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Hello my fellow astrosafarists,

I have a 97 GMC safari, have had the same problem since I bought it, currently at 170k miles, changed tires a few months ago but no variation in speed reading. Is this something related to the speedometer itself ? Or if is not how can I correct this ?

Thanks in advance for your help.
 

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'93 Astro LT Ext w/ FE3 (RWD), 7 Passenger, Dutch Doors, Med Teal, 4.3 V6
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238 Posts
I wonder if a previous owner had it calibrated for larger diameter tires, then put the originals back on it when he sold it.
 

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we've owned it since '99 - lots of miles and memories in The Phantom
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@Dominican - what great timing!

I have a '96 with the same issue - its been driving me nuts (no pun intended.)
I'm not trying to hijack the thread - hoping to consolidate info for everyone that might have this problem.
(moderators/admin - if you determine that i need to post my own thread, just let me know - thanks.)

@Rustbelt_mVan - I have connected to the OBDii port and monitored live data of the VSS reading while having the driver call out speeds in MPH - here are the (approximate) results:

SpeedoOBDii VSS readingGPS (android phone)
302626
403435
504444
605354

Just for giggles, we increased our speed until the GPS read 60 MPH - at that point the VSS was reading 60 and the speedo was reading 69.

All the other gauges read accurate - ECT matched the gauge at the end of the test, voltage gauge was consistent with the reading on the USB charger plugged into the dash and the fuel/oil pressure seemed fine without any huge fluctuations.

So it is not an issue with tire size (they are stock and the VSS is accurate.) I don't believe it is an issue with ground to the cluster, because the other gauges read normally. I am totally at a loss for what the cause is.

We have owned this van since early 1999 the speedo has always been accurate until a few years ago when my son was driving it as a daily driver. The only 'incident' that i can recall that may contribute was when we swapped the battery once - the speedo pegged over to the other side of the "zero pin" and we had to take it up to 70 (speedo 70) to get the needle to swap back over to the other side of the pin. As I recall, the speedo was already problematic when that occurred - but who knows.

Is there a transistor/resistor something within the cluster that may have popped?

Any thoughts/help would be much appreciated - else I keep driving at 69 whenever I'm on the highway. :)

Thanks,
Jon
 

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we've owned it since '99 - lots of miles and memories in The Phantom
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Sounds like someone changed the rear end gearing...
Except, we've owned the van since 99 and have never done anything with the rear end gears.

Also, The VSS is reporting the correct speed, as confirmed by GPS. This indicates the PCM is calculating correctly from the pulses/min it gets from the sensor. If the gears has been changed, the PCM calculation would be off.

I'm convinced it has to be something in the cluster, because the VSS is accurate and the cluster is supposed to simply display the calculated speed.
 

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'93 Astro LT Ext w/ FE3 (RWD), 7 Passenger, Dutch Doors, Med Teal, 4.3 V6
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Except, we've owned the van since 99 and have never done anything with the rear end gears.

Also, The VSS is reporting the correct speed, as confirmed by GPS. This indicates the PCM is calculating correctly from the pulses/min it gets from the sensor. If the gears has been changed, the PCM calculation would be off.

I'm convinced it has to be something in the cluster, because the VSS is accurate and the cluster is supposed to simply display the calculated speed.
Is there a reset procedure? Something like this:


Or maybe just disconnect the battery and wait a few hours for the ECM to reset. I read that on some cars you disengaging the negative battery cable, then step on the brake pedal for a few seconds.
 

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we've owned it since '99 - lots of miles and memories in The Phantom
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No real reset procedure that I can find on the interwebs and I just did the lower intake manifold gasket, had the battery out of the van for almost 2 weeks, no luck. :(
 

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Except, we've owned the van since 99

I'm convinced it has to be something in the cluster, because the VSS is accurate and the cluster is supposed to simply display the calculated speed.
In your case I agree, the gauge itself in the cluster, because of the "incedent" you mentioned. You know the cause, where, when, how and so forth. It has been off since that time.
Dominican has not replied. If he bought the van with that issue it is very easy to determine what gear ratio either by reading the ring gear itself or counting revolutions and doing the math.
 

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we've owned it since '99 - lots of miles and memories in The Phantom
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yeah, I'm not 100% sure the battery swap caused it, I think it was trading off before then.

Thanks for the confirmation that I'm not crazy.

Anyone have any ideas on fixing the cluster? or on what might be bad inside of it?

anyone have access to schematics?
 

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Common Sense + Critical Thinking
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Sure sounds like it's just the gauge, does the mileage match up to mile markers on the highway?

Maybe pull the indicator and pop it back on at the correct speed, on jack stands of course while watching the OBD2 data.
 

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2000 Lifted 4x4 Astro 92 V8-350 Shorty
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Mine is off (reads low) almost exactly by 10% due to oversized tires.
  • If speedo says 50... I'm doing 55
  • If speedo says 60... I'm doing 66
  • If speedo says 70... I'm doing 77 .. and so on (easy to calculate).
To correct, I need to go from 3.42 gear, to 3.73 gear.. almost a matching 10% increase in gearing.
I understand there may be a way to make correction electronically as well.
You can get FREE GPS speedo APPs for your phone if you want to calculate your numbers... they are quite accurate.

Of course, none of this has anything to do with a faulty gauge cluster.

Here's a cool on-line calculator:
 
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