Tachometer? How does it do the things it does?

[DzienManche]

I know I probably have a problem with my decameter, but I’m trying to diagnose and see if it may be an electrical issue instead. When I start my truck, the needle goes all the way and maxes out and stays there until I turn the truck off. Which makes me think there may be a Short or something weird going on electrically. All of my other gauges work but I’m trying to find a diagram for the connection in my 86 for the instrument cluster. Any help is greatly appreciated! I wish I could find a drop-in tachometer that isn’t an upgraded one. I have a newer cluster, but the tachometer has a bunch more different connections and stuff, mine does not have an adjustable screw for the potentiometer. That would make life so much easier. 

[eaglescout526]

Wait. You have an 86. Full cluster package. Is it the blue faces or the later type II that is black? If it’s blue and stock to your MJ, there is not an adjustable pot and one of the resistors  or capacitor on the board behind the tach has failed. 
Now the type II early 87 full cluster(blue face) and later type II have an adjustable pot. 
HO, I have no idea if it does or doesn’t have an adjustable pot. 

[DzienManche]

12 minutes ago, eaglescout526 said:

Wait. You have an 86. Full cluster package. Is it the blue faces or the later type II that is black? If it’s blue and stock to your MJ, there is not an adjustable pot and one of the resistors  or capacitor on the board behind the tach has failed. 
Now the type II early 87 full cluster(blue face) and later type II have an adjustable pot. 
HO, I have no idea if it does or doesn’t have an adjustable pot. 

Yeah it’s non adjustable. But if it’s as simple as just needing to resolder something I can handle that

[eaglescout526]

7 minutes ago, DzienManche said:

 

This one is the one where resistors fail, I wish I knew how to fix these and or make them 4cyl tachs but I do not.

 

7 minutes ago, DzienManche said:

 

This one does have the adjustable pot on top. won't fit in your old cluster housing but swapping to the newer cluster is just a simple matter of changing out the speedo cable to 87-90.

[fiatslug87]

Adjustment screw

[DzienManche]

3 minutes ago, eaglescout526 said:

 

This one is the one where resistors fail, I wish I knew how to fix these and or make them 4cyl tachs but I do not.

 

 

This one does have the adjustable pot on top. won't fit in your old cluster housing but swapping to the newer cluster is just a simple matter of changing out the speedo cable to 87-90.

Yeah, I tried installing that newer cluster into mine everything else fits. It’s just the gas gauge is broken and I think a couple other gauges have issues when trying to use that one.

[DzienManche]

Just now, fiatslug87 said:

Adjustment screw

Yeah, sorry I included a picture of the tach from the newer cluster that I have that has other gauge issues. My stock one everything works perfectly fine except the tach

[DzienManche]

3 minutes ago, DzienManche said:

Yeah, I tried installing that newer cluster into mine everything else fits. It’s just the gas gauge is broken and I think a couple other gauges have issues when trying to use that one.

If I knew what kind of resistor I needed I may be able to just solder it on no problem. Lol

[eaglescout526]

Luckily all the gauges are easy to fix, someone could have over tightened one of the mounting screws and broke the solder and copper wire. I don't know much about the gas gauge but if it functions like the big one, it could have the same issue as above. Thats the down fall between the type II and type I is that the older one just have the prongs that slide into their spot and the newer one has room for more issues if someone who doesnt know what theyre touching could break. As for the type I tach, just find the resistor and replace it. Would be nice to have an adjustable pot on that style too.

 

I vaguely recall sending someone a couple type I tachs to have them fixed/modified with a pot. I don't know who that was or what happened to them.

[DzienManche]

7 minutes ago, eaglescout526 said:

Luckily all the gauges are easy to fix, someone could have over tightened one of the mounting screws and broke the solder and copper wire. I don't know much about the gas gauge but if it functions like the big one, it could have the same issue as above. Thats the down fall between the type II and type I is that the older one just have the prongs that slide into their spot and the newer one has room for more issues if someone who doesnt know what theyre touching could break. As for the type I tach, just find the resistor and replace it. Would be nice to have an adjustable pot on that style too.

 

I vaguely recall sending someone a couple type I tachs to have them fixed/modified with a pot. I don't know who that was or what happened to them.

Yeah I think tomorrow I’ll get to micro center and see if I can’t source one. Assuming it’s on the board that the screw posts are sticking out of on the type 1 pic above? I hate that it’s all welded and sealed, but the spring should be ok because it still moves when current it passed.

[eaglescout526]

Yes. The type I is the one with the screw posts. The nuts hold the board to the back of the tach and do come off to make servicing the board very easy. 

[JYau]

On 10/27/2023 at 12:46 AM, eaglescout526 said:

 

This one is the one where resistors fail, I wish I knew how to fix these and or make them 4cyl tachs but I do not.

 

 

Is there something different about the tachs for 4 cylinder engines? I threw one of these clusters that had the tach in mine that is a 2.5 and seems to work fine.

[eaglescout526]

6 minutes ago, JYau said:

 

Is there something different about the tachs for 4 cylinder engines? I threw one of these clusters that had the tach in mine that is a 2.5 and seems to work fine.

Yes. 84-86 clusters had two different boards for the tach. One for the 4cyl and one for the V6. Granted the 6 should work fine with a 4.0L which are more common than a 2.5L 84-86 cluster. I don't know what the difference is and I would suspect a resistor or two but I am awful with electronics. 

[JYau]

17 hours ago, eaglescout526 said:

Yes. 84-86 clusters had two different boards for the tach. One for the 4cyl and one for the V6. Granted the 6 should work fine with a 4.0L which are more common than a 2.5L 84-86 cluster. I don't know what the difference is and I would suspect a resistor or two but I am awful with electronics. 

Ah I see, I forgot that the 84-86 years were different. The cluster I have is from an 88 and it works fine. 

[zomeizter]

I bought a full cluster from a member here not knowing it was off a 2.5L truck. Once installed, the RPM were high and since there is no easy adjustability with that tach, I bought a converter to make it read correctly.