M38Cdn Refurbishment

[Chris Suslowicz]

Originally Posted by rob love

The gauges take their ground thru the case, or for the later plastic housed gages, thru the clamp and onto the studs on the back of the case.

Many guys test the gages on the bench by putting power to the power terminal and full negative onto the negative terminal. The results are usually quick and not good....you need the case grounded. Cheap insurance for any M-series vehicle is to run a ground wire from one of the screws for the panel lamp (use a longer screw than original so you can get a ring terminal, a couple of internal tooth lock washers, and a nut onto it) and run the ground wire to a screw behind the dash, such as the screws holding the circuit breaker in place. You can put a set of Douglas connectors in between to make it easier to remove the complete panel if that day ever comes, or leave it as a solid wire.

That's a disaster waiting to happen. A lot of vehicle gauges are the "hot wire" type that must have a resistance in series (i.e. the sensor) to limit the current, otherwise the "hot wire" will simply act as a fuse and melt, rendering the meter completely worthless.

Another meter type you cannot do this with is the thermocouple ammeter used in some radio equipment (may be marked "aerial current", "RF amps", "HF ammeter", or similar), where the measuring circuit is a very short piece of resistance wire (so it doesn't waste much of the transmitter power) with a thermocouple spot-welded to the middle of it. The meter movement has no air-damping vane attached (because the thermocouple is also low resistance and provides magnetic damping to the pointer movement), and if you touch a flashlight cell (even a tiny one like an AA) to the terminals, the short-circuit current through the resistance wire will blow it like a fuse.

A dealer did this before my very eyes at the War & Peace show (Beltring) one year, and made contact before I realised and shouted "Stop!"....

"What?"

"You've just destroyed that meter?"

"Nonsense, the needle moved."

"The needle moved _once_, and will never move again because you've blown the wire inside. Try it again and see."

(Dealer tries it, predictably nothing happens.)

"Do you still want it, I'll reduce the price?"

"No, it's beyond repair now." (etc.)

Pity really, it was for the Aerial Coupling Unit 'J' and one of the hard to find plug-in meters.

(I did spend some time explaining why he blew it, I don't know if the lesson stuck.)

This is why I normally carry a pocket multimeter to shows and radio rallies, so I can test them (safely) before purchase.

Chris.
p.s. There exist "Hot wire" RF ammeters as well, and they are just as fragile.