Faulty Tachometer

Before we start'yes I did a search first.Has anybody had their factory tachometer read fast.I hooked up my old scan
tool made by OTC and its a model #4000 "advanced.The scan tool shows the tach reading and its exactly 1000 RPM
lower than the dash reading.When it drops to idle dash tach shows 1100 rpms and the scan tool shows 550 rpms.
Thought that I had lost the TCC lockup on the interstate but scan tool shows the trans shifting perfectly as is the
torque convertor locking and unlocking.Anybody got any idea why the tach is now reading so high?
Thank You aahead of Time
Jim Gordon

Jim, do you have a Caprice or a Roadmaster? If a Roadie, you can read the RPM on the climate control (what the PCM is seeing) to double check everything.

Sorry Jay I have a 91 Olds Custom Cruiser and it has the same dash as the Roadmaster. The only difference is mine has
the red needles instead of white ones.Dont think I can access the tach thru the Climate Control like you can on the
newer systems.
Thank You
Jim Gordon

The electronic board on the back of the cluster is probably toast. I had one acting up, and used the board from a 91-93 RMW/S. They all are the same, except where some of the lights are. Remove/add all the lights that are different. Easy fix.

The face of the dash is different too. There are small lines under the peripheral gauges. The OCC face is more elegant looking.

Also check the larger plug on the ignition module at the distributor for insulation loss. I had 1/2 inch of insulation missing on all 4 wires, and used "liquid tape" to fix it.

I have a similar issue on both of my OCCs. One reads about 2X the actual RPM, the other one pins to the right as soon as you start the key.

As Fred mentioned, time for a new circuit board, I have a couple of Roadie TBI clusters, just haven't gotten around to taking it all apart (will wait until I have another reason to do so).

As long as you know your difference it's still good right? Unless it bugs yyou that much.

jayoldschool wrote:

Jim, do you have a Caprice or a Roadmaster?  If a Roadie, you can read the RPM on the climate control (what the PCM is seeing) to double check everything.

Just a newbie here, hopefully not a dumb question. So how do I display RPM on the climate control? I have a 96 roadmaster.
Thanks

Try this:

https://www.youtube.com/watch?v=npIFgZKYRcI&feature=youtu.be

thanks