fixing the control head and stator tube will be the 3rd item on my fix it list for the car.

Thanks again for the help. I am looking forward to getting these items fixed.

Thanks,

Steve

Which would also leave the heater switch, no? I believe you'll find that it is connected to the fuel gauge, so that one green wire feeds both of them.

For the control head, start by having someone turn the wheel back and forth while you watch the end of the stator tube sticking out of the face of the steering box. If the tube moves, then that joint is loose for some reason. More likely, though, the joint is fine and the stator tube is broken inside the steering column. They tend to break at the weakest point, the base of the slot that engages with the control head. Macy's Garage has new stator tubes.

Probably you should also disassemble, clean and lubricate the control head while you've got it off. I suspect that binding in the control head is what breaks the stator tubes. There are some photos at https://s258.beta.photobucket.com/user/TR3driver/library/Control%20Head which might help.

 

[SteveBones]

Hello,

I do have an update. Here is what I have done. I also need some help and guidance on what to do next which is also mentioned below.

What's been done so far?
----------------------------
1) Confirmed the short is with the fuel gauge wiring. I confirmed this by inserting a new fuse, and with the fuel gauge green wire at the fuse box disconnected, was able to see the turn signals, and wiper motor was working.

2) Next I removed the fuel gauge and disconnected the wiring to the fuel gauge, and with a new fuse, turned the ignition switch on with no issues (meaning shorts and a blown fuse like before).

3) I followed up on this by briefly having the fuel gauge wire ends touch to see if this would cause a short which it did not. I not sure if this would help to remove the actual fuel gauge as the source of the short, but checked it just the same.

4) I also used a 12volt battery from our garage door opener and connected it up briefly to the fuel gauge. The fuel gauge did moved to approximately the 1/4 full mark so it appears the gauge is working. I expect that the gauge reading showed much less than full because the 12 volt battery I used is very small and I expect creates very low current.


What to do and test next to isolate the fuel gauge problem - short?
---------------------------------------------------------------------------

5) So I am wondering if there is something going on with the gauge itself. Before the fuse would blow, when I turned the ignition switch key on, the amp meter would very briefly swing to 30+ before the fuse would blow. Also, a couple of months ago when I was trying to get the wiper motor to run, I believe a lose ground wire had hit the fuel gauge. I believe this because the fuel gauge generated a spark from from the inside behind the black plate. At the time I figured I "blew" the fuel gauge, but it was working. Maybe this current spike and spark weakened the fuel gauge to cause it to finally fail. Bottom line, is there a way to test the fuel gauge by itself to see if it is drawing excess current?

6) Any other suggestions are much appreciated.

Thanks,

Steve

 

[SteveBones]

I went to the link to check this out. In first pass it looks to be extremely informative. I will read thru in more detail. Looks like it might be time to attempt some fuel gauge surgery.

Thanks!

In case you find it useful, here is a website that describes the MGA fuel gauge in great detail. That MGA gauge is essentially the same as the TR3A gauge.

 

[TR3driver]

Yup, it's a great article. Barney really knows how to nail technical subjects. Just don't forget that the resistances are slightly different for a TR3.

 

[SteveBones]

Hello,

I am still trying to figure out the fuel gauge - blown fuse issue. I believe I am making some progress. Here's the latest:

1) Fuel gauge is back in the car but not working with a new fuse. The turn signals and wipers are working with the ignition switch on and the fuse is not getting blown as it was previously.

2) I used a small 12v battery to test the gauge when it was removed from the car and confirmed it is working. As mentioned previously, with the fuel gauge back in the car and hooked up, the needle does not move.

3) At this point I am thinking the issue might be with the wiring at the fuel tank sender unit so I will focus on this and let you know how it goes. I am hoping that the issue is with a loose connection at the sender unit.

Steve