Abnormal CASIS affects speedometer

[Wei2go]

Hi guys:

I've been try to find why my speedometer dies all of a sudden from time to time for months. First I tried to check the ground points but still didn't solve the problem. Lately I found out whenever my speedometer die suddenly, my CASIS light(upshift indicator) on the tachometer also stop working. I know upshift indicator will also comes on when engine speed reach to redline. So I step the clutch and rev it to the redline when the car is moving, and my speedometer come back to alive immediately. So know I know it has something to do with the tachometer and CASIS.

Is there anyone has the same experience as me? and please kindly give me some advice what should I do next. Of course I'm gonna remove the rev counter first and check all the wires and contact point. What else can I do?

Does it has something to do with the voltage regulator as well?

My car is a '89 Carrera coupe USA spec. with no modification on engine and ECU.

Thank you guys!!!

[Chuck.H]

The CASIS is a complicated light that I understand little about; I won't let that stop me from commenting. First, connections:

1) The DME has two wires to the Tach (pin 4 and 11) for RPM and CASIS.
2) The DME does NOT know vehicle speed, nor what gear you are in.
3) The vehicle speed sensor goes to the speedometer AND the Tach.

From what I understand, the DME triggers the Tach to tell it the RPM to display, and a 'CASIS'suggested' trigger. At the same time, the Speedometer tells the Tach how fast you are going. I think the only decision made by the Tach is to not turn the CASIS on if it determines from engine speed and RPM that you are in fifth gear.... otherwise it just lights it up using the signal from the DME.

Remember, the CASIS serves two functions - one to help you upshift for max torque (near readline) and also to help gas mileage when accelerating slowly. Try this, drive slow in fourth gear, see the CASIS, and press on the accelerator. CASIS should go out, thinking you're ready for fun, and it's switching to helping you upshift with spirit.

I'm sure this has errors, corrections welcome!

Now, about your problem -- I would of course check your voltage during testing -- keep a voltmeter in your car for this. Also, check the engine speed sensor in the trans, and it's connection to the Speedo/Tach.

Chuck.H
'89 TurboLookTarga, 369k miles

[McLaren-TAG]

I'd first start with unplugging and cleaning out the connections to the round plugs in the cabin. I've had all sorts of issues from no start to the speedo dying when I've been under there and inadvertently loosened one of them.

The one that I know directly affects the speedo is about 3/4 of the way up on the firewall if you project straight back from around the same level where the heater controls would be.

Unplug it, clean it out, spray it with some dielectric and plug it back in.

[Wei2go]

Thank you, all. I'll work on it this weekend.

[Iracema1]

I have the identical problem with my 1989 Targa. Did you get this resolved? Thanks!!

[dentist90]

The DME processes impulses from the speed sensor attached to the side of the differential housing and converts it into a signal for the speedometer. The DME also uses input from throttle position, rpm and load to determine when to illuminate the CASIS light. Hopefully it is not the DME pooping out. In addition to checking the speedo connections in the front, check the sensor on the side of the tranny and it's connection in the tunnel as well.

This one:

[gamin]

I never thought the casis light gave me any useful information. A waste of space on the tach face. So I sent the tach to N. Hollywood and had a voltmeter installed in place of the casis light. Now I get useful information. Looks really neat too.