Vapor lock in a Jag, electrical issues in a ’Bird and a Bimmer

Dave Ballard writes: I recently noticed that the gas gauge and the temperature gauge in my 1957 Ford Thunderbird do not work. The gas gauge will work temporarily when the car is started, but it soon returns to zero. I replaced the temperature sending unit, but it didn’t help. I cannot find any short. Could it be the voltage reducer?

Since both your gas and temp gauges aren’t working, I’d suspect the voltage reducer or one of its connections is your culprit. Voltage reducers come in a few form factors, but they all should have a 12V input, a 6V output, and a ground connection. To test, use a multimeter and set it to measure DC voltage. Attach the black probe to a good chassis ground and, with the ignition on, use the red probe to check for 12V input to the reducer and 6V output to the gauges. Be sure to verify that the ground connection is sound. If any doubts remain, just replace the reducer.

Jonathan Lentz writes: I have a 1971 Jaguar E-Type six-cylinder that will start and run fine for about 15–20 minutes before it begins to sputter. Upon acceleration, it conks out, and I am unable to start it up for an hour. This problem has been ongoing for years. I once fixed it by replacing an overheating ignition switch, and the Jag ran fine for two years. Now the problem is back. The hotter the car is, the faster the problem will surface. The temp gauge reads normal; could it be vapor lock?

Jonathan, it certainly could be vapor lock—fuel turning into vapor before reaching the carburetors—but it also could be a different fuel delivery problem, or even an ignition issue. The best way to check for vapor lock is to temporarily install a clear fuel hose on the inlet to the carburetors. If, as the car warms up, you see the liquid fuel bubble and vaporize in the hose, you’ve caught it in the act. On the fuel delivery side, it could also be an issue with the SU fuel pump and its internal contact points (like points in a distributor) or a fuel filter that gets clogged but has the blockage relax when it sits for a while. On the ignition side, it could be that your ignition switch has failed again, or that the coil or condenser are failing.

Roger Wagner writes: My sons and I own eight BMWs. One day, when we started up our E36-generation 325iS, the instrument cluster failed—all lights and gauges went out at once. We removed the cluster and sent it off to be rebuilt. Upon return, all was working except the speedometer. It would peg at max speed, and the odometer would register at that speed as soon as you started moving. We sent the unit back, and the specialist said the issue was in the car, not the cluster. We swapped in a cluster from another E36 and all worked as it should. I would think the issue is in the cluster and not the car. Any suggestions?

I believe the odometer and speedometer are fed by the vehicle speed sensor in the differential, so with both of them being pegged high, it sounds to me like a bad sensor. This should, however, happen with whichever cluster is in the car, and you said you tried a different one and the problem went away. Have you tried putting the rebuilt cluster into a different car? If the problem doesn’t reliably follow the cluster or the car, I’d suspect an intermittent connection at the back of the cluster.