QuestionI have a 1967 TR4A. Recently I removed and repaired the heater core. After reinstalling the heater box, my tail lights and dash lights would not operate. I double checked the wire connections to the ignition switch, fuses, and rheostat. Initially this procedure resulted in no change, but at some point both started working again, but I don't know what I did to get them to operate. I've also had the dash lights and tail lights go out while driving in heavy rain; water was also leaking on the drives side floor under the dash.
Upon reviewing the wiring diagram, I can find no relationship of the rheostat and the tail lights. I know my rheostat is of dubious condition and wish to replace it. Are the tail lights in any way affected by the rheostat? I wish to replace the rheostat but found they are either not available, or they are extremely expensive. Can a rheostat from another vehicle be installed in TR4A while still utilizing the original knob, if so, can you recommend one?
Thanks,
Gary
AnswerHi Gary,
Electrical problems use to be difficult for most mechanics and impossible for body men. I was included in those mechanics until I took a course from RCA in Radio and TV repair (never finished the training) The first part was basic Electronics which I did finish and found that all of my electrical repairs on cars went very well after that. I can't tell you where your problem is but I can tell you how to pin it down and fix it.
First, symptoms may tell you an area where the problem MAY be but symptoms are for the most part USELESS. Testing is the ONLY way you will find the problem. And the problem MUST exist at the time of testing. I found that when you had several circuits failing you must pick only ONE of them to test and I found that it is best to pick the simplest one. 90+% of the time when you fix that one it will lead you to all of them. EVERY item on the car has a circuit starting at the (-) post of the battery and ending at the (+) post of the battery. I like to start at the "Load" (item that don't work) and test the ground first.
If the ground is good (not looks good)(is a ground) and then I use the wiring diagram to start testing with a 12v test light (or a Logic Probe if you have one). If you have power at the item (key and SW on) then the item is bad. (that is not what you have wrong but the method of testing all electrical problems) Most likely you will have no power to the item so with your diagram in hand start checking with your test light along the circuit toward the switch and power. With this method, you can not fail to find the problem.
If you go to a doctor and say "My arm use to hurt last week but it is fine now" the doctor can run $50,000.00 worth of tests and may or may not find anything. The same is true of a mechanic (except the price) and he can disconnect and test every connection on the car and test every switch for it's connection and disconnection etc, etc. and he may or may not find any problem. So when it fails start testing only one of the failed circuits in the procedure I stated.
A rheostat is just a variable resistor, any rheostat with the close to correct resistance value and WATTAGE rating will work. This is true of most of the older British cars and you may find that some of them even have the similar mounting and knob. It has been 40 + years since I replaced a TR-4 rheostat but I do seem to remember that the TR-3 and the TR-6 had a similar rheostat as did many of the British car of that era. Most had the same electrical values because they all had the same instrument bulbs and close to same number of bulbs.
A short cut to electrical problems on all British cars of that era is to remove and sand the fuse holder tabs in the fuse box as they are a constant problem.
Good luck,
Howard
