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Old 07-03-2010, 10:33 PM   #1 (permalink)
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Honda VX intermittent speedo issue

Hi Folks,

Looking for some thoughts on how best to debug an intermittently working speedo on my 1994 Honda Civic VX with 320k+ miles.

Basically, the speedo (and odo) will work some of the time and not work the rest of the time. Going over a bump will sometimes change it from working to not working (or vice versa). I have no other issues with the instrument cluster (lighting is fine, no engine check codes, etc).

I'm not clear on the path from speed sensor to the speedo. Does the VSS signal go through the ECU first, or does it branch off before the ECU and essentially go to the speedo directly?

I figure points of failure could be the speed sensor itself (not sure how the ECU would react without VSS, if at all), the signal wiring / connections, power / ground to the speedo, or the speedo mechanism itself. Is there a "common" failure point that you would recommend checking first? (I searched on-line & the recommendations are all over the map, from "replace instrument cluster" to "replace vss sensor")

Appreciate any thoughts from those that are more familiar with Honda Civics than I...

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Old 07-05-2010, 12:40 AM   #2 (permalink)
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Check connections at the VSS on the trans for corrosion, clean them. Then check the connections on the back of the instrument cluster, clean them. You can also check the continuity of the wiring between the sensor/ECU/Cluster.

Corrosion at the VSS connector on the trans is the most common failure point though, followed by a bad speedo in the cluster.
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Old 07-05-2010, 08:38 PM   #3 (permalink)
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Thanks TomO!

Took out instrument cluster and cleaned off the connectors yesterday... no change. Didn't think to check continuity... Doh! Guessing from your description that the VSS signal does go through ECU first, and presumably there is no side effect (no code, etc) if there is no VSS signal to ECU?

Anyway, will see if I can check the VSS itself today.
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Old 07-06-2010, 06:45 PM   #4 (permalink)
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I know that the 88-91 cars with MPFI will throw a CEL at you when the speedometer stops working. But it takes some specific circumstances to make it come on; I think it will come on if the ECU sees you are over 2500 RPM with the throttle closed for more than a couple of seconds, while the speedometer doesn't move. That's not quite it (or you could get the CEL to trigger by revving up in neutral) but it's something like that.

On those older cars, though, there is a spinning cable from the transmission to the speedometer, and the VSS is actually inside the instrument cluster. I think the newer cars have the VSS on the transmission itself, and the speedo is all electronic.

-soD
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Old 07-06-2010, 07:51 PM   #5 (permalink)
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Thanks soD -

In the case of my '94, you are correct that the VSS is on the transmission itself.

I didn't allow myself enough time to really dig into the issue this weekend. Best I could do was check the VSS connector at the tranny, as suggested by TomO. Looked pretty good actually; clean, no corrosion, good connection.

Also managed to find my Honda Service Manual which has some trouble-shooting steps if the speedo doesn't work... first step was to verify I'm getting the proper voltage to the VSS sensor itself when the ignition is on... which I am.

... was time to call it a night right after I did that... learned the hard way that leaving the ignition on overnight will kill the battery...

One thing I'm not sure how to resolve... since this is an intermittent issue; I'm not sure when I'm testing things out if it's in the "working" state or the "not working" state. Just jiggling things around to test things (like continuity) could put things into the "working" state. Guess it's just a matter of working my way through it...
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Old 07-06-2010, 11:23 PM   #6 (permalink)
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When I aquired my 95dx I was having the speedo issue. I popped the VSS out (being careful to get the little "tooth" with it. You can spin the thing and you will get an 5v signal that turns on and off.

Alternately you can check it at the back of the cluster by following the trace on the back that goes from one of the three speedometer screws not labled + or - to the plug and tapping into that wire. Might as well leave leave a long wire attached to the signal wire where you can get to it because you will be needing it if you want to install an MPGuino.

The plus side of leaving a tap into the signal wire is that you can monitor it while the speedo is not working to verify you have a good signal even when the speedo is jumping around and/or showing 0.

U pick a part sells their clusters for about 16 bucks. All are compatible within the 5th gen range (mine is from a 93 EX auto had a tach).
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Old 07-08-2010, 02:30 PM   #7 (permalink)
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Quote:
Originally Posted by bestclimb View Post
Alternately you can check it at the back of the cluster by following the trace on the back that goes from one of the three speedometer screws not labled + or - to the plug and tapping into that wire. Might as well leave leave a long wire attached to the signal wire where you can get to it because you will be needing it if you want to install an MPGuino.

The plus side of leaving a tap into the signal wire is that you can monitor it while the speedo is not working to verify you have a good signal even when the speedo is jumping around and/or showing 0.
An excellent idea; thanks bestclimb! I'll tackle that this weekend.
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Old 07-08-2010, 05:31 PM   #8 (permalink)
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On mine it turned out to be a bad solder joint on one of those 4 long pins inside the cluster that connects between speedo/odo faceplate and the back of the cluster.
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Old 07-18-2010, 04:43 PM   #9 (permalink)
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Finally was able to spend some time on this... attached a wire as bestclimb recommended and (carefully) checked it while driving, both when the speedo worked and when it didn't. I seem to be getting VSS signal always, indicating it is the speedo unit.

Next chance I get, I'll pull out the speedo unit and check the solder connections as mentioned by basjoos.

Thanks for helping to narrow down the problem!!
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Old 08-06-2010, 06:43 PM   #10 (permalink)
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Took out the speedo unit and disassembled it, but couldn't find anything obvious wrong with it... Basically, there are 3 connections, done via screws, from the back of the gauge housing to the speedo unit... (+), (-), and VSS. Those go to a single ckt board on the speedo, which then connects to (I think it was) 4 pins going to the rest of the speedo unit... looked like half go to the odo motor and the other half to the speedo needle motor (Sorry, forgot to snap pix!). I looked over the ckt board pretty thoroughly (IMO) and did not see anything that might be causing the intermittent outage issue.

As an aside, I (FINALLY!!) hooked up my mpguino, using the wire attached to the VSS signal on the back of the gauge cluster housing (as mentioned above), and it has been working flawlessly... in fact, since I now focus most of my "instrumentation" attention on the mpguino display, I'm not too concerned about the speedo not working anymore.

Eventually, though, I'll stop by the Pick 'n' Pull and grab a used speedo from there.

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