Ever wanted to keep better tabs on your Advance Design truck's electrical system but felt limited by what the stock ammeter tells you? Get the goods on what all those rascally electrons are up to by converting to a voltmeter! Doing your own conversion is possible, even for the amateur armature sleuth. And you don't need a court order! Get your gumshoes on and follow Mike as he walks you through ...

Swapping your Ammeter for a Voltmeter in the Advance Design Gauge Cluster ... for $22!
By Mike Bakken
1951 Chevy 3600 Rust Bucket
Bolter 32971

More discussion about this topic in the Electrical Bay Forum

<< Click on any image in the text for a much larger view >>

July 1, 2013

I wrote this Tech Tip in two sections. The first part is why you should consider replacing your ammeter with a voltmeter. The second part is how I actually made the swap. If you are replacing a generator with an alternator in your Stovebolt, you should seriously consider replacing your ammeter with a voltmeter. Those retaining their 6-volt generator can skip the idea.

A big thanks to fellow Stovebolter Gus (DADS50) for the idea of fitting a voltmeter into the gauge cluster. He completed the conversion following a draft version of this article and provided me some feedback, and a photo to add some clarification. Gus's test drive should make you feel a little better about taking on this swap. Thanks Gus!

Why Replace an Ammeter with a Voltmeter?

No doubt this will start a food fight on this website, but a voltmeter will tell most people more about their electrical system than an ammeter. Ammeters also present a certain danger, all of which I will attempt to explain herein.

The ammeter displays flow, or the volume of electricity moving through the meter. Therefore, for an ammeter to function properly all loads (except the starter) must pass through the ammeter as a series circuit. Viewing the shop manual for my 1951 3600, the maximum battery charge is 20 amps while the generator is capable of up to 30 amps load. Certainly General Motors designed the ammeter to handle a greater load but for only short (transitory) periods -- my guess is another 6 amps for a few seconds.

Replacing a 30-amp (or so) generator with a typical alternator of 70 to 100 (or more) amps will exceed what the AD cluster ammeter (and wiring) can handle. On Stovebolt.com folks have posted methods to retain the stock ammeter by adding one or more electrical shunts across the ammeter poles.

First, selecting the value of the shunt takes some time and knowledge. Second, selection of the proper type of shunt as well as the shunt value is critical. Lastly, purchasing a high quality shunt is paramount. The shunt provides a low resistance bypass around the ammeter. Resistors cause resistance, which is electrical friction. Friction causes heat. So you are installing electrical heater(s) behind your dash that does not help your cluster and creates a fire hazard. Additionally, when a low quality resistor fails, your stock ammeter will be fried by the high flow.

Your high amp alternator requires heavy gauge wire to handle high loads. Remember, all the loads except the starter must pass through the ammeter (and shunts). This means running heavy gauge wire and heavy gauge connectors from your alternator, to the ammeter, to the battery. Anywhere along this path, a short or ground can be catastrophic. You just made an arc welder!

Typically, this path is not circuit protected (fused) so the power keeps on flowing until you disconnect the battery and turn off the engine. Understand why you are always told to disconnect the battery before working on your vehicle? Forgetting to disconnect the battery when working behind the dash even for just a bulb change may result in a burnt wrench, screwdriver, finger, truck, garage, and ego.

The ammeter in a vehicle is supposed to tell you the charge or drain to the battery. Indications can be deceiving. Consider a high rate of charge after starting your Stovebolt, which is normal for a short period of time. A lot of charge makes some folks euphoric, but high rates of flow for a long period can be an indication of a bad battery that will later be recognized while untangling your jumper cables.

Automotive manufacturers switched to voltmeters about the time they switched to alternators (a clue). A voltmeter does not measure flow, so it does not require heavy gauge wire to be routed to and from it (parallel circuit). Before the alternator is operating, the voltmeter indicates battery condition or health, which should be (about) 12 volts. A voltmeter will later display the normality of the alternator / regulator by indicating (about) 14 volts.

Okay! That was the first part.

The Swap

Fig 1  
SunPro Style Whiteface Voltmeter from AutoZone  

My first step was trying to find a direct replacement that looked similar to the OEM ammeter. I could not find any at the usual old truck suppliers. Some companies that provide new high-tech gauges have services that modify existing gauges or build a custom gauge to your specifications. I asked for a quote for converting the stock ammeter to a voltmeter. The reply was essentially, "Send us the gauge and then we will give you a quote." Once pressed for an estimate, I was told $400 to $600 -- not for the cluster, just the single gauge module!

I was then inspired by fellow Stovebolter DADS50 who took an off-the-shelf voltmeter and modified the cluster case to make it fit.

I visited a few auto parts stores trying to find something with a similar sweep (90 degrees) as well as thin enough to avoid extensive case modification. I settled on a SunPro Style Line 2" Whiteface Voltmeter from AutoZone, part number CP8205 costing $18 (winter 2013 price) (Fig 1).

Fig 2   Fig 3   Fig 3a   Fig 4
To open the sealed meter case, I ground the edge of the chrome ring with a bench grinder then separated the edges with a prying tool.   Test fitting the voltmeter poles in the cluster case revealed only a little grinding was necessary to allow the pole insulators to fit through the existing ammeter holes.   The gauge-mounting bracket can be used to mark how much case grinding is necessary to fit the new meter poles and insulators.
Thanks to DADS50 for the photo.
  Test fitting the cluster mask revealed the voltmeter was too thick as evidenced by the gap between the edges of the mask and the case.
Fig 5   Fig 6

Using a Dremel tool, the pole insulators' length was reduced. Notice there is a "step" in the insulators -- the narrow section ensures the poles do not contact the case.

First, I reduced much of the large diameter of the insulator to the same as the small diameter. Then I removed about 1/8" of the insulator near the poles. Notice a third center post that needs to be reduced in length as well. Figures 5 & 6 are before and after (respectively) images for comparison.

Fig 7 On the front of the voltmeter movement, above the pointer axis, is a black plastic tongue that also had to be reduced in length (Fig. 7).
Fig 8 With the instrument mask placed over the gauge modules, I could see the edge gap no longer existed, indicating the voltmeter movement is no longer causing interference (Fig. 8).
Fig 9 Placing the voltmeter movement in the case revealed excessive insulation that can be seen protruding beyond the case (Fig. 9). This condition would prevent the nuts from securing the voltmeter movement to the case. So, I did some more grinding of the insulation with the Dremel tool.
Fig 10 Now I started to make the voltmeter appear to be original equipment. I removed the face from the original ammeter by grinding the two rivets on the backside of the ammeter
movement (Fig. 10 -- the right rivet is ground). Once the rivets were ground flush, a pin punch was used to push the rivets out and the face removed.
Fig 11 The original ammeter face has two "bumps" that were eliminated using an anvil and a few light taps of a hammer. I removed the voltmeter face after unscrewing two tiny (easy-to-lose) screws. Then the opening of the ammeter face was enlarged to fit around the voltmeter coil, and allowing the free movement of the needle again, using the Dremel (Fig. 11).
Fig 12 Figure 12 shows how I marked the original ammeter face for drilling of the screw holes, ensuring alignment with the voltmeter. I stacked upon the voltmeter movement first the ammeter face then the voltmeter face. This allowed me to use the voltmeter face as a template to mark the ammeter face for the (tiny) screws. Once removed, I drilled the two holes in the ammeter face with 5/64" drill bit and a wooden backer board to prevent bending the sheet metal face.
Fig 13 A test fit was completed almost successfully (Fig. 13), very little torque is required on these screws -- I split open one of the threaded plastic bosses.
Fig 14

I repaired the split boss by gently holding open the crack, inserting a little Super glue, then clamping the boss with an alligator clip (Fig. 14).

I learned when securing the screws don't feel them seat but watch them seat on the meter face and then gently check the face security.

Fig 15  

Using a computer illustration program, I completed the artwork to print a decal on the ammeter face. At the cost of $4, Fed Ex Office color printed the file then laminated the page with "Label Lam" that provides a peel and stick backing. I cutout a decal and applied it to the (cleaned) original ammeter face (Fig. 15) A PDF file of the decals is available here for you to use

A jpeg file is available here should the colors not match. You may tweak the colors with Photoshop or other software.

Color matching of the face, numbers and scale markings was difficult. Ends up different vendors are using slightly different colors in making replacement decals and gauges. Also, different years used slightly different colors (80 mph speedometer versus 90 mph). Different voltmeters have different scales so I provided a variety of colors and scales on the files.

The laminate is glossier than the faces of replacement gauge and decals but it seems the color differences and glossiness is reduced when viewing through the glass. The cluster mask also provides some contrast between each gauge module further reducing the
ability to discern color differences from gauge to gauge.

Fig 16  

Before installing the decaled face, I painted the orange pointer white. This required gentle sanding of the needle, then cleaning. I masked the needle to protect the meter movement. The mask is just paper with a slot cut in it (Fig. 16).

I first applied (spray) plastic adhesion promoter, then white paint. The pointer is slightly tapered while the stock pointers are not. This was a difference I decided I could accept.

Fig 17   The meter movement was mounted in the cluster case making sure the pole insulators were seated in the case holes and the (orange) paper insulator was placed on the poles before the flat washers and nuts (Fig. 17). I checked the security of the meter movement to ensure insulators were properly machined.
Fig 18  

Before completing the assembly of the cluster and definitely before installing in my truck, I completed a couple tests to check my work.

First, I ensured the poles were not grounded to the case. I used an ohmmeter but a continuity tester could have been used as well. Of course, both poles had to be checked individually to the cluster case (Fig. 18).

Fig 19  

The second test was to check the proper operation of the voltmeter. I used a shop power supply set to 12 volts. You can just as easily use your car, truck, motorcycle, or riding lawn mower. Just use two jumper leads from the battery to the poles (Fig. 19). Make sure you connect the positive battery terminal to the pole marked "+" and the negative battery terminal to the "-".

If the meter indicates less than the lowest position on the meter, you have the poles marked backwards (reverse the orange insulating paper). A properly charged battery will be slightly more than 12 volts. Once the alternator is operating, the indication should be about 14 volts.

There, I just told you how to interpret the indications to ensure your battery and charging system is operating properly.

Fig 20  

Once the testing was complete, I installed the clean glass, a new gasket and new chrome ring.

Our fellow Stovebolter Gus asked, "Does the glossy decal create glare?" To answer I installed two-12v light bulbs (#57) in the stock light sockets then connected them to my power supply and switched off the shop lights. The results look fine (Fig 20).

A nice cluster with the ammeter replaced by a voltmeter!


Wiring your voltmeter is painfully simple. You can use as small as 20 gauge wire as the meter has very little flow. Such a small gauge may be more delicate than you would like. So you can use 18 gauge that may be more common in your shop. Connect the (tested) positive pole to ANY convenient source that is powered with the key on. Then connect the negative pole to a good ground. It is that simple.



Insanity is doing the same thing in the same way and expecting a different outcome.
~ Chinese Proverb

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