I have a 91 z28 that has the common pegged tach at startup and running. The sticky says add a resistor across pins 4 and 10 of the resistor network on tach board.

I did that and it appears tach works now, but speedometer board started smoking and i never touched anything on that side.

Took it out and it appears resistor overheated and burnt up. See the big one thats now blackened.


Looking at the color bands where it was unburnt, it appears to be a 3.9 ohm resistor. I bought some resistor and replaced it. Plug in car and test, resistors get hot immediately when running and saw a hint of smoke again so i assumed board is toast.

My question is what happened/what caused this? Printed board in back of cluster appears to be fine and all other gauges worked. Tach board isnt overheating.

Can i buy a junk cluster and swap speedo boards? If i do that i fear i may burn that up. Can any cluster be used to swap boards or does it have to be matching my speedo which is 110mph?

 

It doesn't necessarily matter if the speedo (daughter) board is on the different side of the cluster. What you have is more of a symptom of a failure and not necessarily the cause of it.

As long as the circuit boards aren't multi-layered you should be able to visually follow the traces/wiring back through to find the other components involved. If you have a multi-meter you can more easily follow as well as measure the short at various points.

You can use the beeper mode with one end on either side of the resistor legs to confirm and/or follow the path to other components. You can either try to visually confirm the condition of components as you reach them or check them as mentioned below.

You can set the meter to resistance to measure the ohms to ground to compare with documented measurements or another known good cluster. You can also generally interpret very low resistance to ground (or positive rail if known) as a short as well. Both of the methods of measurement mentioned are done with the power off.

If you had a schematic you would see that the traces branch off in different directions but ultimately end up at a positive voltage rail on one end and ground on the other. What has happened to the resistor (and whatever else is in it's path) is that there was a failure that led to little to no resistance to either end. That is why it is getting hot and burning up. You need to find the components in-between and consider replacing them, or search for a possible pinched wire or trace that is shorted to either end.

That said, it is customary to re-check or suspect whatever work was done last. The first thing I would do is measure resistance to ground at the resistor and also where work was done to see if they are at the same values. If the resistor you added was done without removing the other from the circuit the two together would be in parallel and equal less than either which may possibly be why this one failed. Scroll down to see parallel and Series Circuit Here> http://www.spec2000.net/06-resistivityohm.htm

To answer the question: Yes, I agree that plugging in another speedo board (also known as a daughter board) will probably cause the new one to fail. I'm going to guess that there is an established short on the main board that is causing this one to overheat. I wouldn't say that the circuit board itself is toast unless you see the resin and/or traces burned or bubbled up. If not, then the components can be replaced and the circuit tested to make sure it's good. Make sure you changed the resistor with one of the same wattage.
Hope that helps.

 

This is what i dont understand.... Speedo worked fine for 23 yrs. all i touched was resisor mod as per the sticky on tach board. Speedo side failed a few min after. I didnt think they would be related.
I have a multimeter but have no idea what to check for problems on the speedo daughter board...
If the daughter board is bad then a new one should fix the issue. If not then it has to be something in the cluster laminated board

 

Any chance your solder on the resistor crossed onto another connector or trace on the board? I went over my soldering with a magnifying glass after I did mine to make sure it was all good. Desolder it and see if it cures it with another resistor in place of the burnt one. If it worked fine before the repair and doesn't now, I would suspect that.

 

Doesnt appear to have crossed over. I'll double check.

Tach works fine without daughter board plugged in and so do the other gauges. Not sure if that means anything or not

 

Yep, it might not be related, but it was the last thing that was touched before it failed. I wouldn't have written as much as I did if I thought that the new resistor was the only issue you have now. All the info is there to help you check it, maybe I didn't write it down well enough.

Like Joe said try removing the one and see if there is any difference. I would check it with a meter though vs. plugging it in again.

 

I already tested it with new resistor as mentioned in orignal post. It did not fix issue so there is something else damaged. I'll have to take a look or just buy a new cluster.

Thats the key statement. Its finding documentation of known values to compare to. I feel this may be a challenge but i havent looked yet.

 

I was referring to the resistor that was originally added to fix the tach. Speaking of the replacement, did you replace the fried resistor with one of the same wattage?

Yes, having the proper information usually is. But the way that resistor fried, the short should be pretty obvious. It should read very low resistance, possibly low enough for the beeper to work. The first sentence was leading up to the more practical approach: " You can also generally interpret very low resistance to ground (or positive rail if known) as a short as well. "

Over all, something shorted out the resistor. So you want to follow the traces back and visually check components along the way. Without a schematic you can do that visually or with a meter or a combo of both.

 

No idea what the wattage is for either of those resistors on either board. I have 1/2 watts with 3 resistors in series on the tach board and 3 1 watt resistor in parallel on the speedoboard

 

If you replaced the damaged resistor with one the same size you should be good to go.

 

3.9 ohms doesn't sound right.

I'd expect 390 ohms or 3.9k ohms, not 3.9 ohms.

3.9 = orange white gold in the 1st 3 bands

39 = orange white black

390 = orange white brown

3.9k = orange white red

4th band is tolerance, most likely is either nothing (20%), silver (10%), or gold (5%)

 

Its definately orange first band. Next appears to be white and the last 2 appear to be gold. From what i can see in the small spot that didnt burn up

Another member has a thread here on the 90-92 speedo fix and he said this same resistor went bad and he found it to be 30 ohms.

So what gives?