Faulty Speedo on Spider

[jamieandthemagi]

Can somebody help me with a lil theory / method.

My Speedo on my 1990 S4 spider has stopped working. I notice on my Alfa Parts CD that this speedo is electronically driven. Therefore has a flexible shaft to a generator which must then convert that to an electronic signal to the speedo.

Question:

How can I go about tracing the fault as I have 3 potential faults causing the problem. The speedo, the electronic generator or cable?
Can I take an electronic reading from the connector at the back of the instrument cluster to prove that the speedo is either good or faulty?

Where is the generator ?

any help would be appreciated

[jamieandthemagi]

Does anyone know where the convertor box is on the car ?

[jamieandthemagi]

found the problem. the wire from the convertor runs behind the pedal box & was trapped, this must have gradually broken the wire. so cut the wire & did a loop round the trapped section - speedo working again.

[Paradiso]

Do you ever get the feeling that you're just talking to yourself?

[jamieandthemagi]

just a tad.

but thought I'd post the fix out of courtesy, incase some one else got a similar problem later.

Talking to myself is good, except when I start arguing !!!

[Paradiso]

and a very courteous thing to do it was. Especially considering that no one here was any help to you at all.

Thank you.

[Capt. Caveman]

I have an S4 and the speedo is approx. 15-20 MPH fast at freeway speeds. Any idea why?
It also sits at 15mph when the car is turned off.....

[Paradiso]

If you look closely at your speedo, you will see that there is a little black pin that keeps the needle from dropping below 15 MPH (It's normal, they're all like that. Well, I don't know about NORMAL, but they are all like that.)

Do you have non-standard wheels/tires?

How do you know that the speedo is 15-20 mph fast? Did you time mile markers?

[Capt. Caveman]

Quote:

Originally posted by Paradiso If you look closely at your speedo, you will see that there is a little black pin that keeps the needle from dropping below 15 MPH (It's normal, they're all like that. Well, I don't know about NORMAL, but they are all like that.) Do you have non-standard wheels/tires? How do you know that the speedo is 15-20 mph fast? Did you time mile markers?

[papajam]

At a quick glance, it would appear that going from 14" to 15" wheels would throw off the speedo because the 15" is obviously bigger. But that's just the diameter of the wheel, not the total diameter which also includes the tire. What affects speedo error is the diameter/circumference of the wheel/tire combo. Here's how to figure out the mathematical diameter of, say, a 185/70-14 (the 185 is the width of the tread in millimeters, 70 is the aspect ratio [the distance (or height) between the rim and the tread; in this case, the height would be 70% of 185 or 129.5mm]), and 14 is the wheel diameter. Multiply the height by 2, then divide by 25.4 to convert to inches. Then add the wheel diameter. In this example, the total diameter would be 24.196". Doing the same calculations for a 205/60-15 and the diameter is 24.685"; a difference of only 2%. In addition, since the 205/60 is 2% larger, the speedo would read 2% SLOWER.

Personally, I never much cared for a speedo anyway. All I use is the tach.

[Favali]

My 93 Spider Veloce came stock with 15" wheels and from the PO with an erratic speedo. Not wanting to remove the cluster and after a new cable and wiring inspection, I learned to drive with the tach. See http://www.favali.com/Fluids.htm for a chart on RPM to MPH for an S4 with 195-60-15's.

I had a car follow me with a cell phone and we tracked the car for every 5 MPH on a highway and I am off by about 5-10 MPH on the high side. But I was able to confirm the RPM to MPH readings

Use the tach ... presuming of course it is correct too!

[ALfemale]

My speedometer is black pinned at 10mph and I believe the Tach is black pinned at 1000 rpms as well (memory is faulty and not able to look at it right now) The tach runs true, but not the speedometer. From reading above messages are all speedometers wrong then? I know the Spider is roughly 10mph (I figure since it starts from 10mph that it's roughly 10mph even with the 16" rims) but starting to have misgivings when I was PASSED by two OTHER yahoos when I thought I was going about 92/93mph. So now I am wondering if it isn't maybe 20mph off. Sure thought I was going faster. Maybe will have to do the cell phone chase car thing as well to get a more accurate idea. What is the formula for tach/speed ? 3000 rpms is roughly 60 mph, yes?

[meatpile]

It took a while, but this thread got kicking.

My '74 speedo acts crazy. Worked until Saturday, other than occassionally bouncing around. Now, It gets pinned at wide open if I'm going 5 mph.

I'm betting speedo cable. Am i wrong?

[BeigeAlfa]

I guess you should have the mechanical (as opposed to electrically) driven speedo, in which case the speedo cable drives a plate, which, by the amazing power of magnets, induction, springs and science drives a second plate which is attached to the needle. The two plates are separated by a small air gap, the size of which is critical. The faster the driven plate goes round, the more the plate with the needle attached works against it's spring to try and follow it, i.e. the further the needle goes round. Simple in theory.

From what you describe, I'm speculating that a bearing has worn out and the two plates have closed together to some extent, possibly even touching.

You should be able to confirm the problem is the speedo, not the cable by ripping the speedo out (easy!) and leaving the cable end showing through the big hole that is now in your dashboard. You should be able to see the cable inner whirring round as you drive along.

Then all you have to do is dismantle your speedo and see what's up inside. I haven't been able to pluck up the courage to do this yet myself. I'm kinda waiting for someone else to do it first, might involve a saw and 4,000,000 very small bits held together with tiny screws and clips...

Dan.

[jamieandthemagi]

Now I have a double whammy problem.

The speedo has stopped working completly & the Tacho is erratic (solved by a quick thump on the dash).