M1008 voltmeter

I've been beating my head against the wall trying to get my M1008 voltmeter to work. I took it to work and attached it to a power supply and it seemed like it was working, but when I attached it to the truck it didn't. So I took it off and traced all the wires around and stuff... well, turns out there's always 25-30 volts at the volt meter connector. So I hooked up the volt meter and it worked... I put the dash all back together and then the next day it stopped working again.



So at that point I was totally confused, but figured the volt meter was intermittent. I saw one on E-bay and bought that, but it doesn't work either, even though there is always voltage on the connector.



On the back of the volt meter there are 3 lugs, the wires are connected to two of them. Tonight I discovered that if I connect 24V across ground and the unused 3rd lug that it goes off the scale +voltage. I probed around some more and also see that there is about 175 ohms to ground from that connector.



I wonder if this meter is really for some other voltage range (12 volts possibly), but they divide the 28 volt voltage down to get into that range. There is a white thing between the the 2 possible lugs, I wonder if it's a resistor to divide the voltage and it's burned out?



That's my long story. My question is, does any know how these things are supposed to work? Or, if someone has a functioning CUCV voltmeter handy, would you mind measuring the resistance between the two lugs?

Well, in case anyone else has this problem some day, here's what I did. I saw infinite resistance between the two non-ground lugs. I think there should be some there, as I explained previously. I went and got some 1 Watt, 100 ohm resistors and put 3 of them in series between these two lugs. No need to use 3 100's instead of one 300, but that's all they had at radio shack. Anyway, with 300 ohms between the lugs, it seems to work as I'd expect. With the key turned on, but the engine off, it goes into the yellow or just to the line at the bottom of the green. With the engine running and one alternator it goes about to the middle of the green and with both it goes to the top of the green. I tried different amounts of resistance, and this seemed best.


I've only had it going for a little while, so I'll report back if it breaks or causes the truck to catch on fire or something.

The meter actually measures current, the resistors mentioned in another post are called current "limiting" or a "balast resistor". The ones I've looked at have a a balast resistor between the second and third lug and adjust the current available to the meter so that around 26 volts the meter should read in the high green.

Milspec

Fort Washington, Md

Oh, okay. In any case , it seems that a failure mode of these meters is for the contact to go away between the lugs and the resistor. I cleaned up one and it seemed to start working. Another I had to replace with new resitors. 350 ohms seems to be the right value.

I am reposting this from another thread...
RE: resistor fix.
I just went to Radio Shack. I showed the guy the picture of ^ that green resistor and they didn't have one that big. They had no 300 Ohm resistors. What they did have was a 5 pack or 150 Ohm 1/2 watt resistors for 99 cents. Well I figured I could waste 99cents on an experiment. I twisted 2, 150ohm resistors in line and hooked them up.... Guess what happened????
It worked! I now have a working volt meter.

Will the batteries still charge even if the gauge stops working. I don't want to have to pull out the dash just to fix the gauge

Yes they will. Do both GEN 1 and GEN 2 light when the switch is on? If so you should be good. Did you check the 10 amp fuse on the bottom of the fuse block?

Yep, both Gen lights come on and go off when above idle, the 10 amp fuse is good and both alts are magnetized at the back when running

My meter quit working and I replaced it with an ISSPRO gauge. I like this better than the military version as it is 24v and has number to show what your running instead of just red, yellow, and green. This is a direct replacement used my factory light looks better works better. I think this is a must do upgrade if you use the truck often.

Okie Cowboy. What do you know I finally gave someone good advise on this website. I have been changing the gauges out when they are inoperable. The actual numbers on the gauge are a real plus. And yes they do look good as direct replacements. I am glad everything worked out for you. I was once in the same situation and all it was was the gauge. I don't even bother trying to repair the gauge. I just toss the defective gauge in the steel dumpster and install the ISSPRO. Thanks for the update.

Got a part number on that gauge? I do like the idea of having the actual voltage readings. The "red yellow green" is only a little bit better than an idiot light, and there's a reason they call them idiot lights!

Is it the same reason they don't put numbers on the OEM voltmeter?
Classic 20-32 Volt DC Voltmeter Gauge - R8661

And while you have the dash out to replace the voltmeter why not add the tachometer.Same size as the voltmeter and fits nicely next to it. 2 1/16" hole and easy to hook up to the tach wire on the alternator.

you say this tach hooks up to alt. directly. it does not need a device to decode the Alt. signal like the Dakota digital DS-1 interface I had to buy for mine?

I will get more information on this hookup and post it at another time. I don't want to get in trouble hacking into this thread with Tachometer installation instructions. Besides someone else on this site knows more about tachometer installation then I do and I don't want to be corrected for simple errors. If you buy the 2" VDO 4,ooo rpm diesel tachometer the instructions will come with it. I did use the alternator wire and then had to calibrate the tachometer to the correct engine RPM's. It was easy. I did it long ago 5+ years and never bought anything but the tachometer kit.

That looks like a sweet pair. Do you happen to have the model number for the tach? I've been searching the ISSPro site and can't find it.

I found the box my tachometer came in. It is a VDO 333-965 Tachometer , alternator,4000 RPM 12 volt Made in Switzerland. I also have the instructions. In short. Use of the VDO tachometer with an alternator or other type of ignition that provides a signal in pulses per revolution may require calibration of the pointer. You will need an accurate rpm reader to adjust the potentiometer on the side of the gauge. The instructions are very clear. I hope that helps. And it does complement and fill the empty space to the left of the volt meter. Also remember the ICE diagnostic bracket at your right knee area is the perfect size for any other 2 1/16" gauge you may want to mount. At least it has some use if nothing else. Good luck and keep the CUCV's rolling. Big trip for today.