1987 Mercury Cougar SPEEDOMETER

MALLEN11050

MEMBER
1987 MERCURY COUGAR
6 CYL
2WD
AUTOMATIC
126,000 MILES

MY CAR HAS THE DIGITAL SPEEDOMETER WITH THE ANALOG SIDE PANELS. ALL OF THE DIGITAL PARTS WENT OUT. I CHECKED ALL FUSES INCLUDING THE FUSEABLE LINKS. CHANGED THE DASH PANEL, CHANGED THE SENDING UNIT IN THE TRANSMISSION, THE INGNITION SWITCH AND CHASED EVERY WIRE FOR BREAKS. THE ONLY THING I DID BEFORE IT WENT OUT WAS CUT A DIODE OUT OF THE LINE GOING TO THE AIR CONDITIONER COMPRESSOR UNDER THE HOOD TO HOOK UP A DIRECT POWER FEED TO THE COMPRESSOR. THIS IS DRIVING ME CRAZY! EVERYTHING ELSE ON THE CAR WORKS GREAT. EXCEPT FOR THE CRUISE CONTROL (IT DID WORK BEFORE THE SPEEDOMETER WENT OUT). All back lights work properly on dash and I have no tach in dash to read. I did not reinstall the diode because it was only going to the air compressor.

1 Reply

KHLOW2008

MECHANIC
41,814 POSTS

Hi MALLEN11050,

Thank you for your donation.

You mentioned the diode and did you reinstall it?

The cruise control needs signals from the speedometer to function.
Is the tachometer working correctly?

Is there any backlight when the ignition is turned to "RUN" or when the headlights are turned on with dimness control turned to maximum?
If no, remove instrument cluster and reconnect battery negative cable.
Measure the voltage to bulbs, if reading is more than 10v, there is an open in illumination ground circuit, PIN 12B.
If reading is below 10 v, repair illuminations circuits, fuses or relays.