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Speedo electric diagram for S4s

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Hi All, 

Does anybody happen to have an electric diagram (chart, schematic) for S4s which has the speedometer in it? The Service Manual has only 1 sheet - Sheet S3 in Section ML that ever shows a speedo, and it's for Sport 300. It appears there is no official sheet for S4s. The previous page - Sheet 3 - for MY 95.3Y - doesn't have a speedo, because, as I understand, with S4 (MY 94) and prior SE's (MY 93 and earlier) the speedo was mechanical, not digital. S4s (MY 95) seems to be the only year with the 4-cylinder engine when the speedo became digital. 

I need this diagram as I'm trying to solve a wiring problem. The speedo on my S4s is not working. I had it checked by a speedo repair shop just now and it's fine. Connectivity from ECU's connector J2-B8 to the speedo's orange wire is fine. Connectivity from VSS (vehicle speed sensor on rear right wheel hub) towards the harness near the ECU is fine. But after that the wires go somewhere, before they reach ECU, so there is no connectivity between that point and ECU - connectors J3-C2 and J3-C8. Sheet S3 shows that the wires from VSS go directly to ECU, but, as I described, I found it to be not true. At worst, I can try to trace that wiring myself, but of course having a true diagram would help! 

Thanks very much in advance. 

Anton 

'95 Esprit S4s

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Greetings to All, 

The speedometer on my S4s is still not working. I ruled out the speedo itself (had it checked by a speedo shop, it's working OK), the connectivity between all known 'external' points, and finally the VSS (vehicle speed sensor) - I replaced it with a new one. 

So has anybody ever encountered a problem inside the ECU (a burnt circuit, internal cracks etc) that may cause the speedo not to work?

Any insight will be appreciated, thank you in advance!

Anton.

 

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1995 onwards S4 and S4s vehicles came with the electronic speedometer.  Wiring diagram Sheet S3 is valid for all electronic speedometers.

Have you checked the whether the ECU is seeing the VSS signal by checking the MPH using Espritmon, TunerPro or Freescan?

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Hi Derek, thank you for replying. I thought that too, but the S3 sheet diagram may have some inaccuracies (in general) re: S4s, for example it has a tachometer amplifier that S4s doesn't have, as far as I know... 

I checked the signal continuity everywhere I possibly could - in the speedo-related wiring/circuits. 

I guess what you are saying is that I should try to run Freescan while driving (that is, when the rear wheels are rotating) to see if it it 'registers' the speedo, that is, the latter shows non-zero speed; is that what you meant? 

I'd like to be able to do that, but it may be a problem at the moment: I don't have a laptop, only an old PC with a serial port (via which I connect Freescan), hence I can run it only when the car is stationary. I was advised to try to run the wheels (with the car's rear being up on the stands) with the engine on and at low speed, but I don't feel comfortable doing that yet. 

Are there any other ways to diagnose this kind of problem? I was told I should get out and open the ECU to try to see if there is any soldering going bad, any (microscopic) cracks etc. 

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If it is the speed sensor - I’m pretty sure that would throw up a CEL - it certainly did on mine. Can’t remember the error code it threw - but the ecu used data from this to adjust feeling as well - sure went a hell of a lot quicker once that sensor was changed

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Apart from the S300 tachometer amplifier I do not think there is anything else different on the sheet S3 diagram.

I personally would jack the car up and run the engine in 1st gear to check for the ECU seeing the VSS signal. If you are really uncomfortable doing this try spinning the right hand rear wheel by hand. Maybe if you can do it fast enough for at least 3 mph (1 revolution per second should be more than enough to exceed 3 mph with a 245/45r17 tyre) you might be able to see if the speed signal is OK going to the ECU and it is processing the input.

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Barry, exactly that happened to me - the VSS threw the code 24. So I ended up replacing the VSS with a new one. No code 24 anymore so far; I guess the old sensor was somehow bad. 

Derek, yes, I thought about that and actually, with the wheel taken out, tried to rotate the brake disk and saw a multimeter register some small voltage upon rotation. However I measured it only on the VSS contacts. So the next question is: where should I measure the same voltage (when rotating the wheel) on the ECU? The input contacts there are J3-C2, J3-C8. The continuity between them and VSS checked out. The output contact of ECU (towards the speedo) is J2-B8, the continuity between that and the speedo checks out as well. However, as I come to think of if, I never measured the voltage between the output contact (J2-B8) - and the ground, I guess (when rotating the wheel); is that what I should try next?

BTW, thanks very much for replying.

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Have you checked with Freescan or Espritmon if the ECU actually sees a speed signal? If it doesn't, it wont transmit it to the speedo either.

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Filip, thank you. Well, first, I checked continuity between VSS (speed sensor) and the ECU.

Now, how do I exactly check what you are suggesting - that the ECU sees the signal (from VSS)? It sounds like you are implying I should run Freescan when the wheels are rotating? That could be achieved by either 1) driving the car and having a laptop with Freescan hooked up (unfortunately I can't do that since I don't have a laptop), or 2) as Derek suggested, jacking up the car and running the engine in 1st gear (that I can do since I have a stationary PC with Freescan). I still don't feel comfortable enough to do the latter, but perhaps I'll finally get to doing it - or trying to rotate the wheel by hand - to see if the speedo in Freescan reacts to that. 

I tried the latter only to check that there is some small voltage on the VSS contacts (that's 'the beginning of the line'). I also know there is no signal at 'the end of the line': the speedo doesn't receive the signal (even though I had the speedo itself checked by a speedo shop so I know it's working). And also there is continuity between the ECU and the speedo. So it feels like something inside the ECU is either not getting the signal from VSS, or getting it but not passing through to the speedo. 

Anticipating the outcome, what would be the causes in each case: 1) if Freescan doesn't register speed? 2) if it does? Please advise. 

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Driving with a laptop hooked up is the obvious way, I try to log a run a couple of times each year,  just to make sure all readings are as they should be.

But without a laptop, jacking up the rear wheels will work as well. You can try with manually turning a wheel (make sure the other one can't turn the other way). I think you should be able to get it up to speed for a reading from the ECU. If that doesn't work, you'll have to start the car. No real risk if you make sure the front wheels are properly chocked and the axle stands are stable. I agree it doesn't feel comfortable, especially the first time you try it. If you're in the car, even if something should go wrong, you'll be able to stop it right away.

You've been very methodical in your approach, so the ECU seems a plausible culprit. Or the connectors, as your checking continuity from the cable side, not from the ECU side.

Another thought, is your speedo an original one, or an aftermarket? I had to be creative with the wiring of an aftermarket one to get it to work in my SE.

Good luck,

Filip

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Jacking my old one up and spinning the wheel showed the faulty sensor straight away - also proved the new one worked 👍

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Hi Filip et al, thank you for your advice. I just found - by rotating the r.h.s. rear wheel by hand (and blocking the l.h.s.) - that Freescan registers the speed up to 3 or 4 mph! So I guess that means that the ECU 'sees' the signal, but - doesn't transmit it to the speedometer. (and BTW the VSS on my car is new). The speedo itself is original, it's a Caerbont unit. I made sure (multiple times) the wiring is connected correctly (both in sync in how it was on the car before the speedo started misbehaving, and in sync with the schematic in S3 chart). 

So what do you think the problem may be now, and how can I diagnose it further? 

I was advised to take the ECU out and inspect it - for bad soldering, mini-cracks... Could it be that the ECU itself is faulty (and if so, in what sense? it's just the speedo that isn't working; the rest of the car systems seem to be working OK), and if so, what can be done about that? 

Please advise, thanks very much for your help. 

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Try swapping pin B8 over to pin B14 and/or B16. The speed signal is inverted on these pins and also halved on pin B16 but should still show on the speedometer. This should help narrow down where to investigate further.

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I tried both combinations advised - with a paper clip: 1) connected B8 to B14; 2) connected B8 to B16 (I've put the paper clip on the smooth - female - side of the connector, bent it to reach for both pin holes, cut it to the right length in both cases, gently re-installed the connector/plug back into its place). In both cases Freescan showed the speed, but the speedometer still didn't. 
So what does that indicate and what should I try next: take out the ECU and open it? Please let me know your thoughts, thank you. 

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