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pulse generator

speedometer

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#1 scott87star

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Posted 08 September 2013 - 03:47 PM

87 Flatty with the digital dash, my speedometer has been intermittent at times over the past year, usually during cool, damp weather.  Well, I now live in Washington about 300 yards from Bellingham Bay, so its non-existent at the moment.  I pulled the dash to service the ground behind it but that didn't help, I see in the FSM a pulse generator which must be the interface between the physical speedometer cable from the tranny and the digital dash since the speedo cable didn't run to the back of the gauge cluster like the fatty does.  How do I get at that little monkey?  It appears to be mounted just above the bell housing?  I'm guessing that if I can get to the connector at least I could make sure that's all clean.  It's a very stock motor so the secondary air tube is also back there, getting that out of the way will help?

Thanks in advance.
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#2 ucw458

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Posted 08 September 2013 - 04:46 PM

I have a digital dash and there is no speedo cable.  Not even from the trans.  The sensor on the trans tail housing is the speedo signal generator.  It replaces the usual signal generator for ABS that you find on ABS equipped cars.
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#3 scott87star

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Posted 08 September 2013 - 05:02 PM

Interesting.  OK, under the car I go.
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#4 ucw458

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Posted 08 September 2013 - 05:19 PM

FYI that's why digital dash cars didn't have ABS.  There's no way to stack the 2 signal generators.
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#5 ZacMan

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Posted 08 September 2013 - 07:54 PM

Yeah, I've noticed the signal generators for the LCD dash and the ABS have two different part numbers, which is weird that they couldnt share the same signal.

I'm betting the sensors for the electronic speedo's output a square wave, whereas the more common ones used for the ABS output an ac waveform.

#6 scott87star

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Posted 09 September 2013 - 06:14 PM

I'm betting you're correct Zac, I crawled underneath to service the connector and it's a three wire.  I have the old ABS sensor off my fatty and its a 2 wire so its VR while the three wire is likely all positive square wave.  I'll let you know soon enough, servicing the connector didn't bring the speedo back to life, I've got to pull the pulse generator out to test it.
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#7 ZacMan

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Posted 09 September 2013 - 11:19 PM

Cool. Would be awesome to know how many pulses per revolution they are. When you're testing it, remember to tie the output signal wire high with a 10k or similar 1/4W resistor.

#8 scott87star

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Posted 10 September 2013 - 01:03 PM

Posted Image
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#9 scott87star

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Posted 10 September 2013 - 01:05 PM

My normal picture host is down so I uploaded a screen shot to this site, very small.

Anyway, it is 0-12V square wave, 4 pulses per revolution.  Actually wish it had been the problem because now it looks like the digital dash itself is the problem.
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#10 ZacMan

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Posted 10 September 2013 - 08:56 PM

Hmmm, shitballs. Can you see if the signal is apparent at the dash cluster? Could just be a loom problem? I want one of those sensors now, would be a nice easy way of interfacing to later model ECU's.

#11 scott87star

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Posted 12 September 2013 - 04:04 PM

Nope, not a loom problem, I checked this morning and it was a nice strong signal at the plug into the dash itself.  I also spent some time poking around in the guts cleaning up whatever corrosion/leaching I could find (and replaced some burnt out bulbs!) but it didn't help.  I need to pry apart the circuit board, it's glued as well as screwed on, and trace the input signal to check the actual components.  I'm guessing it has to be a component because the display fires up just fine when you turn the key on.
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#12 ZacMan

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Posted 14 September 2013 - 08:41 PM

Any electrolytic capacitors that you can see looking swollen or leaky? They've got to be about ready to pop by now. Keen as to know how you get on dude, I'd like to get one of those dashes one day :-).

#13 scott87star

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Posted 17 September 2013 - 12:45 PM

LOL, Zac, the moment I read electrolytic capacitor the light bulb went on.  It took me a couple of days to get back to it but I managed to separate the boards this time for a closer inspection.



Posted Image


And.......



Posted Image




Lovely!  I count 15 10 microfarad 50V capacitors, at least 6 that have deposited their guts onto the board.  I've ordered the parts but I'll have to drive to the Mesquite meet without a speedo, odo or cruise control.  When I get back I'll be swapping out all of the caps, there are the 15 mentioned, one 100 microfarad 50V and one 2200 microfarad 50V.
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#14 ZacMan

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Posted 17 September 2013 - 03:33 PM

Wow, they puked really good! Sweet dude, hope replacing them sorts it out :-).

#15 scott87star

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Posted 05 October 2013 - 06:45 PM

Pooh!  I just spent the day swapping out all the electrolytics on the board and it still doesn't work!  I thought after driving to Mesquite and back that the caps were the problem because after 100 miles or so into the trip the speedo started working again and it did fine the next 3000 miles to Nevada and back.  It seemed once everything warmed up and pushed the condensation off it came back to life.  Even the cruise control was trying to work, even though it couldn't hold speed very well.  Now I'm back to square one on this.  I guess I have to take the flatty out of DD duty so I can properly trace the circuits to see where the problem may lie.
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#16 ZacMan

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Posted 06 October 2013 - 04:21 AM

Bugger! That sucks dude. Those caps stuffing out could definitely bugger some of the IC's :-(. How did you go about cleaning their puke off the boards? That stuff can be pretty corrosive, maybe it ******** some tracks somewhere? I still think they'll be your problem.

Have you got a wiring diagram for the electronic dash cars? I've got one I can try and make a digital copy of if you run into problems finding one.

#17 scott87star

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Posted 06 October 2013 - 11:16 AM

The stuff came off pretty easily just poking at it with a very small screwdriver.  I have the FSM diagram which only covers up to the dash, if you have something that actually covers the board itself that would be very helpful.
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#18 scott87star

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Posted 10 October 2013 - 09:01 AM

Haven't had time to pull this apart again but I can report that my gas gauge is now working properly again.  Before the cap swap it was randomly moving up and down, at one point I stopped the car and checked under the hood because it seemed I was pumping fuel out onto the ground.  Now since I swapped all those caps the gauge is rock solid and accurate again.
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#19 ZacMan

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Posted 10 October 2013 - 02:33 PM

Small victories! Bonus :-).

#20 scott87star

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Posted 20 October 2013 - 04:32 PM

Whoo-hoo!  Success!  Thanks for the support ZacMan, you were absolutely correct, I hadn't done a good enough job of cleaning the old electrolytic snot off various components.  I ohmed out several yucky spots the last time I had it apart, when I actually replaced the bad caps, and got no conductivity anywhere so had stopped short of totally cleaning up the board lest I ruin something.  This time I cleaned EVERYTHING including the sub-boards as was having some random issues with things like the BRAKE light coming on for no reason.  Every board, I mean EVERY board had flux leaching on various connectors:

Posted Image

Posted Image

Posted Image


So I scrubbed the H-E double hockey sticks out of everything and hit them all with some conformal coating to extend the life:


Posted Image



Tested every bulb, changed the seven burnt ones I found.  Got it nice and tidy:


Posted Image



And now it works like a CHAMP!  BEER ON ME!
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