Solenoid / Isolator Replacement


With our 97 U 320, when turning the ignition key to the right, nothing would activate. No speedometer or tach needle movements. No back lighting. Nothing that normally came on would work. All dead. Turning key further to the right, to start, would do nothing. This started happening after a hours long runs. Usually just a few minutes sitting, and it would work again. Very unsettling the first time. A call to Foretravel, a few instructions on crossing the solenoid, brought us back to life. My thinking is the solenoid gets to hot, quits working, then cools, and works again. Raymond & Babette Jordan

There was/is a solenoid problem on the mid nineties Foretravels. Mine did the same as yours. James Triana let me know there was an upgrade/remedy for this.

On my 97, just in front of the passenger seat, the dash panel removes, and there is a solenoid near the bottom. It looks like a Ford style from the sixties/seventies. When my coach stopped, as yours, I crossed the solenoid, everything started. Again, Foretravel has upgraded the system. I do not know how far back the problem went. Sounds like this may be your problem. Raymond and Babette Jordan

What was the UPGRADE, did they go to a higher amperage solenoid or just improve the quality of the solenoid vendor? My front solenoid died on me in Yuma early this spring and we had a hard time finding a continuous duty solenoid that handled 100 amps. JON TWORK

The upgrade is two solenoids that replaced the one solenoid. There is a kit that can be purchased from Foretravel. It takes about an hour to put on. Hope this helps. Keith Risch from Encore.

I took off our dash cover and checked our ignition relays located on panel bottom center. I don't know if our coach has been upgraded. It appears to have been manufactured with these two relays to control ignition loads. There are two identical relays, each powering different circuit breaker bars.

One ignition relay powers 14 circuit breakers, the other powers 11 circuit breakers.

bosch_relay Tyco # V23232D0001X001 12 volt d.c. / 75 amp power relay. Bosch sold their power relay line to Tyco and this replaces the Bosch # 0 332 002 150 relay. Enclosed single pole single throw relay has one set of contacts that close when power is applied to the coil terminals. It can be used to turn on 12 volt loads of up to 75 amps. The power terminals have #10/32 screws and the coil terminals are 1/4" male quick disconnects (spade terminals). The nominal operating current is 300 ma. ($18.75)

 They are rectangular in shape. Two terminals have bolt terminals and two terminals have push on spade connectors. Terminals #30 (bolt) are connected to a large start-battery connection with heavy wires.

Terminals #87 (bolt) are connected to different circuit breaker bars. Other two push-on spade connector are hard to follow but are relay's coil connections that are live when ignition key is turned on.  by Barry and Cindy 1997 U270 36'

We had the same problem, and after replacing two of them that went bad and were working right at their design limit, we switched to the one below: PN 9012 - Solenoid Switch L-Series 12-24V


Input Voltage 9-36 Volts DC
Terminal Stud Size M8 (5/16")
Power Consumption - inrush max, 130ms 3.80 Amperes
Recommended Torque 80-100 in-lb
Power Consumption - Holding 12 Volts - 0.13 Amperes / 24 Volts - 0.07 Amperes
Contact Form SPST-NO
 Inrush Rating: 2.5 sec. 2000 Amperes
Weight Lb (Kg)  1.00 (0.45)
Maximum Voltage Rating 60 Volts DC

Put it in in 2001. No problems since. Brett Wolfe 1993 U240

This is the type of heavy-duty solenoid we should also use for start-battery boost switch to flow significant house battery amperage to engine starter. by Barry and Cindy 1997 U270 36'

There are two different solenoids in discussion:

1. Ignition solenoid. Located under the dash (at least on ours).

 Because of the high amp draw that can be run with the ignition on/driving (lights, dash A/C fan, wipers, etc), a solenoid is used, since running all that current through the ignition switch would burn it out in short order. That is the one I replaced with the Blue Sea unit which is more expensive, but rated for SIGNIFICANTLY higher amp loads than the OE solenoid.

2. Battery "Combine/Boost" solenoid. Located near batteries. when the dash "Combine/Boost" button is pushed, this solenoid connects the house and chassis battery banks. Brett Wolfe


1. It doesn’t matter how many batteries you have in a bank (Lets call Starting one "bank" and Coach another "bank") if the batteries in a "bank" are connected plus to plus, negative to negative then it doubles the available amperage (if they are both 12 volt batteries then the result is still 12 volts) but the amperage is the sum of the batteries in the bank added together, you could have 2 or 200 batteries in parallel, It doesn’t matter. The voltage stays the same the available amperage increases.

2. On the coach "bank" it's common to have 2 or more batteries to give you more AMP hours (Amps divided by load equal AMP hours available) IE: if I have 1000 amp hours capacity and use 10 amps of current per hour I have 100 hours use. But in reality I can only use 50% because we only want to discharge our batteries down to 50% to ensure long life.

3. On the starting "bank" the reason we like more AMPS is that AMPS crank the motor over quickly. The voltage would drop considerably if we didn't have lots of amps to drive the starter. Starting batteries have different discharge rates than "coach " batteries, IE: a starting battery can dump 600+ cranking amps to get the starter going but don't have long term reserve capacity.

4. When you see a starting battery it says "reserve minutes" that’s the time that you supposedly can drive your car with the alternator failed and still have your headlights on to drive home at night and still keep the car running.

5. The battery has NO bearing on the Isolator needed. The Isolator is only there to ensure that if one "bank" discharges due to use or failure – it doesn't take out the other bank down also, hence isolates the discharge of the "banks" You just don't want the isolator smaller than the rated output of the alternator for obvious reasons.

The isolator can be tested with a multimeter set on continuity; it should "beep from the inner terminal out to each outer terminal. Should not beep across the two outer terminals and should not beep between the outer terminals and the inner terminals with the leads reversed from step one. The Isolator is a simple one way "valve" for electricity.

Here's how to check the isolator using a DC voltmeter.

First, measure the voltage at all three terminals of the isolator with the engine completely off. You should be able to read each battery voltage independently on each of the outside terminals. If there is voltage on the center terminal, one of the diodes in the isolator is faulty and the complete isolator must be replaced. (Some VOMs do have a diode test circuit to double check the diodes in the isolator). If voltage is only read on one of the two outside terminals, there is an open in one battery system.

Next, start the engine and check the voltage at the center terminal of the isolator; it should be at least 13.8 volts and there should be an increase in the voltage measured at the two outside terminals, though it will be less than the center terminal. If you have access to a clamp-around DC ammeter, you will be able to measure the charging current on the alternator output wire.

Additionally, be sure to check all the cable connections at each battery & in the circuit very carefully. I just ran across the same symptom as yours and found a corroded ground cable from the engine battery to the frame ground. If you are sure the alternator is functioning as it should and the isolator tests good, you just may have a cable problem."

After checking the Isolator, I recommend getting out the wiring schematic and following each wire one by one to ensure that it is where it should go. Also check the continuity of the alternator Voltage Sense lead and make sure that it gets from the alternator to the positive terminal of starting batteries.

Whether directly to the starting battery, the start battery terminal of the isolator or the start battery side of the Boost Solenoid, it doesn't matter which. Then check that the alternator has not lost its ground due to corrosion. Also remove the nut from the alternator excite terminal on the alternator, clean the connection thoroughly, tighten the nut that connects the stud to the alternator case and use an internal star tooth washer and a NYLOCK stainless steel nut to re-secure the Voltage Sense wire to prevent loosening and use Dielectric grease to prevent corrosion. In fact it's a good idea to clean each alternator terminal and use a star washer on each.

If you need to replace the Isolator there is no particular order to removing or replacing the isolator. I would disconnect the negative on both the starting bank and the coach bank prior to any work, and make sure you are not on shore power, just to prevent arcs and sparks. The coach batteries are powerful enough to weld a wrench to the frame if the positive gets shorted to ground, that can be very dangerous.

After getting the isolator replaced, then reconnect the negative battery terminals. You only need to disconnect the one negative terminal at the battery that goes to the frame on each bank. Not all the wires.

You can use a three post isolator. It doesn't matter how many batteries you have,  Just hook your alternator to the center post, the starting batteries to an outer post and the coach batteries to the other outer post. This is ALL you need. If the alternator needs remote volt sense then connect to start battery positive terminal . The boost solenoids "join" the battery banks external to the isolator. Just trace the wires one a time and you will be fine. All you need is a three post isolator that exceeds the capacity of your alternator. IE: 160 amp alternator - 200 amp isolator.

A 200 AMP isolator is fine, if you have a 180 amp alternator, only alternator current goes through the isolator, You will NEVER put 180 amps into the batteries, for one most batteries cannot handle 180 amp charge, and your alternator really only produces about 1/2 the rated output.

I have been running for over two years with a 3 post 250 amp isolator- no problems.

6. The electricity you use to start or light your coach does not go through the Isolator. Only juice from the Alternator to battery "bank" goes through the Isolator.

7. When you hit the Boost Solenoid it Parallels up the two bank's (Starting and Coach) to give you a huge boost in AMPS to get you started. It does this by when you activate the "boost" the solenoid (switch) closes and adds in the "coach" batteries to the positive terminal of the starting batteries before it gets to the starter. Since everything on the coach is a common ground, the negative sides of the coach batteries are already "paralled" up with the starting batteries.

8. The reason your "Starting" bank does not charge from your inverter charger is that the same wire that connects the coach positive from the "coach" batteries to the Inverter to turn 12 volt to 110 is the same wire that charges the batteries when the inverter has shore power. It just reverses direction on the same wire. The inverter / charger does this internally.

Take a look at this for some good info.

Probably more than anyone wanted to read.  Greg Jones 2/27/08

© Barry Brideau 2003-2017