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I need your help badly.
My speedometer has a Gremlin causing it to read over 100 when traveling around d 60!
This car is a bone stock 3.8 with automatic transmission
I notice that the odometer is working correctly when measuring out miles.
I suspect the signal from the VSS is getting corrupted somewhere in the solid stat circuit on the cluster.
I base this conclusion on the following observations:
VSS is providing correct signal thru PCM based on the fact that the odometer is working correctly.
I am just stumped as how to diagnose the electronic components on the cluster. Especially since I can find no detailed schematic diagram outlining the gauge cluster in detail.
Please help.
Last edited by wssix99; Dec 22, 2023 at 08:03 PM.
Reason: removed quote from old thread
I am just stumped as how to diagnose the electronic components on the cluster. Especially since I can find no detailed schematic diagram outlining the gauge cluster in detail.
Please help.
If the car had a cable connecting to the speedometer, the odometer might be useful. In this case, its all electronic and these pieces are not necessarily tied together.
The best route in diagnosing this is to use a real-time scanner and compare the RPM and speed readings from the PCM to what you have on the cluster. Take readings across a wide band of speeds and graph it out. This should indicate what is happening.
If the car had a cable connecting to the speedometer, the odometer might be useful. In this case, its all electronic and these pieces are not necessarily tied together.
The best route in diagnosing this is to use a real-time scanner and compare the RPM and speed readings from the PCM to what you have on the cluster. Take readings across a wide band of speeds and graph it out. This should indicate what is happening.
I hope to be getting a more advanced scan tool at Christmas.
I’ll be curious to see if the OBD2 and info from the PCM on this old car will provide such info.
So let’s say all of the info coming from the VSS thru the PCM is correct. Then what?
I will still need to look at doing a repair to the cluster.
Agree? My suspicion is that one or more components are bad. It’s pretty clear to see that there are a series of just a few controlling the stepper motor signal.
A bad stepper motor is not out of the question, but I haven't seen one on this site. An out-of-alignment needle is the more common cause. Either way, a real-time scan will indicate what is going on.
When the needles are put back on the cluster, the car needs to be on so the cluster is energized and the speedo can properly be set to zero. (If pressed on while off, the needle can be way off zero.)
I used an advanced scanner to confirm what I already thought.
The VSS is providing the correct info to the PCM. Vehicle speed showing on the scan vs actual speed measured by GPS and MPH vs miles traveled is dead on.
Tried at multiple speeds and distances.
The actual speedometer however was all over the place. At 60 read 100 +- same at other speeds. At no time did the actual speed shown thru the PCM OB2 to scanner match the speedometer readings.
However, when measuring distance and matching the odometer reading, it was dead on.
The cluster has either a bad resistor, capacitor or other electronic component.
Which one of the components shown in the pic would cause such a condition if it was failing?
The Air Core gauge motor has already been confirmed as good by switching it with the tac.
My challenge goes out to any electronic geek or tech out there that can lead me in the right direction.
The small electronic components are rather cheap and I can easily remove and solder a replacement in. I would just like to have an idea of where to start first.
Thanks to all.
With the information we can see here, there's still no proof that the speedometer is bad. Do you have a full data set of the real speeds vs. the speeds the speedometer is showing you? "all over the place" is not something we can analyze.
Having the PCM reporting the same speed as GPS calculates is good and rules out most causes away from the dash.
Has this car ever behaved normally since you have owned it? Is this something that just started recently? Did you change anything on the car or repair anything at the point the problem started? (If this car is new to you, there are other questions we might ask.)
With the information we can see here, there's still no proof that the speedometer is bad. Do you have a full data set of the real speeds vs. the speeds the speedometer is showing you? "all over the place" is not something we can analyze.
Having the PCM reporting the same speed as GPS calculates is good and rules out most causes away from the dash.
Has this car ever behaved normally since you have owned it? Is this something that just started recently? Did you change anything on the car or repair anything at the point the problem started? (If this car is new to you, there are other questions we might ask.)
Speedo has not behaved normally since owned. I spoke to previous owner and found it started about a month before I bought it.
OK. Did the Previous Owner give you any information about things that might have changed when it started acting up?
Can you share the data you collected? Is the behavior linear? The spread of the actual vs. indicated speed over a range can give some insight to what is broken. If the gauges are the problem, I would guess that it's more likely that a stepper motor is bad vs. something on the circuit board. (The circuit board is very simple and there's nothing fragile there.)
OK. Did the Previous Owner give you any information about things that might have changed when it started acting up?
Can you share the data you collected? Is the behavior linear? The spread of the actual vs. indicated speed over a range can give some insight to what is broken. If the gauges are the problem, I would guess that it's more likely that a stepper motor is bad vs. something on the circuit board. (The circuit board is very simple and there's nothing fragile there.)
As far as the data what would you like to see and in what format?
Stepper motor is not used on this. All the motors are Air core type. I eliminated that possibility by switching the tac with the speedo which were the same part numbers.
Once the motors were switched nothing changed. The tac worked fine and the speedo was the same.
Update: I had an electronic specialist checked out the circuit and confirmed that all of the resistors were good. He said he couldn’t check out all of the other components without removing them from the board.
He also switched out the black IC Chips from the tac side. No change when installed back into the car.
At present I am considering the purchase of a good user cluster. If it takes care of the problem then I will take meter readings across the boards and compare in an attempt to determine what if anything is going on with the original board.
The speedo saga continues.
As far as the data what would you like to see and in what format?
However you have it. When you drive the car multiple times, are the actual vs. indicated readings the same?
There are specialist companies that check clusters out and repair them. If yours is really broken, you'll need one of these shops do do your odometer and provide your State certification for that work.
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