Voltage spikes and relays

Bernie Benz b.benz at charter.net
Sun Mar 2 10:33:58 PST 2008

IME, all of the several relays used for the A/C clutch contained coil  
suppression diodes, but of poor quality.
They would either fail open or short out. They are easily  
replaceable, BTDT.


On Feb 29, 2008, at 10:28 AM, Brett Dikeman wrote:

> On Feb 27, 2008, at 7:50 PM, Scott Elliot Gilman wrote:
>> My '91 200 TQ has developed intermittent electrical spikes, which
>> causes problems with the audio system.
> What kinds of problems, and when do they occur and how often?
>>  The radio and speakers were
>> checked out, and they are not causing the problem.  The audio tech
>> believes the problem may be a result of a faulty relay.
> There was a TSB on the A/C relay many, many years ago- it didn't have
> a flyback diode to arrest the inductive pulse from the clutch, which
> would cause a loud "snap" noise in the stock Bose speakers.  If your
> noise occurs only with the climate control in one of the air
> conditioning modes, then it might be the clutch relay. (If it's below
> 38 degrees, the compressor won't come on, so this is not a great test
> in the winter.)
>>  Where can I find a schematic diagram showing the relays?
> The factory repair manuals.  Maybe Alldata, which your installer
> should have.
>>  How many relays are  there on this model?
> There are something like 3-4 relay boards in the car- the main
> fusebox, the driver's side kickpanel, the driver's side knee panel,
> and buried in the dash on the passenger side above the glovebox.  The
> biggest are the ones for the cooling fan- they handle a hundred amps
> or so.  There are relays for the stock audio system, seat heaters (I
> think), fuel pump, ABS, you name it.
> If you are having general weird problems with wiring or voltage
> issues, check the condition of the battery cable. It goes between the
> starter and battery, but via a splice- the cable itself goes from the
> battery to the main fusebox.  It is a well known problem for the
> splice to corrode and become a major voltage drop.  The splice is in
> the front passenger footwell.
> Brett
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