328 speedo problem followup - still unsolved

For those unfamiliar with my situation, the short of it: speedo stopped working completely after intermittent viability, and the problem is not the sensor. Voltmeter confirmed ~13+V to the sensor.

Sent it to PAS and they bench tested it today using the new sensor I bought and found the speedo works, no problem w.it or the sensor. I would say then it's pretty obvious there's a connection issue but the the connections behind the speedo were tight and clean, and nothing was disconnected when I removed it to send to PAS. The blocky connectors under the dash are all tight and clean (don't know which is associated with the speedo). The connxns at the sensor and harness were tight and clean. So it's en route back to me but I don't know specifically where to focus.

I wasn't able to trace the harness past the few inches I could see going up into the engine bay - anyone familiar with the routing of this wire to know if there are pinch points or other vulnerable spots between there and the speedo?

Thanks for your replies.

 

Steve, thanks for checking in. I'm not very good with electrical issues and hardly know how to use my multimeter :embarassed: But, to answer your questions:
1. Yes
2. I believe so. The speedo is en route back from vacation in CA and I will confirm upon reinstallation but as I recall the speedo lights worked so I don't think the ground connection is involved.
3. Err, can't quite interpret this one but when I ran the tests you so graciously laid out the other week I got the same +13V on both tests, i.e. power to sensor and then from the sensor+ while car in motion.

I'll try to tap one of my e-savvy friends to lend a trained head to this investigation. Stay tuned....

 

Hey Steve, I've been examining the wiring dgm tracing from the sensor connection back to the head end. I've traced it in green (but cut off the rightmost side which illustrates a path through a connector 'between dashboard and rear section' behind the passenger footrest). It 1st goes to #169 which is a 'connector for the engine wiring harness'. From there it goes to a connector between the dash and inst. panel (#125). From there, there's one unidentified/unlabeled connection and then to the speedo.

I figured at the very least I will try to get at each of these connections and inspect their integrity. In the absence of any other electrical troubleshooting acumen does that seem like a sensible next move? I.E. am I going about this the right way since I can't seem to make any practical sense of your suggestion/question #3? I will also try to identify the ground associated with this run (hell I don't even know if that's how the grounds work...sorry! Wish I were more electrically savvy ) and see if those are intact. Ah hell who needs a speedo and milage logging anyway?!

Thanks for your help!
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Yeah I was kind of hoping (despite the expense) the problem was in the head and could be fixed and returned. Me no likey hunting down wire troubles Anyway, are there particular points where these straps are or are you just saying wherever they might occur check the wires they restrain? My biggest unknown is where to find these connectors, well the ones not explicitly located (e.g. behind the pass. footwell).

 

Update: I tested continuity w.ohmeter just now at the connector behind the pass. footwell and interestingly here are the results:
at the harness-side (i.e. footwell-mounted half of connector) RN (red striped black) I got a reading of 552 but no audio whereas if I tested any of the other leads in the connector I got a # as well as audio. Does that mean anything? The meter was on the right setting.

When I pulled the connector apart the RN male (well-mounted half of connector) was only ~half way into it's spot but even when I pushed it back through and reconnected the halves - and verified it hadn't pushed back out - still nothing. I was hopeful that was the issue but neigh. All connections are free of corrosion or fester.

I did not test at the head but will do that when the irritability of this whole g-d thing subsides a bit. I spent a few hours w.it today and I am not in a good mood about it.

 

To what on the head end (back of speedo) would I connect the two wires to? The RN lead coming from it's connector wire bundle?

 

Is there another sensor besides the one that screws into the engine, i.e. the 'speed sensor'? PAS just bench tested both the speedo and sensor, both checked out.

 

***** UPDATE *****

The mothergrabbing issue is fixed!! The problem was.....simply the engine harness connection. That was the one connection I did not get to until yesterday to test voltage there, and after re-rigging the extended jumper from the sensor connection to the cabin - and reconnecting that engine harness connector (#169 on the wiring diagram) after visual inspection - the speedo worked as I pulled out the driveway. Sonofa... Anyway glad it's back. The wires appear ok - it's possible I jostled the connection weeks ago when I changed the gear oil...? Who the hell knows. Anyway, done. Lots of hours and dollars wasted on this

Thx again for all of your replies and suggestions.

 

See pic - driver side fwd of the rad expansion tank. The connector contains the white-striped green and red-striped black sensor wires, which in the pic is the larger bundle of wires.
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