Technical Speedometer Reset

[PandaEs]

I have a 2004 Panda 169 JTD. About 10 years ago I had a dead battery and had a tow truck jump-start the car. The car started and recharged the battery, but the speedometer was then reading 10 kph less than my actual speed (compared to my Garmin GPS). Six months later the battery died again and after the battery was jump-started the speedometer was now reading 20 kph less than the actual speed. I took the car to my local mechanic (not an official Fiat mechanic) and he sent the speedometer to a specialist to fix it. After a week the speedometer was returned, functioned perfectly, and set me back about 150 euros (I live in Spain).

A month ago (and two batteries later) my battery died again. After jump-starting, the speedometer again read 10 kph less than the actual speed. Two days later the same thing happened again twice. I had the battery replaced and the car starts perfectly, but now the speedometer reads 30 kph less than the actual speed.

Does anyone have an idea what the root problem might be? Speed sensor? Other? Thanks.

 

[koalar]

Sounds like the pointers need moving round



Only spin the points the correct direction

As in this part of the video

 

[PandaEs]

Thanks, Koalar---
Yes, I watched this video earlier and considered resetting the speedo pointer, but that won't solve the underlying problem. Why is the speedometer resetting the position of the pointer when the battery is jump-started?

 

[koalar]

The problem is caused by the voltage seen by the body computer falling below 6.5V, normally during cranking, stopping the correct information being sent to the instruments. Not the jump start in itself, time normally resets and starts flashing also

Could be

Battery
Alternator
Earth lead gearbox
Earth lead chassis
Dirty or corroded terminals
Auxiliary belt
Parasitic drain

I'd start with

Visual inspection of the aux belt
Battery resting voltage
Battery load test
Voltage drop test on the leads
Voltage test on the alternator at idle
Voltage test on the alternator at 2.5K RPM


If you have a scan tool it's quite useful to graph the voltage while cranking

Here, my car with a weak battery, you can see while the starter is stationary the voltage drops to 7V for a brief fraction of a second, far to quick to be measured by a multimeter

Another volt and I would have had problems starting clock would have reset and possibly problems with the instrument pointers

 

[DaveMcT]

Our 1.2 had steering issues due to poor battery. Fitted a new battery only for that to struggle a week or three later. After the usual tests, the problem turned out to be alternator that was not charging well enough. New alternator and no more problems.

My suggestion is to properly test the alternator, and accessory belt.

A problem in UK is cracked alternator stator, due to rust expanding the laminated iron core. Check the casing for cracks.

 

[PandaEs]

The problem is caused by the voltage seen by the body computer falling below 6.5V, normally during cranking, stopping the correct information being sent to the instruments. Not the jump start in itself, time normally resets and starts flashing also

Could be

Battery
Alternator
Earth lead gearbox
Earth lead chassis
Dirty or corroded terminals
Auxiliary belt
Parasitic drain

I'd start with

Visual inspection of the aux belt
Battery resting voltage
Battery load test
Voltage drop test on the leads
Voltage test on the alternator at idle
Voltage test on the alternator at 2.5K RPM


If you have a scan tool it's quite useful to graph the voltage while cranking

View attachment 458509

Here, my car with a weak battery, you can see while the starter is stationary the voltage drops to 7V for a brief fraction of a second, far to quick to be measured by a multimeter

Another volt and I would have had problems starting clock would have reset and possibly problems with the instrument pointers

Wow, Koalar! A great diagnosis and an incredibly complete set of tests. I'm researching how to do these tests today. Thanks. Once I've solved the low battery voltage/alternator problem I'll be resetting the speedometer pointer as shown in the video you posted earlier. Thanks again!

 

[PandaEs]

Our 1.2 had steering issues due to poor battery. Fitted a new battery only for that to struggle a week or three later. After the usual tests, the problem turned out to be alternator that was not charging well enough. New alternator and no more problems.

My suggestion is to properly test the alternator, and accessory belt.

A problem in UK is cracked alternator stator, due to rust expanding the laminated iron core. Check the casing for cracks.

My batteries have been lasting about two years before I need to replace them. I thought that was a short lifespan, but compared your one to three week battery life, I guess it's pretty good. I will definitely check the alternator and accessory belt, although the key word, as you say, is PROPERLY! Thanks for your advice.

 

[the green vanper]

Two years battery life? There must be something wrong.

I've replaced the factory installed battery in the Ducato after 9 years. And it was parked for almost a year before I've purchased it. I've bought a good quality aftermarket battery, the van is parked most of the time, after accidently resolving the issue with something draining the battery (I assume the aftermarket tow hook installation) I have no longer any problem with it. 3 years and a half, not more than 2000 kms done. It can be left parked for 2, 3 months and starts with no problem. Not a diesel though

On the daily non-FIAT car the 1st battery was replaced by the dealership by the OEM battery. This one lasted 11 years. It has 2 years now, the car is sometimes parked for a week, even with such weather (snow, humidity, subzero temperatures) I have no problems starting it.

 

[PandaEs]

I finally had the time to do a bit of work on the car.

Battery when the ignition is off: 12.57 volts. So the new battery is good. No surprise there.

Getting to the alternator is a pain, so I just measured the voltage at the battery when the car is idling at about 1000 rpm: 14.20 volts. So the alternator should be good, too. Right?

Regardless, I'm taking the car into one of the local shops. Today I was able to remove the speedometer from the dashboard and remove the clear plastic cover over the instruments. I reset the speedometer pointer to zero (with the ignition ON) and reinstalled the speedo. I turned off the ignition and turned it back on. Still reading zero. I started the car. Still reading zero. I did this three times. The third time I heard a "click" and the speedo needle reset itself to MINUS 40 kph when the car was stationary.

Seems like there must be an error in the speed sensor?

 

[irc]

Still sounds like an earth problem.
I had a speedo needle problem a couple of months back, had never had one before in quite a few years od Panda-ing.
Then I had another 2 weeks ago (not my car). Car had new battery, good secondhand alternator (from me, so I know it was good when supplied!), and it still randomly reset the clock when starting. This was its first visit to me, so I was a fresh pair of eyes on it.
It had the main earth lead replaced by the owner a month ago, and I double checked it was good.
Decided to re-check everything anyway.
The car had a new clutch last year and someone had left the small earth cable to the ECU holder loose.
It was the only thing out of place that I could find.
I fixed the speedo needle, and it hasn't done it again since (but its only been 2 weeks).

 

[PandaEs]

Thanks, irc---
I'm going to need the Panda for the next two weeks, but then I'm taking it to a mechanic. This is getting beyond my feeble skills. Amongst other things, I'll have him check the earth. You sound as if you are a very conscientious mechanic, checking and re-checking everything. I have doubts about the average mechanic (mine). I can only hope he takes an interest.

 

[irc]

.You sound as if you are a very conscientious mechanic, checking and re-checking everything. I have doubts about the average mechanic (mine). I can only hope he takes an interest.

Sadly the reason I end up checking and re-checking everything is often because "average mechanics" have been there first.

 

[koalar]

Battery when the ignition is off: 12.57 volts. So the new battery is good. No surprise there.

Resting voltage is good

This does not test the battery under load

What's it drop down to while cranking

Red line starts at 12.6V but drops to 6.9V during cranking, this car has a faulty battery that fails in cold weather

1000 rpm: 14.20 volts. So the alternator should be good, too

14.2 @ 750 rpm is correct

Switch the main beam, rear heaters, fog, blowers on full,

It should lower a bit, and if you raise the revs to 2000K it should come back to 14.2V

The third time I heard a "click" and the speedo needle reset itself to MINUS 40 kph when the car was stationary.

Seems like there must be an error in the speed sensor?

Nope

It's a problem with voltage, it messes up the CANBUS signals and forces the needle to bounce on the stops

Still sounds like an earth problem

Correct

The potential between ground and power has to remain above 6.5 volts

Either the battery or cables aren't supplying enough volts while under the heavy load of the starter

Or the ground has risen above 0V due to a poor return path

The car had a new clutch last year and someone had left the small earth cable to the ECU holder loose.
It was the only thing out of place that I could find.
I fixed the speedo needle, and it hasn't done it again since

Slightly strange, but nothing surprises me, I guess if it there but loose it could cause noise on the 0V line

Fiat does everything it can to supply the ECU with power while cranking, It requires slightly less than the body computer, that includes several earth's in the wiring harness as well as the black cable either on the valve cover or under the ECU screw, I have seen an extra one fitted for some reason

If you set the time correct does it's starts flashing after cranking