Dealer says my ecu (computer) is bad on my 1999 jeep grand cherokee 4.0 and that a used one will not work. do I really need to buy a $700 one from the dealer?

Could be a sensor shutting off the car, engine overheating, ECU overheating.
You may need to pull codes from the OBD/onboard computer.

Are you sure it's not low on fuel? Any error lights?

Try a repair manual for 'troubleshooting steps'
http://onlyrepairmanuals.com/jeep/jeep-grand-cherokee-1999-2004-repair-manual/

Presumably you are referring to the engine management ECU rather than any of the other black box computers scattered around the vehicle?

It is usual for cars produced in the last few decades to be fitted with an immobiliser. If your car has an immobiliser you need to take advice from the dealer about whether the replacement ECU is ordered with your car's code already installed, whether it arrives with a random code requiring the security system to be recoded or whether the ECU arrives blank and the code is programmed into it as part of the fitting process.

ECU TOPIC (swaps)
why ask us? we can't see car, or see what drive line is there. nor VIN code.(telling same)
why not ask JEEP.?
amazing ?

ever seen this?

http://www.factorychryslerparts.com/productSearch.aspx?&ukey_make=1064&ukey_model=15529&ukey_trimlevel=0&ukey_modelRange=0&modelYear=1999&ukey_category=0&ukey_driveline=0&isOnSale=0&searchTerm=



computer, ,that is the ECU, and is not the only computer.
here are the ECU rules. (not full, only jeep knows all rules)
it must have same base number.
then needs to be programmed to match your cars, drive line and federal smog laws and year.)
end story.

i want a ? xxxxxxxx
the dealer sells them. call them and ask, em.?
fleabay has them used.

or the hope thing.
http://www.ebay.com/sch/i.html?_sacat=0&_from=R40&_nkw=1999+jeep+grand+cherokee+operators+guide&_nkwusc=1999+jeep+grand+cherokee+opeators+guide&_rdc=1

Unshort the shorted circuit.

Usually a bad crank sensor or a bad ecu computer.

If the numbers on the two computer are the same, it should work. If they are different numbers, you may need to take it to a mechanic shop with the proper equipment needed to reprogram it.
I am curious as to why you bought a new computer; Did the old ecu die and not want to light up the check engine light or start the jeep any longer? If so, you could've had it repaired instead of buying another one and now possibly having to pay even more to reprogram it.

The Coolant Sensor is inside coolant bottle; in this models is common ths message failed; clean the plug and reset your ECU alarms, if the message come on again, replace your coolant sensor. And, really, one message about a bad sensor/signal is one bad work engine.

Good luck.

Yes perfectly well.

you cannot remove the covers.
you will have to buy new headlights.
sorry