Gauge Cluster short 1996 Ranger

[98v70dad]

I've been troubleshooting a blown #25 fuse for a good while now. My gauge cluster has a short in it and I've spent some time looking for it. The MOV in the circuitry near the odometer has continuity across its terminals so I guess thats my short. I don't have a real good wiring diagram so I don't know if the would cause my #25 fuse to blow but the cluster is on fuse 25 so I'm pretty confident I've found the source of my problem. My odometer was working intermittantly before the short showed up. Also a burned smell is coming from the area of the circuit board with the MOV. The MOV is a black disck that sort of looks like a disk type capacitor except that its there to protect the circuit from transients. I don't know much about electronics and had never heard of an MOV until a few hours ago

So, can anyone confirm that a good functional MOV shouldn't have continuity? Also, there is no writing on it so I have no idea how to source a new one (both place to buy one and rating, etc). I would really appreciate any help with that.

 

[RonD]

And did you test if fuse doesn't blow, no short to ground, with C216 connector unplugged, so not a wire short between fuse 25 and the cluster?

Here is the wiring diagram I have for 1996 cluster, but not board level wiring, C216 pin 7 is the speedometer power from fuse 25

Also fuse 25 powers GEM module, seen on second diagram

A 1996-2003 Ranger/Mazda B-series cluster is pretty much plug and play, and they are not too expensive

 

[98v70dad]

And did you test if fuse doesn't blow, no short to ground, with C216 connector unplugged, so not a wire short between fuse 25 and the cluster?

Here is the wiring diagram I have for 1996 cluster, but not board level wiring, C216 pin 7 is the speedometer power from fuse 25

Also fuse 25 powers GEM module, seen on second diagram

A 1996-2003 Ranger/Mazda B-series cluster is pretty much plug and play, and they are not too expensive

Thanks. I appreciate the information and comments.

I get no short to ground with C216 unplugged. I know that the cluster is not that expensive but they aren't that cheap either. I am mainly trying to avoid having to mess with setting the odometer to the right mileage. I know that you can swap the odometers between the old and new clusters but then you have recalibrate the speedo and I'd rather not do that either if I could just solder in a new MOV. Also, a junkyard part means the MOV's in that part might be about to fail. I read that they fail from age and voltage spikes. If a junkyard part ends up being my only good option that's what I'll do - I've put plenty of junkyard parts in old cars. I'm watching a bunch of them on ebay already.

 

[RonD]

OK, had to look up MOV, lol, its a Metal Oxide Varistor

And how to test one seen here: https://ourpastimes.com/test-metal-oxide-varistor-7643295.html

So if ohms are under 100 then replace it, but if its on/between the speedo motors power wires the motor would read under 100ohms I would think, so you would have to unsolder one lead of the MOV to test it

 

[98v70dad]

Thanks for the link. I looked a long time for the test and found nothing that good. I compared what I got from the odo motor MOV with an ohm meter reading to what I got from the fuel circuit board MOV and the two were completely different but the MOVS at least look the same. I only know enough about electronics to be dangerous, but I figured the one I don't seem to be having trouble with acts different than the one I am having trouble with so one of them must be bad.

I've already removed the motor because I thought the worm gear might be eaten up. The gear is fine for now but I might replace it since its a lot of work to get in that deep and according to what I've read its a common failure item. A new worm gear is made from better material and is pretty cheap.

With the motor removed I get 0 ohms across the MOV. Mouser electronic sells MOV's but you need to know the operating voltage (12V?) and clamping voltage (I'm thinking 10% higher maybe) to pick a replacement. Guessing is probably not much better than just jumpering over it though. Guess too low and it will fail frequently. Guess too high and whatever its meant to protect will fail eventually.

 

[RonD]

You could just remove the MOV and then power up the cluster, if it was blowing the fuse then all would be well with it removed

Yes, its 12v and yes 10% would be safe bet, it sends voltage spikes over to ground as far as I can tell, so not used like a capacitor to smooth voltage, but not quite a resistor either

 

[98v70dad]

You could just remove the MOV and then power up the cluster, if it was blowing the fuse then all would be well with it removed

Yes, its 12v and yes 10% would be safe bet, it sends voltage spikes over to ground as far as I can tell, so not used like a capacitor to smooth voltage, but not quite a resistor either

Thanks. You might remember that my alternator failed. Odo working intermittantly started a few weeks later and fuse 25 blowing started a few weeks after that. I suspect that one is probably the result of the other.

I also thought about removing the bad MOV as you suggested as a test - I might do that. Also, I have the one good MOV on the fuel gauge circuit - I'm wondering if there's a way to measure the clamping voltage on that one?

The mouser website lets you filter what they have available. Once you specify the voltage (12V), the application (automotive) and the form factor (disc type) there aren't many left to pick from so I may be lucky and be able to get to the answer that way. I think there was one with the clamping voltage over 13V and under 14V and nothing lower. The circuit board is tight with traces pretty close together, though. I'm going to need a better soldering iron.

 

[98v70dad]

And did you test if fuse doesn't blow, no short to ground, with C216 connector unplugged, so not a wire short between fuse 25 and the cluster?

Here is the wiring diagram I have for 1996 cluster, but not board level wiring, C216 pin 7 is the speedometer power from fuse 25

Also fuse 25 powers GEM module, seen on second diagram

A 1996-2003 Ranger/Mazda B-series cluster is pretty much plug and play, and they are not too expensive

Thanks also, for the wiring diagrams. After looking at them I see that the speed sensor goes directly into the gauge cluster. Is it possible that the speed sensor has a fault that would have killed the MOV in the gauge cluster? If so, how would I check that? I want to avoid blowing the cluster again if something else is causing it.

To get my truck back on the road, I've decided to buy a used gauge cluster and keep the old one to fix later if I can igure out what MOV to buy. MOV's cost less than a dollar but I spent a long time looking yesterday and wasn't able to figure out what I need,

 

[ericbphoto]

I would think the failing alternator probably killed the MOV. As RonD mentioned, removing the MOV is probably your best test. You definitely want a new one installed for long term operation though.

Mouser is a good source. You may also try Allied Electronics or All Electronics or Digikey. ( pulling this from memory)

 

[RonD]

I don't think the MOV is on the speed signal wire, its an ACvolt signal, I think the MOV is on the 12v/ground circuit for the speedometer motor

Speed(and ABS) sensors are variable reluctance sensors that generate their own voltage, they "read" a tone wheel, as a metal tooth on the tone wheel passes close to the VR sensor it generates an AC pulse, these generally stay between +5 to -5 voltsAC
Only the + side of the AC volts are used, so 0 to 5 volt AC, because this has to be a Pulse signal to work a voltage stabilizing circuit wouldn't be used, but I too am ignorant in the use of the MOV

 

[98v70dad]

I don't think the MOV is on the speed signal wire, its an ACvolt signal, I think the MOV is on the 12v/ground circuit for the speedometer motor

Speed(and ABS) sensors are variable reluctance sensors that generate their own voltage, they "read" a tone wheel, as a metal tooth on the tone wheel passes close to the VR sensor it generates an AC pulse, these generally stay between +5 to -5 voltsAC
Only the + side of the AC volts are used, so 0 to 5 volt AC, because this has to be a Pulse signal to work a voltage stabilizing circuit wouldn't be used, but I too am ignorant in the use of the MOV

Thanks. RE: the MOV, me too! You know way more about it than I do, though. I've learned a lot from you.

 

[98v70dad]

I would think the failing alternator probably killed the MOV. As RonD mentioned, removing the MOV is probably your best test. You definitely want a new one installed for long term operation though.

Mouser is a good source. You may also try Allied Electronics or All Electronics or Digikey. ( pulling this from memory)

'

I agree about the failing alternator. Also, I'm planning to remove the MOV when I can clear some work space. My wife just "requested" that I get my crap (disassembled gauge cluster and tools) off the kitchen table. My workbench is cluttered with stuff from two other projects right now.

I was home sick yesterday and spent hours on mouser looking for something that might work and didn't find much. When I get the MOV out maybe there will be some identification on it. I don't see anything written on it but its hard to see. The one on the slosh module MOV which is easier to see doesn't look like there is any info on it, though.

Looking around the internet yesterday I learned that MOV's for auto applications are usually custom made for the application for the auto company. The TDK DC varistor I found on mouser that is rated for 14 volts has a clamping voltage of 37V. I think that's way too high but really have no idea.

I've read that a certain kind of diode can do the same job but I don't know enough anout electronic to pick a substitute.

 

[ericbphoto]

'

I agree about the failing alternator. Also, I'm planning to remove the MOV when I can clear some work space. My wife just "requested" that I get my crap (disassembled gauge cluster and tools) off the kitchen table. My workbench is cluttered with stuff from two other projects right now.

I was home sick yesterday and spent hours on mouser looking for something that might work and didn't find much. When I get the MOV out maybe there will be some identification on it. I don't see anything written on it but its hard to see. The one on the slosh module MOV which is easier to see doesn't look like there is any info on it, though.

Looking around the internet yesterday I learned that MOV's for auto applications are usually custom made for the application for the auto company. The TDK DC varistor I found on mouser that is rated for 14 volts has a clamping voltage of 37V. I think that's way too high but really have no idea.

I've read that a certain kind of diode can do the same job but I don't know enough anout electronic to pick a substitute.

Stick with an mov

 

[98v70dad]

Stick with an mov

Yeah, I came to the same conclusion but good luck finding a suitable MOV replacement, especially if you don't know much about electronics.

 

[98v70dad]

Stick with an mov

I found this link below which is a little helpful if you know some key info (I don't). In order to pick an MOV you need to know the rated voltage, energy dissapation ability, surge current and clamping voltage. Voltage is 12V, so a 14V MOV would be OK. Energy dissapation is related to diameter so I can match the diameter of the failed one. Surge current and clamping voltage are the two I don't have any idea how to estimate.
I'm thinking jumping another car would be the worst normal case for surges. Suggestions?

*** Link below ***
MOV design guide for automotive applications