Solving Problems Using Relays Organization (ISO) And Its .

Technical Document MS-0908AProduced by the Electrical Rebuilder’s Associationwww.electricalrebuilders.orgSolving Problems Using RelaysOrFour Handy Uses for BoschRelaysOrganization (ISO) and its technical name is ISOMini-Relay. ISO Mini-Relays are built into a 1”square cube, and the pinouts are standardized asshown in the following diagrams. In the rebuildingindustry, ISO Mini-Relays are usually referred toas “Bosch Relays”.A relay is simply an electrical device designedto switch a large amount of current using asmaller amount of current. Some, like thefender-mounted starter relays on Ford vehiclesof the 60's through 80's, were made to carryseveral hundred amps for a short duration.Figure 3 – Single-Pole Single-Throw with one #87 terminalFigure 1 – Early (left) and Late (right) Ford Starter RelaysOthers, like the smallBosch relays, (nowmanufactured by Tyco),are designed to carrysmaller loads of up to40 amps for unlimitedlengths of time and areknown as continuousduty relays.Figure 2 – Mini-RelayThis article will dealwith a few commonelectrical problems youcan easily solve usingrelays.There are many different relays available in theaftermarket, in a variety of amperage andvoltage ratings. Many have a mounting bracketbuilt onto the case. The pinout numbers andinternal diagrams are usually printed on eachrelay. The relay shown in Figure 2 is the stylemost commonly used by rebuilders. It is basedon standards set by the International StandardsFigure 4 – Single-Pole Single-Throw with two #87 terminalsSPST (single-pole single-throw) Relay:Terminals 85 and 86 are connected to the coil.Terminal 30 is common and usually connected toB through a fuse. Terminal 87 is where the loadis connected. When power is applied across thecoil, the contacts close to connect 30 to 87. Somerelays have two 87 load terminals (Figure 4).SPST relays may have four or five terminals.Created by Wes Grueninger and Bob Thomas for the Electrical Rebuilder’s Association - 2008, Electrical Rebuilder’s Association – All rightsreserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consentof the Electrical Rebuilder’s Association .1

Technical Document MS-0908AProduced by the Electrical Rebuilder’s Associationwww.electricalrebuilders.orgwith the coil (see Figure 6). The purpose of thediode is to bleed off voltage spikes generated eachtime the relay is switched off. If there is no diodeconnected in parallel with the coil, polarity doesnot matter. But when a diode is used in the relay,power must be applied to only the negative side ofthe diode, or terminal 85. The diagram printed onthe relay will show a diode if one is used.Figure 5 – Single-Pole Double-ThrowSPDT (single-pole double-throw) Relay:This isn’t a style of relay that you are likely to buyfrom any rebuilder supplier, but you should beaware of its existence, especially if you are wiringup a used relay. Hooking it up the wrong way willcause a direct short between terminals 85 and 86.Terminals 85 and 86 are connected to the coil.Terminal 30 is common and usually connectedto B through a fuse. Terminal 87 is where theload is connected.Terminal 87 will beconnected to 30 when power is applied acrossthe coil. When the relay is inactive, terminal 30will be connected to 87a. SPDT relays alwayshave five terminals. This type of relay issometimes referred to as a changeover relaybecause the current flow changes from terminal87a to terminal 87 when the coil is energized.Figure 6 – SPST Relay with DiodeSpike-Suppression Diode:When a relay is switched off, the collapsingcurrent in the coil can cause a small voltagespike. If the relay is controlled by a solid-statedevice, such as the PCM, this spike can causedamage. One method of protecting the relaycontrol circuit is to install a diode in parallelFigure 7 – What’s Inside a SPDT RelayCreated by Wes Grueninger and Bob Thomas for the Electrical Rebuilder’s Association - 2008, Electrical Rebuilder’s Association – All rightsreserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consentof the Electrical Rebuilder’s Association .2

Technical Document MS-0908AProduced by the Electrical Rebuilder’s Associationwww.electricalrebuilders.orgProblem Fix #1:Low Voltage at Starter S TerminalPROBLEM: There is low voltage at the Sterminal of the starter solenoid due to worncontacts or poor connections in the switchcircuit. This could be caused by the ignitionswitch, neutral-safety switch, old harnessconnections or a combination of more than onesource of resistance. Often on older vehicles,the solenoid only clicks. This happens whenthere is not quite enough voltage at the Sterminal to close the contacts every time.SOLUTION: Install a SPST relay to activate thestarter solenoid, and control the relay using theoriginal start circuit. The relay needs only afraction of an amp, so the voltage drop becomesnegligible, and full battery power will be appliedto the S terminal every time.CAUTION: Be careful not to bypass any safetyswitches when installing a relay. All dequipment-disengaged switches should beincluded.NOTE: This is a simplified diagram and does not show all circuits present on the equipment/vehicle. Always install a fuse on all circuitsthat carry current to the relay terminal 30.Figure 8 – Amplifying “S” Terminal VoltageCreated by Wes Grueninger and Bob Thomas for the Electrical Rebuilder’s Association - 2008, Electrical Rebuilder’s Association – All rightsreserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consentof the Electrical Rebuilder’s Association .3

Technical Document MS-0908AProduced by the Electrical Rebuilder’s Associationwww.electricalrebuilders.orgProblem Fix #2:Create a Starter Lock-OutPROBLEM: The starter is being accidentallyengaged while the engine is running because ofa fault in the control circuit or by the operator.This can quickly destroy a new starter. Becauseit can be intermittent, the exact cause may bedifficult to pin down.SOLUTION: Lock out the starter when theengine is running by placing a SPDT relay in theswitch circuit using terminal 87a, which isnormally closed. Wire the relay coil to a diodetrio in the alternator to activate the relay once thealternator begins charging. If the alternator doesnot have a trio in it, you can add a small diode triojust to run the relay. You cannot use a trio that isattached to a charge-light circuit, because thecurrent passing through the bulb is enough toactivate the relay and prevent starting. Once therelay is activated, the starter-switch circuit isbroken, preventing accidental starts.NOTE: This is a simplified diagram and does not show all circuits present on the equipment/vehicle. Always install a fuse on all circuitsthat carry current to the relay terminal 30.10SI One-wire alternatorshown hereFigure 9 – Starter Lock-OutCreated by Wes Grueninger and Bob Thomas for the Electrical Rebuilder’s Association - 2008, Electrical Rebuilder’s Association – All rightsreserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consentof the Electrical Rebuilder’s Association .4

Technical Document MS-0908AProduced by the Electrical Rebuilder’s Associationwww.electricalrebuilders.orgProblem Fix #3:Control Charge Light in Positive-Ground 10SI SystemPROBLEM: You sold a 10SI alternator toreplace a generator on a positive-ground system,and now the charge light won't work. The triooutput is positive voltage, which is now groundon a positive ground system.SOLUTION: Control the charge light with aSPDT relay, and control the relay with the trioterminal on the 10SI alternator.NOTE: This is a simplified diagram and does not show all circuits present on the equipment/vehicle. Always install a fuse on all circuitsthat carry current to the relay terminal 30.Figure 10 – Positive Ground Charge Light CircuitCreated by Wes Grueninger and Bob Thomas for the Electrical Rebuilder’s Association - 2008, Electrical Rebuilder’s Association – All rightsreserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consentof the Electrical Rebuilder’s Association .5

Technical Document MS-0908AProduced by the Electrical Rebuilder’s Associationwww.electricalrebuilders.orgProblem Fix #4:Limit Voltage Drop to AccessoriesPROBLEM: A customer has installedaftermarket driving lamps, fog lamps, coolingfans or horns that are drawing more amperagethan the switch circuit can handle. The result islow voltage to the device and possibly lowvoltage on other accessories too. This can alsobe a problem with headlamps and electric fanson older vehicles when voltage drops throughthe various switches, and harness connectionsbegin to add up to several volts.SOLUTION: Use a SPST relay to operate theequipment suffering from low voltage, and controlthe relay with the original switch circuit. Mountthe relay as close as possible to the power sourceand the device.NOTE: This is a simplified diagram and does not show all circuits present on t he equipment/vehicle. Always install a fuse on all circuitsthat carry current to the relay terminal 30.Figure 11 – Reduce Voltage Drop To AccessoriesCreated by Wes Grueninger and Bob Thomas for the Electrical Rebuilder’s Association - 2008, Electrical Rebuilder’s Association – All rightsreserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consentof the Electrical Rebuilder’s Association .6

Technical Document MS-0908AProduced by the Electrical Rebuilder’s Associationwww.electricalrebuilders.org Using Parallel Relays to Increase Current Capacity Exceeding the Relay’s Amp RatingWhen adding a relay to any vehicle or piece ofequipment, always consider the amperageneeded to carry the load. Most Bosch-stylerelays are rated for continuous duty. If the loadis over 40 amps, you'll need to use a highercapacity relay, or use two relays wired in aparallel circuit to split the load (Figure 12).If you are using the relay to control a shortintermittent load, like the starter switch circuit,you can increase the rated amperage. See Chart1 for load and time limits. It is also highlyrecommended that you always install a fuse in thewire going to terminal 30 of the relay to protectthe wiring and relay. Of course, any wiring youadd also needs to be heavy enough to carry theamperage.Overload Current1.35 x Rated current2.00 x Rated current3.50 x Rated current6.00 x Rated currentTime1800.0 sec.5.0 sec.0.5 sec.0.1 sec.Chart 1 – Tyco Relay Overload Capacities and TimeNOTE: This is a simplified diagram and does not show all circuits present on the equipment/vehicle. Always install a fuse on all circuitsthat carry current to the relay terminal 30.Figure 12 – Parallel RelaysCreated by Wes Grueninger and Bob Thomas for the Electrical Rebuilder’s Association - 2008, Electrical Rebuilder’s Association – All rightsreserved. No portion of this guide may be reproduced in any way, or stored in any electronic retrieval system without the prior written consentof the Electrical Rebuilder’s Association .7

SPDT (single-pole double-throw) Relay: Terminals 85 and 86 are connected to the coil. Terminal 30 is common and usually connected to B through a fuse. Terminal 87 is where the load is connected. Terminal 87 will be connected to 30 when power is applied across the coil. When the relay is