Fusing your WUR (CPR)

[Bitsyncmaster]

I had one owner just install a new RPM relay with it burning up on first power up. He sent me photos and it looks like the MOSFET driver for the WUR is cracked. Also had another owner asking about this fuse modification because it was documented on the old forum.

The RPM relay powers three things. First is the fuel pump but it also powers the heater in the WUR and the lambda relay signal wire (coil power). We have seen a few WUR short circuits inside on the heater circuit and that still does not blow the 20 amp fuel pump fuse. This may also be a reason some fuel pump fuses get overheated.

My solution is to fuse the WUR heater with a 3 amp fuse. I only fuse the WUR so if the fuse blows (your WUR shorted) it will not affect a long drive home. If I fused both the WUR and the lambda relay wire with one fuse your engine would run rich or lean on that drive home. You would know when the WUR fuse has blown when your engine takes a long time to warm-up so checking the fuse is not necessary.

The problem with this installation is they used two yellow/red wires. One powers the WUR the other the lambda relay signal. So I guessed and cut one of those yellow/red wires. Then measure the resistance of that wire (not the RPM relay side) to ground. If it reads about 20 ohms then you have the correct wire to fuse. Another check is to pull the connector off the WUR and the resistance should go to infinity.

Get a fuse socket with wires at the auto parts store and some 3 amp fuses. Install that fuse between the two yellow/red wires you cut that goes to the WUR. One end to the RPM relay wire and the other end to the wire to the WUR.

Note my photo. I guessed wrong and my first cut wire needed to be spiced back.

[Chris 16409]

Another way to determine which is the correct wire is to unwrap the wiring harness. One of the yellow/red wires goes towards the engine compartment and the other goes to the relay sockets. Just find and follow the one that heads back to the engine compartment, and that is the one that you cut to install the fuse. This method is for people who don't have multi-meters. If I recall, I didn't have to unwrap much of my harness.

[sdg3205]

Another way to determine which is the correct wire is to unwrap the wiring harness. One of the yellow/red wires goes towards the engine compartment and the other goes to the relay sockets. Just find and follow the one that heads back to the engine compartment, and that is the one that you cut to install the fuse. This method is for people who don't have multi-meters. If I recall, I didn't have to unwrap much of my harness.

This is how I determined which wire to cut too. I didn't want to remove the whole vertical parcel shelf wall to get to the grounds. I hate taking that thing out!

Saves potentially cutting 2 wires too.

[TTait]

you can also get a Fox and Hound or similar electrical circuit finder - you hook the "fox" up to the wire at the wur and it sends a low power radio signal down the wire. The "hound" has a small metal pointer antenna that you touch against each wires insulation to listen for the signal, when you find the right wire you hear a tone.

Search for "Fox and hound wire" on ebay - they go for about $30 a set.

It might also be worthwhile to get one of the light up fuses like the ones Hervey sells.

[JIMJAM]

Back to sqaure one.
Yesterday replaced the fuse hold and 20 fuse. Car started right up 1st turn. Yaaa
Had to stop for a bad storm. I just went out and tried started. Noticed the missing f pump whirrrrrr. Yup, Will not start.
Fuse looks good,replaced it anyways. Checked v at pump. Nothing!
Back at the rpm relay 12+. Across the fuse terminals-Nothing Jumped rhe relay,nothing
Did a all over jiggle test but it seems overnight I lost my voltage to the pump.
I did fool around with the ineria switch but dble checked it. Unless its possible to pop in that brown relay or what ever it is, I am stumped.
My battery is showing 12.8 but I just put it on the charger to top it off. And I recheched my solder job.

[Agent Smith]

Rather than post a lengthy treatise on the technical details and Bosch specifications of the WUR I'll simple say that, as long as the WUR remains in contact with the engine, its heating elements are not needed for very long.

What that means is they can be shut off. I do it using a cheap and common-as-dirt N.C. snap action disc thermostat. Works like a charm without effecting control pressure operation in the slightest and automatically resets when needed.

The benefit of this is not only a reduction of load through the RPM relay but it also resolves the most common cause of wiring/element failure internal to the WUR. Put simply there's no need to power the thing at all times.

Fwiw...

[Bitsyncmaster]

Rather than post a lengthy treatise on the technical details and Bosch specifications of the WUR I'll simple say that, as long as the WUR remains in contact with the engine, its heating elements are not needed for very long.

What that means is they can be shut off. I do it using a cheap and common-as-dirt N.C. snap action disc thermostat. Works like a charm without effecting control pressure operation in the slightest and automatically resets when needed.

The benefit of this is not only a reduction of load through the RPM relay but it also resolves the most common cause of wiring/element failure internal to the WUR. Put simply there's no need to power the thing at all times.

Fwiw...

I would agree that after the engine has warmed up, you can shut off the WUR heater. Where are you mounting your thermostat and what temp does it open?