348 Speedo Burned Internals

One possible source of the strange voltage at R/W wire(s) is the ABS ECU. Although the diagrams do not show any link between the Speed Sensor and the ABS system, they might be connected. For example, the ABS light on my friend's Volvo 940 once came on and the fault code pointed to the Speed Sensor (a sensor on the differential), not to any ABS sensor. So, one more ECU to unplug and check the mystery voltage again.

 

Pin 2 test results (as expected): 0 volts with battery on, key off, speedometer safely ensconced in front trunk.

In fact, I got zero volts at all nine pins under the same conditions. I double checked the meter and its ground connection by touching the positive battery connection, at which time the meter showed 12.83 volts, so I'm confident in the results.

As a side note, my connector is installed with pins 1-3 across the top, instead of rotated 90 degrees as yours is. Which means you can't necessarily use pin position for comparison tests.

 

For the archives, here's a link to the discussion about the pin-out and voltages for the engine bay "shock tower" (115) connector: http://www.ferrarichat.com/forum/348-355-sponsored-bradan/505341-348-quick-voltage-check-connector-115-pin-2-a.html

 

The USPS got the large IC to me a day early, so last night I went ahead and changed it and both topside transistors on my speedometer board. I also swapped in better quality, higher rated capacitors for the ones I'd previously changed. The components I replaced are indicated by the aqua colored arrows in the attached pic.

It's not a professional quality job, but acceptable considering the tools I had available. In retrospect I wish I'd bought a better soldering iron just for this purpose, but I may not be completely done yet so there could be an opportunity to pretty things up a bit in the future.

The major goof was that I unknowingly bumped the hot iron into what I thought were ceramic resistors while cleaning solder from the pin holes. Turns out they're plastic instead of ceramic. Oops. (See red arrows in attached pic)

Can anyone tell me what those are and their values? One bears the marking ".33J50" and the other is marked "22(small R) J100". Are they caps, and if so, what type? I'd kinda like to replace them and tidy up the other soldering at the same time.

Anyway, the upshot of this work is that the both the speedometer and odometer worked flawlessly during a 12 mile test drive this morning! I'm not sure which of the components I replaced was the bad one but I plan on testing the old transistors versus one of the new spares, so hopefully that will eventually tell us which specific component was bad.

Speaking of spares, I did buy some of every component that I've collected so far and can make a few sets available to anyone looking for one-stop shopping. And I'd still like to pick up some spares of the smaller IC, since it's pretty tough to find and will presumably become even tougher to get in the future. Miroljub, did you ever hear anything from the Italians regarding whether or not 92693 crosses to the more common op amp? I do have a '91 ECG Semiconductor Master Replacement Guide on the way to me, so maybe that will provide a clue. Either way, I'd like to pick up some spares but the more common part number is *lots* cheaper.

Anyway, very happy that the speedometer is back working again. In fact, the car ran flawlessly, without even turning on any left bank SDLs (still need to replace the one remaining original thermocouple to fix that). But I gotta say, after all the hassle I've gone through with the speedometer, I was even more paranoid than usual driving the car.
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Thanks, Wade! I'm really happy to have (mostly) completed that detour along the path of getting my car into shape to enjoy now that cooler weather is here. This morning's drive was glorious-- temps in the low 60s with the roof off and a clear sky!

Still want to find a replacement for the smaller IC, even if I don't change it now. For one thing, those seem to be getting scarce so I'd like to have a couple of spares. But I'd also like to be able to supply those in the spares kit, as well as maybe the components I dinged with the soldering iron, assuming someone can help me ID them.

So I'm not quite through learning about 348 speedometers, and I'll of course be here to help you in any way I can. Thanks to you and Miroljub for your support and info. Here's hoping this thread will help shortcut the diagnostic process for others in the future.

Best of luck finding your wiring issue. Let me know if I can help.

Cheers,

///Mike

 

Those are capacitors:

.33J50 = 330nF 50V

22.J100 = 22nF 100V

I didn't get any info from the Italian components supplier, they just sent me a PayPal invoice which I paid and the parts are on the way. Most probably 92693 is actually CA3140 (as what the Italian website shows). Nowhere else I could find any more info on this.

 

That's awesome!!! Perhaps a fault in one or both of the seatbelt modules?

Now you can swap out the burned parts on your board and re-test the speedometer. Fingers crossed that it will work with no further effort on your part, as long as those pesky seat belt modules are disconnected. I totally ditched mine because I hate the mouse belt system and I didn't want it to wear itself out since parts are spendy. Also, at some point in time someone had hacked up the ECU mounting board in order to install an alarm in the center console. So if you wind up replacing your seat belt modules with used ones try to find some still on the mount and I'll buy the mount from you just to have it in storage for the future. If you need modules for testing purposes I could likely be convinced to loan you mine.

When I get some time I'll try to remember to plug mine in and see what happens to the pin 2 voltage...

Congrats on some great progress! Today is a good day to be fighting seat belt gremlins.

 

Thank you, Wade and Miroljub! I really did not recognize those as caps and when Wade had previously referred to the part number I thought it was something on the bottom of the board.

Miroljub, do you know what type of capacitor they are? I'm guessing a standard ceramic cap should not be used as a replacement, otherwise they would have been used in the first place. So if I want to replace them, are there options other than chasing down the original part numbers?

Thanks again for your help on all of this. If I even could have made it this far without the info you've provided it certainly would have been a much more difficult process.

Cheers,

///Mike

 

Are the wires at the seat belt modules red/black or red/white?

 

They are just standard capacitors. The one on your board is 50V and I noticed that in the Wade's new speedo the 330nF (.33J) is 63V and the 22nF is 100V. These days, the capacitors usually come in the shapes as shown on the pics (another way of marking is, for example, 330nF is marked as 334 = 33 plus 4 zeros in pF = 330 000 pF = 330 nF; 22nF is thus 223).
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Thanks, Miroljub! So I can just pic up some quality caps from Mouser & I'm good to go, huh?

I think I might just replace those two before I button it up for good since, at least in the audio world, caps seem to be failure prone due to age (or at least electrolytics do).

 

Aha-- now my R/B confusion makes sense. Funny, but when I used to work on the wiring in my X1/9 I had memorized the Italian for each color and used it because that's how the wires were shown in the diagram (only a single page, included in the owner's manual of my '76!). But I'm very rusty on the colors after years of disuse and I figured most people here would be comfortable with using English for the colors, so that's what I've been using.

Interesting theory regarding the seat belt system causing speedometer issues... and worse. I wouldn't be at all surprised, since if I'm reading you right *both* of your modules have failed in such a way as to introduce voltage to the "wrong" side of the speed sensor.

Like you, I'm wondering why the seat belt system was tied to the speedo circuit in the first place. My belts were working when I got the car but I pulled the modules almost immediately. If my terrible memory serves though, the belts operated when the key switch was in the on position. I don't recall the motorized belts taking any action as a result of the car beginning to move.

Best of luck in the next steps. Hopefully you're close to having a working speedometer with fully captive smoke.

 

Awesome! Congrats! Now replace the burned components on your old speedo board and see if it works. There's a good chance you can keep your old speedo and odo reading.

Driving my car tonight, thanks to the fact that the odo is now working. Soooo awesome to be back in it now that the weather is better.

 

Going to go ahead and update this thread to say that my speedometer has been working flawlessly since I replaced most of the components on the top of the speedometer drive board inside the housing. The top pic shows the board with most of the components swapped, the red arrows indicating the two that did not get replaced.

The bottom pic is of the replacement parts kit I put together in case anyone wants to swap out those parts on their own board without having to source and order parts from all over the place. The pile of three transistors toward the top right and the pair of ceramic caps toward the left are duplicates of one another, so you actually get spares of those two parts. The ICs are direct replacements for the originals but most of the discrete components are uprated from their original spec and are from high quality manufacturers. I spent hours trying to find the best replacement components that would physically fit in hopes of not having to do this again anytime soon. I only have a few of these kits but will make them available to the Brotherhood for as long as they last.
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This IC 92693 is also used in the Alfa Romeo 33 speedo and I have serviced a lot of these as the 6.8 ohm resistor and the BDB02C get burned up and also the 92693 goes defective.
After your very interesting thread which suggests that the 92693 is equivalent to the CA3140
I have found that on the speedo board of the AR33 pin 3 is not connected.I did change the 92693 for the CA3140 but the speedo did not work either.
PS
I change 92693 from other scrap speedo boards.

Hoping one day I will find this equivalent.