QuestionHi Howard - this is follow up to your help re: ignition \ overheating question. My suspicion on electrical remains - starter sluggish 1 out of 5 times on turn over, car runs strong but bogs on acceleration, will backfire. Temp stays well below middle N on gauge but after 30 min creeps to middle and then throwing out very hot air into cabin. Summer outside temp is only 70F. Cooling system seems OK, and oil cooler is new.
I have run test of alternator ( 13.57V @ 1500 rpm ) and will do voltage drop test next, engine ground strap to body ( location? ) A few questions: when rendomly testing voltage at accesible spade connections( fuse bx, relay, ) I find that some 10 ga wires ( purple, white, but not brown ) read less than 12V ( 10.2V ) with engine running... significant? All electrical items work except turn signal indicator that between flashes still glows dimly. Should I be looking at the coil? ( very hot to touch ) points \timing, or even thermostat failure? Sorry for all the random questions! Paul in Toronto
AnswerNot always, but often when I received a car with several odd problems they all stemmed from a single problem.
Too often symptoms lead me down wrong roads so I keep symptoms off to one side and only use them to test systems or sections rather then parts.
It is time consuming and troublesome to take a ground strap off and clean every surface and strap ends to see if it is a problem. So all you have to do is a "Voltage Drop Test" on the negative cable system. Just put your negative probe of the voltmeter on the negative "Post" (not cable end) at the battery and put the positive probe of the volt meter on a good clean metal part of the engine and set the volt meter on a low volt scale and hit the starter motor and watch the meter. (I like to use an analog meter because a digital meter will flash a bunch of numbers that are not readable unless you have an expensive digital meter that can average it out)
All voltage drop tests must be conducted with a load so your purple wire can be tested the same way. You need to look at your wiring diagram to see where to put the test lead probes. Then turn on what ever that wire operates to have a load on the wire. Keep in mind that an unusual load will make a wire fail a voltage drop test. For example a 14ga wire will normally be used to power a heater fan motor and not be a problem but if the fan motor is shorting out and causing the motor to run slow and you do a voltage drop test on the wire it will probably fail because of the excess load. The same is true of a starter load on a ground cable system. If a starter motor is "dragging" it can put an undue load on both the positive and negative cables so it is a good idea to run a second test with just the head lights and some accessory on without using the starter motor to see if you still have a voltage in the ground strap system. Don't for get that the battery to car ground cable and ends are in that ground test.
I don't remember where the 74 ground strap was but most MGs had it on the bell housing to frame or over the front motor mount.
The voltage with the head lights and maybe a fan motor on should be below one volt. I usually see from .2v to .5v and if there is a problem you will see well over one volt. A starter motor test may show more but not much more.
Some mechanics do a simple test and that is to clean the negative battery cable end and post and then run the voltage drop test and then connect a jumper cable from the negative post to the engine block and run the test again and if they show a large difference between the two tests they start cleaning cable ends and connections.
All the years I spent in dealerships I found most electrical problems on all British cars were connections. I would guess that nine out of ten electrical problems were connections. And most of them were the fuse to fuse box connections and next were all the round bullet connectors. Switches were next. All of this was blamed on the "Prince of Darkness, Lucas"
I worked on many different brands of cars other then British cars and in defense of Lucas I can say that Lucas was very good with consistent color codes and even though the switches failed often most could be taken apart and fixed. To appreciate Lucas all you need to do is work on a car with Magnetti Morelli wiring and you will love Lucas.
let me know how you do on it.
Howard
