My 1991 Cadillac Seville speedometer sometime reads high or double. Shows MPH not Metric? Is it controlled by ECM?

More than likely a broken wire, corroded connector or a chafed wire.

What module did you replace ? Was the vehicle transmission shifting OK ? The gauge is controlled by a stepper motor inside the instrument cluster , GM had bad stepper motors 03 to 06 .That is probably your problem .

Speedometer
The instrument panel cluster (IPC) displays the vehicle speed as determined by the engine control module (ECM)/powertrain control module (PCM). The ECM/PCM converts the data from the vehicle speed sensor to a 4,000 pulses/mile signal. The IPC uses the vehicle speed signal circuit (4,000 pulses/mile), class 2 message from the ECM/PCM in order to calculate the vehicle speed. If the IPC detects a malfunction in the vehicle speed signal circuit from the ECM/PCM, the IPC uses the class 2 message from the EBCM, indicating the vehicle speed as a backup. If the speedometer reaches (108 mph) the fuel will shut-off, and it will turn back ON when the speedometer is at (106 mph). The speedometer displays either miles or kilometers as requested by the activation of the english/metric button on the driver information center (DIC). The associated indicator (km/h or mph) illuminates.

How to repair GM Instrument Cluster Speedometer Gauge Stepper

Scan the system for fault codes, you could have a defective speed sensor or a defective speedo both are possible.

b120 121 & 122 are a twilight delay & panel dimming codes. Probably a problem with the switch. b 441 is a cooling fans problem. b 552 is caused when the battery cables have been disconnected or power loss to the ECM. Keep alive memory error.

i have seen this happen in other makes also, that fast idle is controlled by the ecm, if it annoys you talk to the dealer, otherwise its not hurting anythig

is the battery weak?if good ck all grounds,then ck voltage from batt. down to starter.also make sure solenoid isnt loose on starter case.If you did bypass and it cranks then neutral switch or ignition switch could be worn.good luck

What is the engine RPM when this happens?

You need a new speedometer cable

Replace the insturment cluster or send it out to be repaired. 88 89 90 91 will fit.

It may sound like a bunch of different problems but when in reality it could be just a VSS - vehicle speed sensor . The PCM/ ECM use's this information for shift point's , speedometer setting an someother systems on the vehicle . When you say it cut's off an will not start like it has no fire , what do you mean . Explain ! When you turn the key to start the starter engages the engine an spins it over but does start an run ? Or when you turn the key to start nothing happen's ? Traction control wouldn't work either with a bad VSS . You should take it to a qualified repair shop that has a professional type scanner to view codes in all the modules . If when you turn the key to the start position the starter does engage the engine an spin's it , no key or lock cylinder problem