Voltage spikes and relays

Brett Dikeman brett at cloud9.net
Fri Feb 29 10:28:43 PST 2008

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.


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