Can anyone tell me how I would go into Diagnostic Mode on an 88 Deville? I want to be able to look at the engine rpms.

Thanks

Press "off' and 'warmer" at
the same time and hold till the codes display. Then use hi and low on the fan speed to say yes and no. Hi is yes , low is no. Best if car is running when you do this . Turning the car off will return to normal climate control display. Buy a factory service manual

OK thanks, I had gotten into the diagnostic mode before, I just don't know what the code was that I was supposed to press "yes" on for the tach.

Actually, you need a little more/different info on the procedure to access the RPM data on an 88 Deville.

Enter diagnostics mode as described by simultaneously pressing and holding the OFF and WARMER buttons while the engine is running at idle in park. After any codes display you will see the "system ready" indicator which is .7.0 on the display.

After the .7.0 displays, press the LO button. The display should respond with E.9.0 which indicates it is in the "PCM data" mode. After you see the E.9.0 press the HI button to get into the "PCM data-parameters" mode. This will signal with the display showing P.0.1 . Each of the data parameters has a number such as P.1.1 which is RPM on your 88. You scroll thru all the different data parameters from P.0.1 to P.1.7 using the LO and HI buttons. Here is what some of the data parameters you'll see are:

P.0.1 Throttle position in degrees throttle angle
P.0.2 MAP or manifold absolute pressure in kPa
P.0.3 computed barometer in kPa
P.0.4 Coolant temp in degrees C
P.0.5 MAT or manifold air temp in degrees C
P.0.6 injector pulse width in milliseconds
P.0.7 oxygen sensor voltage from 0 to 1.0 volts
P.0.8 computed spark advance commanded to distributor
P.0.9 ignition cycle counter since last reset
P.1.0 Battery volts
P.1.1 Engine RPM
P.1.2 vehicle speed in MPH
P.1.3 oxygen sensor cross counts which indicates O2 activity
P.1.4 closed loop fuel integrator - how much correction is used. 128 is
good. Less than 128 indicates leaning, more than 128 indicates richening.
P.1.5 trans temp
P.1.6 E-cell
P.1.7 Prom id.

You can drive around in the PCM data parameter mode while looking at any of the data without affecting anything else. To exit this mode press the OFF button repeatedly until the system exits diagnostics or just turn off the key if you are stopped and finished driving.

thanks a lot! ill go test it and see if i can get it to come up.

it worked, thanks again

It's always nice when things work like they are supposed to.....LOL.