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Old 09-07-2012, 06:06 AM   #1
willo1976
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Engine shakes when brake lights on

I have a strange problem, in that, when my brake lights come on, the whole car shakes, and revs drop. its a mk1 1999 1.6 16v zetec focus.

List of things i have done.

Brand new alternator (80amp)
Brand new battery leads & and new live lead to fuse box
Renewed all earths around car
New battery (silver calcium)
New MAF sensor
New idle control valve
New throttle housing
New wiring loom to rear lights
New Coil pack
New Leads
New Plugs

Reading from battery using multi meter

Battery: 12.8v
On Tick over: 14.5v
Everything switched on: 14.v
Smart charge disconnected: 13.9v

I am at a loss now, on what to do next.
Also if car is at stand still say at traffic lights, and window wipers are on intermitent setting, everytime wipers start, revs drop and engine shakes.


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Old 09-07-2012, 06:18 AM   #2
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That would seem to be an "earthing" problem to me. Just because the battery/alt are good doesn't mean that you're getting all the power you should everywhere else. There are supplemental earth connections to the body of the vehicle and the engine that need to be maintained. I'd pull the cowl and see if I could find the earth for the wipers first, verify it was good, then start checking other earths. Ford should be running a separate earth to all the electronic parts to cut down on this- so you might start at the battery negative terminal first. Check the wires that come off that terminal to be sure those are well connected at both points. One should go to the engine block, the big one, and then there should be 2: one will go to the chassis in the engine compartment, and the other will go in the harness for everything else in the car. This one is typically bonded to the chassis at various points- like near the fuel pump module under the back seat.

A less likely possibility is to check that individual circuit for problems. You might have a nicked wire that doesn't short out, but is using more power than it should be and dragging the whole system down. There is also a procedure for using the electronic odometer to monitor battery/alt voltage while driving. Your should be the same as ours- or so I've read: engine off, key off, press and hold odometer reset button, turn key on, hold button until odometer reads [test], release. Now each time you press and release, you'll scroll through one of 30 test functions. You should look for the one that reads [bat 12.3] and expect the voltage it reads to be about .2v less than a reading taken from the battery terminals. The odometer will remain in test mode until the key is turned off, so you can start the car and watch the voltage while you drive- or turn on the wipers.
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Old 09-07-2012, 06:42 AM   #3
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Thanks for your replie.

The earths are all good, all new leads under bonnet, and new one from battery to engine block. and added more.

All earths in kick panels are all good, earths in boot are sound.

What i have noticed tho, is if i disconnect earths on back lights, the lights still work.

I would post pics, but it wont let me upload them.
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Old 09-07-2012, 06:58 AM   #4
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Old 09-07-2012, 01:29 PM   #5
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Under the increased electric load placed on the alternator, it is trying to stall the motor at idle.

The PCM is supposed to actuate the Idle Air Control during such events.

Does the idle speed drop during other events where the IAC is supposed to compensate for the increased load, or does it maintain idle speed fine?
-If the power steering pressure is high but engine speed is low.
-If the AC is turned on
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Old 09-07-2012, 04:06 PM   #6
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I am having the same issue, I will report my results when I get my car back into the yard.
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Old 09-07-2012, 04:25 PM   #7
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Check any wire loom going from the car body to the trunk/hatch. You drive a 99, so likely there are broken wires in the loom, or atleast cracked insulation, and when the brake lights are powered on, there may be arcing causing your problem. We have seen it many times here.
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Old 09-07-2012, 06:47 PM   #8
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Quote:
Originally Posted by elsolo View Post
Under the increased electric load placed on the alternator, it is trying to stall the motor at idle.

The PCM is supposed to actuate the Idle Air Control during such events.

Does the idle speed drop during other events where the IAC is supposed to compensate for the increased load, or does it maintain idle speed fine?
-If the power steering pressure is high but engine speed is low.
-If the AC is turned on
On tick over, if i switch on heater full or headlights, revs drop to point of stall but pick stright back up, and ticks over, all be it with a little engine shake.

if on tick over i turn power steering engine revs drop and pick straight back up.
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Old 09-07-2012, 06:48 PM   #9
willo1976
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Quote:
Originally Posted by f250lightnin View Post
Check any wire loom going from the car body to the trunk/hatch. You drive a 99, so likely there are broken wires in the loom, or atleast cracked insulation, and when the brake lights are powered on, there may be arcing causing your problem. We have seen it many times here.
I have renewed wiring loom, so that eliminates that.
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Old 09-07-2012, 06:48 PM   #10
willo1976
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Quote:
Originally Posted by WrinkledCheese View Post
I am having the same issue, I will report my results when I get my car back into the yard.
Thank you.
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