So I know I have a short somewhere. But it’s intermittent. Sometimes my tach works, sometimes it doesn’t. But it ALWAYS works when my foot is on the brake, same with the horn, and apparently the radiator fans. I overheated in my garage because my fans didn’t kick on and would like to avoid this in the future. I know the answer is “find the short” but I figured I would ask if anyone has had this particular issue and knows what may cause it off hand. Why would it work sometimes but not others? Why is it always willing to work when my foot is on the brake? I have checked the fuses. They appear intact

 

If it is short, you should be blowing fuses. This sounds like bad collective ground to me, and when you hit the brakes, somehow, the ground problem corrects, suggesting that the braking action is completing the ground. Look for common ground across those circuits or the primary ground in the engine bay to ensure it is appropriately attached. Also, look for a loose wire near the brake pedal assembly. Good luck!

 

I agree, likely a bad ground somewhere. Beside chasing the grounds around the engine be sure to check in the area behind the instrument panel as there are some ground junctions in there too.

 

You don't have a bad ground, each of these things is grounded to a different point, so don't get lost in that rabbit hole. And if you had shorts, you would also mention blowing fuses. What you do have is a problem with your fuse panel. ALL of the circuits you mention come off fuses 6 or 7. Although you AREN"T blowing fuses, one possibility is that these fuses are going non-conductive, which is common enough with the crappy repro Lucas fuses that people insist on using. The end caps lose contact with the fuse strip, and all sorts of intermittents result. Another possibility is that the fuse clips have surface corrosion or have lost some "spring" due to metal fatigue. It's possible that the incoming wiring has corroded connectors. Finally, it's possible that the real culprit is your ignition switch, since all of these are the switched circuits coming off lug 2 of the switch. OTH, you didn't say the car is stalling, as it would be if lug 2 wasn't supplying power.

 

I don't think we disagree on the fuse panel, but how do you explain the problem correcting when he brakes? The brakes would have nothing to do with non-conductive fuses; that would just result in a sustained failure, but he has an intermittent failure (or intermittent success) when the brakes are used, so I don't think we should ignore the brake link. And you are right; it could also be on the power side because cutting the power would have a similar effect (these cars were hand-built and often modified over time, so they all aren't always wired identically. I pulled the entire fuse panel on mine and upgraded it to modern fuses, for instance (I started working on Jaguars in the '70s and just grew to hate looking for intermittent electrical issues). You might want to start out determining if, when the instruments aren't working, whether their fuses have power, if the fuses have power, then (you shouldn't need to check the guages because they are pulling from two fuses not one, but your car could be miswired, and the check is easy, so I'd do it regardless. If there is power at the fuses and to the instruments, it is likely a ground; if they don't, then the power side has been compromised. (These things are kind of hard to diagnose without having the car. You could also just wire a ground to one of the gauges not working (multiple grounds on the same circuit aren't a problem) and see if the gauge comes on that'll also confirm whether you have a ground problem, and all you'd need to do to get them working is make a permanent redundant ground (or if you want it to be stock, you'd have to trace down the disconnected ground(s).

 

A ground problem is unlikely. Each of the failing components grounds to a different point, and the brake lights ground to the worst ground point in the car, behind the tail lights. How do I explain tach working with the brake lights? I don't. It's serendipity or stupendously munged wiring. The car isn't cutting out, so the ignition switch must be in some sort of functional shape. So that leaves the fuses, the fuse holder itself, and associated wiring.

So let's concentrate on 6&7. What do we know? The white wire coming from lug 2 of the ignition switch must be powered, because the ignition coil, fuel pump and charging system haven't been mentioned as problem areas. That's about the whole of the unprotected side of those fuses. On the protected side, we find:
Fuse 6: Horn relay winding, wipers, fans, heater blower, stop lights, windscreen washer
Fuse 7: instruments, wiper motor, reverse light, choke warning light, tachometer

Which seems to cover just about all his intermittents, and a couple that may not have been tried. The fuses (particularly 6), holders and connectors are the prime suspects.

 

My E has been relatively elec issue free, considering his age. However, I had a 62 MGA that would exhibit similar behaviour. Checked connections, nothing conclusive. Feeling very discouraged I was looking very dejected and my husband asked me why.. After explaining, he commented... "well, a socket rolled under the car and I noticed a huge gash in the battery cable. MGAs have their battery behind the seat and a very long cable up to the front. I looked and it was a HUGE gash... all green and nasty. I took it out and had a new one built at the auto elec shop. The flaky instrument behaviour, starter, etc. all straightened out. Sometimes just unplugging and the replugging in a connection is all it takes. On my other BCs with less complicated fuse boxes, I just bought new ones and put them in, immediate improvement.. brighter lights, etc. On the TR4a someone just twisted wires together and then taped them. I took those all apart, cleaned and soldered them, shrink wrapped them, that too corrected some issues.

 

Just to add my 2 cents, I believe mxfrank has zeroed in on the problem and given a succinct explanation. As far as the brake light switch, here is a theory that does not stretch credulity too much. The 2 filaments in the stop light are powered by different fuses, 6 and 3 I think. An intermittent short between them (easy to occur inside the socket) would power the circuit 6 (and 7) components whenever the brake pedal was pushed and the side lights were turned on. This may not be the actual cause but illustrates the kind of thing you might be up against.