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99 Prizm Cruise Control, Mph Won't Stay

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My 99 prizm cruise control(CC) isn't working. When I press the engage button at 50mph, the CC light will stay on but my speed drops

as if I don't have the CC on. Could a simple dashboard light bulb or missing abs relay cause it not to work? Solutions?

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trap

When you say engage button, do you mean the "set"? The only button on the stalk - is to turn on cruise control (you'll see the cruise indicator on the dash come on) - but it won't set any speed. To set your speed, you don't push any buttons, you just tug the cruise control stalk downwards to set the speed. Pull down and hold the stalk to decrease the speed, push up and hold to increase speed. Tapping upwards will bump the speed up in 1 MPH increments or so. Pulling the stalk towards you will cancel the set speed.

 

Sometimes the speed will drop slightly, before it will hold speed. You could try accelerating, via the control stalk, after you set a speed to see if it is talking to the ECM. Tapping the brakes or if there is a signal from the ECM that the brakes are engaged, will automatically cancel the speed setting. Double check that the brake switch is not engaged by accident. Sometimes the switch can be faulty - brakes will still work fine, but the brake lamps might be on all the time.

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When you say engage button, do you mean the "set"? The only button on the stalk - is to turn on cruise control (you'll see the cruise indicator on the dash come on) - but it won't set any speed. To set your speed, you don't push any buttons, you just tug the cruise control stalk downwards to set the speed. Pull down and hold the stalk to decrease the speed, push up and hold to increase speed. Tapping upwards will bump the speed up in 1 MPH increments or so. Pulling the stalk towards you will cancel the set speed.

 

Sometimes the speed will drop slightly, before it will hold speed. You could try accelerating, via the control stalk, after you set a speed to see if it is talking to the ECM. Tapping the brakes or if there is a signal from the ECM that the brakes are engaged, will automatically cancel the speed setting. Double check that the brake switch is not engaged by accident. Sometimes the switch can be faulty - brakes will still work fine, but the brake lamps might be on all the time.

 

I've tried the "just tug the cruise control stalk downwards to set the speed. Pull down and hold the stalk to decrease the speed, push up and hold to increase speed. Tapping upwards will bump the speed up in 1 MPH increments or so." But it does nothing. Just the CC light is on.

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Sounds like something is cancelling the cruise control set feature or it could be an issue with the cruise control stalk itself or some electrical issue. There is a remote chance it is related to the actuator under the hood - but that will require a significant amount of debugging to find the source. I'd start with the simple stuff you can check quickly - then go to the harder stuff.

 

Question: When you try and set the cruise speed with the stalk - does the cruise control lamp "flash" when you do so? If the system is OK, you will never see the cruise control indicator flash or blink, if there is a problem, the indicator will flash or blink.

 

Easiest item to check. Double check that the brake switch is not engaged all the time - can verify by looking at the brake pedal assembly - the switch is located about halfway up. Should be a slight gap between the switch and the brake pedal assembly. Can also verify by stepping on the brakes and noting the brake lights turning on and off.

 

The control stalk and electrical checkout is a little more involved. You'll have to short the Tc and E1 pins on the diagnostic port under the hood of the car (little box close to the master brake cylinder - says diagnostic on the lid). This will trigger the DTC self check on the Cruise Control ECU and flash the trouble code via the cruise control lamp on the dash. Ex. If there a code "21" stored - it will flash the cruise lamp three times in succession - first two flashes will be about 1/2 a second apart, a 1 1/2 second pause, a single flash, a four second pause, then on to the next stored code or repeats the same code if only DTC. To test the operation of the stalk - need to hook up a breakout box to the diagnostic port, set the cruise ECU into test mode and follow a prescribed test. Unfortunately, you'll need access to Toyota test equipment to do this yourself - otherwise, you'll have to drop it off at a shop to have them test it.

 

You can test it yourself with the built-in test feature - but I'm hesitant to post that info, as it could be pretty dangerous. Basically involves jacking the front end of the car in the air while the wheels are powered. Could try holding the stalk up or down, while pressing in the cruise control button ON/OFF button on the end while looking for the cruise lamp to flash several times, all while the car is moving more than 15 MPH - but that only tests a single input to the cruise ECU. But again, pretty unsafe to do on the street - as you'll have to be looking directly at the indicator lamp.

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If it doesn't flash - means that the cruise control ECU didn't see any problems. This is sounding more like an electrical issue to me. Problem is, it could be anything from the ECU logic to the control stalk to the cruise control actuator under the hood and any number of the wiring/connectors in-between.

 

Power (fuses and relays) seem to be OK, as the system is lighting up the cruise lamp. But I would double check the relays and fuses, as well as chassis grounding points - as it could be electrical noise that is causing the problems.

 

This definitely needs to be buzzed out with a Toyota Handheld tester - as you need to get the cruise ECU into diagnostic mode and verify the signals coming from it.

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Forgot to mention, when I press the CC button, I hear a click or a engaging sound but still doesn't work, as in staying at a constant speed, etc...

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That's weird - clicking sound from the button itself or from something under the hood? When pressing that CC button, you shouldn't hear anything, as it is all logic controlled. If you hear anything - that is likely pointing to an electrical issue. Not a whole lot you can diagnosis at this point, without tracing back the wiring by hand. Need to throw a Toyota test tool at it, exercise the CC and see where there is a disconnect in the logic.

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That's weird - clicking sound from the button itself or from something under the hood? When pressing that CC button, you shouldn't hear anything, as it is all logic controlled. If you hear anything - that is likely pointing to an electrical issue. Not a whole lot you can diagnosis at this point, without tracing back the wiring by hand. Need to throw a Toyota test tool at it, exercise the CC and see where there is a disconnect in the logic.

 

The clicking is coming from the CC module. I looked at the module and I don't see anything burnt or that needs soldering.

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