348 Speedo Burned Internals

Thanks!

 

Way cool! Please post your sources when you figure them out. And I'd appreciate it if you would take a look at the mechanical component in hopes you can see where that bloody escapee part might go. I couldn't find anyplace that looked like it was missing a small "L" shaped copper tab, but I also can't imagine that it came from anywhere else. It looks like it could very well have been inside of the speedo, but just examining from the outside I sure couldn't tell where.

 

So you didn't notice anything that looks like that copper tab inside your speedo, huh? Freakin' odd-- I wonder where that part came from. Especially since my speedometer *and* odometer actually worked for the duration of a trip around the block, after I replaced the sender with a new one. Only it didn't work at all a few minutes later when I took the car out for a longer test. Wiring seems to be okay, at least according to the schematic Mitonian posted some time back. So now I'm left trying to figure out if there's a bad component in the speedo itself or???

I can't identify the component in your pic-- I quit hobbying with electronics about the time that surface mount devices were beginning to show up in consumer electronics. I'm guessing it's a transistor, based on the fact that it has three pins. I only see two components on the back of the board that have that form factor and three pins. There is another on the corner of the board with the cutout in it but I have no idea if it's the same as the one that appears to be burned.

Seems like the next step for me is to remove the speedo again and have another look. May try to take some better lighted photos this time. Let me know if you need anything specific & I'll do my best to get it. Might try to compare those two 3 pin SMDs to see if they seem to be the same type of transistor, assuming that's what they are.

If this were a German car I think I'd just send the speedo out to be tested and repaired if need be, but not sure if there's anyone out there who truly knows these instruments well enough to trust them. Think I'm gonna try to test the signal at the speedo connector to ensure it's actually getting through, and if it is I guess I'll have to start calling speedo repair places. Has your research led to to a place you'd trust? I've seen Palo Alto Speedometer mentioned here, but no idea if they can fix ours.

Oh, and what USB microscope are you using? Been thinking to pick one of those up for times like this.

 

It is a transistor, BCX 19 (or BC 337). Its code is U1 and the "p" stands for Philips. See the attached datasheet and the list (meaning) of the U-codes.
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Correction: Philips uses letters "p" or "t" (after the "U1") to indicate whether the transistor is made in Hong Kong or Malaysia respectively.

 

Thank you for the information, Miroljub! Can you tell us anything more about the red disc component marked H P2 22Z1? It would be nice to figure out as many of these components as possible while we're at it, for the archives if nothing else.

Also, do you know if there's a way to confirm that the speedo head is getting a signal by using a DVM alone? It's pouring down rain here so can't get the car out to test it, but currently trying to figure out if a 'scope is required to confirm that the speed sensor & wiring are working okay.

 

22Z1 is a Varistor, also numbered as V22ZA1 (see the attached datasheets). It appears to be available from Littelfuse.com or you can search the web for V22ZA1.
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The speedo sensor should be producing a small AC signal. I believe that you can detect it with an ANALOGUE multimeter if you set it to its most sensitive volt or ampere range (mV, μV or μA). Try with DC and AC selections (the needle should move up-down if DC is selected). So, jack up the rear wheels and give it a try.

 

Oops, forgot to reply to a few posts above.

First off, thanks to Miroljub and Wade for answering my questions, as well as contributing to the knowledge base for the 348 speedometer. Thanks to Miroljub, we can now identify most of the parts on the front of the board!

Wade, that's a cool USB microscope-- I'll need to grab one at some point. Nice soldering station as well. I'm about to replace the caps on my board using just a plan old 40 watt soldering iron, so I have tool envy.

BTW, the wiring connectors for the pod aren't that tough to remove. The three larger ones are simple, in that the release tab only needs to be pressed down with your finger, although access to the one for the tach is a little tight and might be tough for larger fingers. The tabs on the two flat connectors need to be pried up with a very small flat blade screwdriver while simultaneously pulling outward on the connector housing. Having three or four hands would help here, but it's certainly possible using only two. Placing a towel over the steering column shroud is probably a good idea though, because the pod will be moving around a bunch as you remove those connectors.

Incidentally, I made labels for each of the connectors using my Brother PTouch tape labeler, just to take the guesswork out of reinstalling the pod. I find it easier to stand outside the car with the door open and look through the windshield and side glass to re-plug the connectors.

No real progress on mine, but I did get replacements for all of the topside capacitors in the highest voltage rating I could find that would still fit on the board. As you guys no doubt already know, but for the archives, the voltage rating on a capacitor does not change its function, so you want the highest voltage rating that will fit, obviously staying with the same capacitance rating of the originals.

Since capacitors are the only components on that board that I know for a fact degrade over time, I figured I'd start there since it's good preventative maintenance in any case. Not sure where to go if that doesn't fix it though, since none of the components on my board show any visible signs of failure and all of the solder joints look okay. I did buy a handheld oscilloscope that I intend use to ensure that the speedo is getting a clear signal since the wiring tests fine for continuity. At least that should confirm that the problem is indeed in the speedometer itself.

Wade, for whatever it's worth I think I'd replace the caps if I were you. They're easy to find and to install, so may as well put fresh ones in place for good measure, and to eliminate them in the diagnostic process. Past that, seems to me you're left either replacing components until you find the bad one(s), finding a friendly electrical engineer or hobbyist to reverse engineer the circuit, or just punting and sending it to Palo Alto. I'm leaning toward the latter if the 'scope test proves out and the cap replacement doesn't fix it, but I'll be keeping an eye out here in case you come up with another option.

Good luck!

 

In many cases, when you find a burnt (say) transistor, it is because a failure of another component caused it to burn, not that it just decided to burn.

In your case, since the odo is counting without any speed input, it suggests that there is something wrong with the big IC (HCF 4024 BE or MC 14024 BCP). This IC is the brain of the speedometer - it is a so called "ripple counter", i.e. it counts the pulses from the sensor and processes them to suitable signals for the speedo/odo gauge. It might have gone crazy and it is counting by itself or some other bad component is causing it to do that.

So, it is possible that the failure of the big IC (some kind of short), or of some other component, caused the transistor and the resistor to be overloaded and fail. A big suspect is that big black capacitor which appears to be linked to the transistor. You may make another try by replacing the big capacitor but I would also have a couple of new MC 14024 BCP-s on standby.

 

So the same components fried during testing, but not immediately?

I pulled all of the topside caps on mine last night. Replaced all of the standard can type but screwed up and got the wrong value for the axial lead cap, so no in-car testing today. Sadly, putting an ohm meter on the ones I removed didn't reveal any open ones, so I doubt this will solve the problem (not that I really thought it would).

 

I'll try to remember to check the resistance of the motor windings and post the number here. Checked it last night just to make certain it wasn't open, but can't remember the number. Keep thinking I have an intermittent issue somewhere since it hasn't burned any components yet, but everything I've tested so far seems to be okay. Looks like we're each chasing different problems, but the end result is the same. The weather here was perfect for driving today, but I can't enjoy the 348 because I don't want to put undocumented miles on it.

Any idea what the three wire connector that sits behind (relative to the driver) the motor connector goes to? I'm guessing it's a sensor that confirms the amount of needle movement to the circuit, but that's a WAG at best.

How long was yours working before it toasted itself the first time?

 

Wade, you have a serious short somewhere in the line --Resistor--Transistor Pin1--Transistor Pin 3--. Trace this line further from the resistor on one side and further from the transistor on the other and see where it starts and ends. It does not seem to be the speedo motor as you said it was turning; the smell from it could be the residue of the smoke that came out of the transistor and resistor.

On the other hand, the short could be caused by a short in the transistor Base (the middle pin, pin 2) causing high current flow through pins 1 & 3. The transistor middle pin is probably linked to the big IC.