Bye Bye Idiot Light, Hello Voltmeter

[Blue Chips 284]

This was a simple idea, which, as usual, I made more complicated. 

The battery indicator light bulb (idiot light) on my GT18 was missing, and after installing a bulb, I found that the battery light was always ON, even though the charging system checked out perfectly. A quick check led me to a faulty battery light module, which contains a voltage comparator circuit. The failed module is probably why a previous owner removed the bulb. Here's the culprit:

 

 

A new OEM battery light module was WAY too expensive. I saw some generic aftermarket modules for less cash, but instead of staying with the old idiot-light system, I decided to have some fun and replace it with a voltmeter. The voltmeter would have to fit in the little plastic trapezoidal cavity that was the former home of the battery light, so I poked around a bit and found some miniature, low-cost, self-powered, two-wire voltmeters. There are tons of these on eBay and elsewhere. Be careful to check the specs, as there are a lot of variants.

 

 

The ones that were already potted (above right) wouldn't fit unless I modified the dashboard, due to the wires coming out of the side, and I'm trying not to be too 'invasive' as to any upgrades, so I opted to use one of the other voltmeters and pot it myself. One of those would easily fit into the battery light cavity if I clipped the two mounting ears off the circuit board. I did a preliminary potting step by applying a dab of non-corrosive, non-conductive silicone onto the little circuit board, being careful to squish the silicone to fill in any voids. I didn't think it was worth setting up a vacuum-impregnating rig.

 

 

The next step would be to make a cast around the voltmeter in the battery light cavity, using some two-part urethane rubber casting resin that had been sitting around on a shelf for a while, and I wanted to start using it before it went past its use-by date.

 

However, before making a cast around the voltmeter, I decided to make a test casting. I applied a coat of PVA mold release on the inside of the battery light cavity.

 

 

I then mixed the urethane rubber resin and poured it in. After it cured, the test piece came out fairly easily and looked good.

 

 

The next step would be to do the actual casting around the voltmeter (see my next post).

 

Edited 21 hours ago by Blue Chips

[Blue Chips 284]

I recoated the inside of the cavity with mold release, sealed the hole around the wires, suspended the voltmeter in place with some tape, and poured the resin.

 

I removed the casting from the cavity and washed off the mold release. I was pretty happy with the way it turned out.

 

 

 

I removed the old battery icon from the dashboard decal by glass bead blasting to remove the translucent red and a bit of acetone to remove the black battery image. I tried blasting a test piece from an old dashboard decal first, and was pleased that it didn't warp the plastic, which is almost paper thin.

 

 

I painted the newly-cleared area black, cut out a small rectangle to match the voltmeter, and attached a small piece of clear plastic sheet behind the cutout. The reason for the cutout is that the decal sheet has some significant surface texture on the front surface (NOT from the bead blasting) that produced a fuzzy/halo effect around the numbers, and the cutout eliminated that problem. I then mounted the light assembly and decal sheet on the tractor and installed the dashboard 'lens' over the whole thing.

 

The wiring was easy, as it only involved rerouting the wires that were previously connected to the old battery light module. Just some cuts, crimps, heat shrink, and a couple of cable ties.

 

It works great and allows me to check the voltage at a glance. When I turned on the ignition without starting the engine, it read 12.3 volts, which was accurate and in the proper voltage range. Here you see it with the engine running and charging, and it's showing what I consider to be within the proper charging voltage range.

 

 

 

 

 

Edited yesterday at 02:06 PM by Blue Chips

[pfrederi 19,084]

Good Luck.  I bought a set of 3 of the cheap voltmeters just like yours from amazon to use on my big tractors.  They last a few weeks all three failed 

Edited 23 hours ago by pfrederi

[Blue Chips 284]

18 minutes ago, pfrederi said:

Good Luck.  I bought a set of 3 of the cheap voltmeters just like yours from amazon to use on my big tractors.  They last a few weeks all three failed 

 

We'll see how it goes. A friend who has an old Dodge M37 like mine installed one a few years ago, and it's still in operating order. I'm guessing that potting it should help a great deal, since the circuit board and its components look quite delicate to me, and I think it could be subject to failure from vibration. The non-corrosive/non-conductive waterproof potting should also protect it from corrosion-related failures.

 

 

Edited 23 hours ago by Blue Chips

[ri702bill 10,282]

And the potting helps to seal out moisture too... Excellent alternative choice, excellent results. And a full scope post too.

[kpinnc 15,953]

8 hours ago, Blue Chips said:

works great

 

Looks great as well. 

 

8 hours ago, Blue Chips said:

We'll see how it goes.

 

I've had mixed luck with the digital volt meters, but I agree- they work for years or quit in a couple weeks. The round ones I've been using seem to hold up well for a long time. 

 

 

 

[Blue Chips 284]

3 hours ago, kpinnc said:

I've had mixed luck with the digital volt meters, but I agree- they work for years or quit in a couple weeks. The round ones I've been using seem to hold up well for a long time. 

 

The components, traces, and IC legs on the little voltmeter circuit board are so close together that even the slightest bit of corrosion (or wetness, especially with any ions in solution, such as from salt) might cause a failure. And vibration could break the leads if they're not secured. I think that potting or otherwise sealing or isolating the circuit from the elements is very important for it to survive in an outdoor setting. The PC boards in the round instruments are probably sealed or potted somehow.

 

 

 

 

Edited 11 hours ago by Blue Chips

[Handy Don 14,884]

4 hours ago, kpinnc said:

I've had mixed luck with the digital volt meters, but I agree- they work for years or quit in a couple weeks. The round ones I've been using seem to hold up well for a long time. 

I went with a marine model round digital voltmeter thinking it would have better sealing against dust and maybe handle vibration. Three years and still going. 

[Blue Chips 284]

Bending the lead wires a few times while handling one of the un-potted voltmeters was enough to break off the red lead wire at its connection point, where solder in the copper strands creates a stress riser.

 

 

To protect against this kind of damage, I potted the voltmeter in such a way that the leads are well secured against bending anywhere near the solder joint on the board.