Speedo cuts out...

[1989k2500]

during a drive, the speedo will just stop working. It's a bit like someone pulled out a pin and the display would just sea-saw down until it totally disappeared. (i don't know why this type of cluster is called, but instead of dials it has these graphs that would show the temp or fuel or speed.

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so question number one is... what could cause the gauge to just drop out? It works again when I stop and restart the truck.
I don't think it's the sensor , as a sensor would usually just stop entirely. Also I took it out and cleaned it. It looks to be alright.
I think it's the gauge itself. But what causes it and how to remedy the situation?
question number two is. Can I replace the cluster for a later model that has actual dials with a pointer?
Like the one below? is it a straight swap , if possible?

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[1989k2500]

anyone?

 

[Bryan Wochna]

When the Speedo cuts out, does anything else happen? Like does it drop to first or second gear and not go higher?

 

[1989k2500]

When the Speedo cuts out, does anything else happen? Like does it drop to first or second gear and not go higher?

I have a manual transmission, so nothing really happens but the speedo dropping. I noticed that after a while the check engine soon light would come on. (probably as the ecu detects no speed. Another thing I noticed is that the speedometer graph would spin 3 to 4 times pretty fast, when I turn the key.
Maybe it's a faulty cluster? in that event Can I replace it with one from a later model that has dials instead those moons?

 

[PlayingWithTBI]

i don't know why this type of cluster is called, but instead of dials it has these graphs that would show the temp or fuel or speed.

They're called "Moonie" gauges. You can replace them with the newer style but I think you will have to re-pin the connector. You can also go with Dakota Digital gauges but they're pretty expensive. I got this one when my Moonies went out.

https://www.dakotadigital.com/index...t_id=159/category_id=784/mode=prod/prd159.htm

 

[Mark Gilbert]

What might be happening is the DRAC module also known as a VSSB module could be going bad.

The speed sensor on the transmission sends the signal to the DRAC and then that translates it (there are jumpers on it that can be manipulated to to account for tire size and gear ratio) and sends a signal to the PCM and the gauge cluster.

http://www.gearhead-efi.com/Fuel-Injection/showthread.php?1734-VSSB-module-pinout

That will have the pinout for the DRAC/VSSB

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That should also be the pinout for your PCM

The Permanent magnet generator in the drawing is your spped sensor in the transmission

EDIT:

I did a little more reading and had forgotten that some of the clusters had the DRAC as part of the speedo cluster instead of a separate component... So this might be useless info for you

 

[PlayingWithTBI]

Yeah 88-91 have the signal converter as part of the speedometer. It's behind it.

http://67-72chevytrucks.com/vboard/showthread.php?t=313815

 

[DerekTheGreat]

If you hook a scan tool to the OBD-I port you can see if the speed signal is reaching the ECM when the drop out happens, and see if that code corresponds to that. The signal goes from the sender to the cluster, then the ECM. If you find out the cluster is bad, you can try swapping a different one in from a junkyard to see if that fixes it.

You can convert to the needle style gauges but you need the 1990.5 specific cluster to do it. What is special about it is that it retains the same harness as the moonie cluster and also has an internal DRAC module. They're pretty spendy and hard to look up via Ebay. I snatch them up whenever I find them in yards. If I had two, I'd offer you one.

 

[1989k2500]

@DerekTheGreat that 1990.5 dash , does it look the same as the moon circle cluster, but with dials? I think I saw one at the local parts guy.

 

[Mark Gilbert]

Could you also buy the newer gauge setup and then wire in a DRAC module?