Every once in awhile, a question comes up that I actually feel qualified
to chime in on. I am an electrical & control system engineer with 25
years in power plants, and relay logic is a second language to me....
Check your wiring descriptions again- the description for the 'new'
relay doesn't match the terminals you said it had....
Where did you get the information that this relay was a drop-out on low
voltage? If you are translating, you should double-check. The
information I can find only deals with the standard VW setup that
disconnects the aux battery when the alternator is not providing
charging current. Does the 11 volts refer to the alternator output?
That would make more sense, but after looking at the codes and wiring
description you gave, there is still some confusion. I pulled out one
of my generic 30 amp replacement relays, and it has 85 & 86 for the
coil, 30 for the common of the relay contacts, 87 for the normally open
terminal (not connected to common when the coil is not energized) and
87a for the normally closed relay contact (connected to common when the
coil is not energized).
The DIN codes for these are:
30, input from + battery terminal, direct
85, Output, actuator (end of winding to ground or negative)
86, Start of winding
87, Input
87a, 1st output (break side)
In the Westy 2-battery wiring, 86 goes to the 61 (alternator light)
terminal; 30 comes from the B+ or hot of the main battery; 87 goes to
the aux battery; and 85 is grounded. This disconnects the aux from the
main battery when the engine is off, connects them to charge the aux
when the engine is running. 87a is not connected.
I looked up the code numbers on your relays in the list of standard DIN
designators. VW has used these designators forever, every one I have
worked on uses the same codes and most modern Japanese and American cars
do too:
Original DIN Codes:
51, DC voltage at rectifier
61, Alternator charge-indicator lamp
86, Start of winding
87, Input
86 should be the coil- are you sure this one is connected to the main
battery positive? This seems like it would always be energized....
There is no 85 terminal- presumably it grounds through the mounting tab
(or other screw) to the body. Alternatively, it could be that the 61
terminal is the other end of the coil- the alternator/generator light
terminal is ground when the engine is not running, which would energize
the relay when the engine is off. Possibly, this would connect all
loads to the aux when the engine is shut down, but I don't get how the
charging takes place... without the rest of the circuit, I can't be sure.
If the wiring is as you say, I'd have to say that this may be a
'special' relay, usually called a voltage sensitive relay, that has a
fixed drop-out voltage of 11 volts and a pickup voltage near the
charging voltage, so would disconnect the aux when the system voltage
dropped to 11 volts and reconnect it when the system voltage was about
12.8-13 volts. It would not be replaceable by a 'standard' disconnect
relay. I have never seen this arrangement on a VW, though.
New DIN designators:
30, input from + battery terminal, direct
85, Output, actuator (end of winding to ground or negative)
86, Start of winding
87, Input
The new relay uses codes that tell you what the relay does, it is
presumably generic for the relay. The 85 & 86 terminals are the coil.
One (85) needs to be grounded, the other needs to go to the charge
indicator light (old 61). 87 should connect to 30 when the relay
energized- 87 would be the aux battery, 30 the alternator output (or
main battery positive terminal, which are electrically the same. (this
is where your description is confusing- looks like you have more than 4
terminals?? Is the one you called 51 actually the 30 terminal?) This
arrangement is then just like the late bus, with the aux disconnecting
from the main battery when the engine is off. This arrangement works
very well and would be a good alternative. Power all the auxiliary
loads from the aux, and they will be supplied by the alternator when the
engine is running, but won't pull down the main battery when the car is off.
Hope this helps- if you can find out any more, let us know and we can
try to refine this.
Glen Hadley
On 3/15/2013 10:11, Bert Knupp wrote:
Battery isolation relay
Volks,
I need some help. Various VW models over the years have used
dual-battery systems: campers, sound trucks, fire engines,andpolice
cars to name a few.The two batteries are connected via an isolation
relay (in German it's a "Batterie-Trennrelais"). The idea is to permit
both batteries to charge from the generator or alternator, but to
"uncouple" the #2 equipment battery when the voltage drops below 11
volts, making sure that the #1 or starting battery doesn't get pulled
down by the equipment. For example, in the police cars, it allowed the
car to sit working an accident with the blue light, flashers and radio
running but the engine off. If the available voltage dropped below 11
volts, the starting battery would disconnectso the car could start
when done.
So I'm recreating the Copbug's two-battery system. I've mounted the
#2 battery under the left rear seatand found a 75-amp Bosch isolation
relay on-line. The problem: the four terminals on the new Bosch
relay don't match the four terminals on the VW factory bulletin for
police-carwiring. I'm usually pretty good at logicking-through
circuits, but I'm stumped here.
The factory bulletin shows an isolation relay with four terminals:
51, 61, 86 and 87.
61 comes from the 61 terminal on the voltage regulator. Skinny wire.
51 comes from the B+ terminal on the voltage regulator. Fat wire.
86 goes out to the #1 (starting) battery(+). Fat wire.
87 goes out to the #2 (equipment) battery (+). Fat wire.
The new isolation relay comes with four terminals also: 85-, 86+, 30
and 87.
The 30-to-87 circuit seems to be the switch that opens and closes.
The 85-to-85 circuit seems to be the coil activation.
The 87 and 51 terminals are high-amp screw terminals.
The85 and 86 terminals are low-ampFastonslip-on tabs.
But I can't figure what's what. Can anybody help me? How do I hook
up the new relay to do the job?
I've written to Bosch, but I won't hold my breath. The last time I
asked them for help, the reply came 5 months later -- and they said
they didn't have information on the old equipment. Aargh!
Bert Knupp in Music City USA
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