348 Speedo Burned Internals

Difficult to follow the lines as the circuit board is double sided and components on top mask the view to some of the lines. If you remove the big capacitor, it will expose the lines to the left of the transistor (and you can check the cap for any internal short). Is your speedo motor good, no short?

 

A few random thoughts on the latest discussion regarding Wade's speedometer...

Obviously I'm not a trained electronics guy by any stretch of the imagination, but it was a hobby in my youth and I still have some vague memories from that time. So keep an entire salt block nearby while considering these:

*Assuming* that the "replacement" by the PO's mechanic included the board, it would seem that this car could have smoked components on the board a minimum of three times. I do not believe that the axial lead cap on the board is original, so it is conceivable that the board has been repaired before. It is also conceivable that the previous board had been repaired before it was replaced. We do not know 100%, but since the same components have burned a minimum of twice it does seem reasonable to consider the possibility that the same issue has arisen in the past.

To me, that points toward a problem with something in the car itself or components in the speedometer that are original, or that at least date back to before the problem presented itself. Apparently Wade has changed the sender at least once. *If* different senders were installed each recent time that the components burned, that virtually eliminates the sender. But we can go further by each of us measuring the resistance across our senders and posting the results here. I have the original from my car and a brand new, just installed 348 style sender that I can measure.

If the schematic posted elsewhere by Miltonian is correct, there are only two wires to consider here, but obviously they are suspect. Again, we can compare test results. In my case I got +12V on the correct wire and continuity between the engine bay and the speedo connector on both wires. My board isn't burning, so I did not check for a short between the wires, or to ground. But something for Wade to consider checking, if he has not already.

That pretty much brings us back to components common to the speedo during the time that the problem has been occurring. Wade posted that a mechanic replaced speedometer components with those from another speedo, but retained the original odometer reading. In order to replace the speedometer motor, it has to come apart pretty far-- I don't see any way to switch motors without pulling the needle, face, and other bits. Maybe said mechanic was that thorough, or maybe he just switched boards and left the rest of the speedo alone.

It seems to me that a true dead short would cause the smoke to escape from the components (Lucas electric joke reference) immediately. But that apparently isn't happening. Wade got some movement from the speedo needle and had the odometer turning for a time (makes me wonder if the odo is driven by a separate motor). The components eventually burned, but not right away. To me that point toward high resistance someplace, rather than a direct short.

Since I would think that most of the components on that board would either short or go open when they fail, based on my very rudimentary knowledge of electronics that would seem to point toward either a resistance change in windings (motor or sender) or maybe somehow a capacitor is going wonky. Considering that the board is a ways away from the motor and the the burned components are on the opposite side of the board from the motor, the fact that Wade says that the motor has a burned smell does make it seem as though the motor could be the culprit, especially if it is original to the car. Perhaps the windings are partially shorted across themselves or to ground, and now getting worse due to heat. One way to test this would be to compare the resistance across the windings to that of another speedo, only the guy that was supposed to get that measurement last night just plain forgot. Wade, feel free to remind me via PM tonight if I don't post up the spec here.

Of course, the sensor windings are also suspect, assuming the same sensor was in place each time the smoke escaped. So I'll try to get resistance measurements on those as well, and suggest that Wade measure his and post the results for comparison purposes, along with the motor winding resistance.

Anyway, that's my half a penny's worth on the subject. Now back to scratching my head about why my speedo/odo worked perfectly after the sensor swap, and then abruptly stopped...

 

10.8 is what I got as well. Does not seem to be any continuity between any of the leads going to the three pin connector. Since you've got yours apart, could you post pics of that and a guess as to its function?

Will try to post sensor resistance later. About to test drive the car after the capacitor change.

 

Thanks.

You don't happen to have a hard copy of the repair manual, do you? Just got back from a test drive and the speedo needle never budged from what I saw. The 'scope shows a square wave pattern across the green and red wires at the speedo head, so about all I have left to try is to see if I can figure out if the other two wires that go from the harness to the board are doing what they're supposed to. If you've looked at the online manual you've seen that the speedometer section of the wiring diagrams is missing, so hoping to get a scan of the relevant pages from another source...

 

Are there any more speedometers where you got yours? If so, please PM me the source.

I'd still be concerned that there's something in the car causing the speedometers to blow, in which case your replacement is very likely to suffer the same fate as this one. IMO it would pay to do what I'm trying to do and try to be a sure as possible that the wiring in the car and the sensor are okay before connecting up the new instrument.

I seriously doubt your board is beyond repair. Doesn't look too hurt in the pics, but no telling how many components need replacement. Please take high res pics if you strip the board-- they might help someone duplicate it in the future. PC boards aren't all that tough to make.

 

Even if there is a dead short somewhere, the big resistor (80W heater element in this case) will not burn immediately as the current is limited by its 6.8 Ohm resistance so it will take some time. Otherwise, I don't think the speed sensor can cause this kind of damage as it only generates a very low current signal which is sensed by the big IC.

 

One point to check: The black round component (marked with red line) appears to have moved to the right; its left solder looks like it was melted at some point and its shape suggests that there was some movement (spreading of the solder) to the right. If this was the case, the left and right solders under the black component might have connected to each other. This would put the big resistor in direct contact between "+" and "-" of the power supply which I believe is as I have marked it on the picture.

I can't tell what the black round component, with light blue and green stripes, is. It could be a diode which failed and heated up first melting and shorting its two solders. If the circuit board, when fitted inside the speedo, sits in a vertical position, i.e. in a position where the gravity would move the black component in the direction as suggested if the solders are softened, it would make this theory possible.
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The black thing shown on my previous picture (having light blue and green stripes) is most probably a Zener diode. If this is correct, than it can easily be the culprit (in Wade's case) if it shorted. The probability is that the diode is there to stabilise the power supply the the circuit at some voltage lower than 12V by causing voltage drop across the big resistor. If it blows short, the resistor will get 12V at its terminals, heat-up and burn.

Now the problem is to determine the type of diode, i.e. what is its stabilisig voltage if it is a Zener. The colour stripes are difficult to decode as they mean different things with different manufacturers. For example, the picture below shows a 6V Zener but Zeners for other voltages from the same manufacturer have the same colour stripes.
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See if there are any other markings on the diode, perhaps stamped into the metal ends?

 

Awesome thread!

 

Done.

In the DIY sticky now.

 

I tend to be a perfectionist and not to tackle the same problem the second time. I would go through the trouble of eliminating the zener diode and resistor. Augment the circuitry with a voltage regulator module similar to something like in the attached photo. Just cut some traces and tack it on. The speedo will last a lifetime. But then, if you're purist ignore this post and plow on.
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... I admire your tenacity. Double-up the zener to increase the current sink and increase the resistor wattage rating to something like 5 watt. That should harden the speedo a bit.

 

Thanks for the info, Miroljub. You description of the resistor's effect does make sense, at least as far my limited knowledge goes.

Regarding the sensor though, since +12VDC is applied to one side of it, couldn't a dead short in the sensor winding cause burned components on the board?

BTW, the board in the speedometer sits horizontal. However, the diode to which you refer is on the bottom of the board. (Once again, your contributions are most helpful-- I had no idea that component was a diode)