M38Cdn Refurbishment

[rob love]

You are lucky they work. I found that usually on those gages, the fuel in particular, as well as the temp and oil pressure, would burn out without a ground.
The later M-series added a short ground wire from the panels to the dash which helped prevent this. There are times that you have the dash backed out (cable or gage replacement) that turning on the ignition switch can have bad results.

Same goes for testing gages on the bench. You must provide a ground to the case or the negative stud before applying power and sending variable negative thru the wire.

[Peter Phillips]

Originally Posted by rob love

You are lucky they work. I found that usually on those gages, the fuel in particular, as well as the temp and oil pressure, would burn out without a ground.
The later M-series added a short ground wire from the panels to the dash which helped prevent this. There are times that you have the dash backed out (cable or gage replacement) that turning on the ignition switch can have bad results.

Same goes for testing gages on the bench. You must provide a ground to the case or the negative stud before applying power and sending variable negative thru the wire.

Interesting Rob, never knew that bit of info...thanks . I haven't tried the fuel gauge but the ammeter, oil pressure and temp works fine. Interestingly I don't remember seeing a ground symbol coming from anywhere on the dash....unless I missed that?

[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 sending unit 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.

[Peter Phillips]

Great advice Rob, thanks. I'll install the ground mod as I have some spare Douglas connectors and wiring.

Got the tub stripped down and only broke one footman loop screw and have to drill one stripped screw out. My original sandblaster guy is AWOL now so the hunt is on to find someone that is competent to blast sheet metal

[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.

[Peter Phillips]

Blasting is complete...now it's time to get to work. Not too bad of shape overall, thankfully. I don't like all the braze repairs as I'm going to have to grind them out and replace with welds if they are compromised. I included a picture of 2 x 3/8" holes that were not supposed to be there that I filled in last night, not too bad if I say so myself

Sadly the motor has developed a small oil leak at the rear after running it for an hour or so. I'd hate to have to take it out again if it's the rear main or welsh plug. It did seem to slow down once it got hot. I'll let it go through numerous hot and cold cycles and see where I am prior to putting the tub back on in a month or so.

[rob love]

There were warnings about military vs civilian rear main seals, but as I recall the civilian ones were too tight. So tight in fact, they were causing rear bearing problems and leakage because of the amount of heat. Since then, some dealers have had proper rear main seals made up.
That warning aside, I have had it where the rear main seals take a little bit of time to set in. And they can be replaced while on the vehicle. I used to do them in about an hour, although it almost takes me that long to get onto the floor and back up again these days.

[Peter Phillips]

That's good to know, Rob...I'm really hoping this thing just needs to "run in" to seal up. You cracked me up about taking an hour to get down and up again, lol.

Got the carb kit from Brian today so that's on the list to do soonest.