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Variable Intermittent Wiper mod..?


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Most of you guys probably won't care about this, and it's not really a perf mod so this could be moved if this isn't the right spot for it...

But one thing that's really missing on the 850 and '98 S/V70 cars IMO is a variable intermittent wiper control!  Yeah I know, you can get a wiper relay from ipd and others where you can change the intermittent interval time but the UI is terrible; it's a PITA to change the intermittent time interval; you gotta get all jiggy with wiper switch to modify the time.  Link for ipd relay



The last car i owned with a variable intermittent wiper control was a 2001 MY saturn L200; it just had a knob at the end of the wiper stalk that you twisted to get different time intervals.  Very nice and easy to change interval times.  I don't know about you guys but where i drive the rain/mist rate does not stay constant during my drive; and of course it changes with your speed too.  So (barring rain sensors automatically controlling your wipers..) the knob is a very nice UI.

So I dug into our wiper relays and did some reverse engineering.  There's a RC network that controls the time interval and it's locked down to a 6 second interval.  Actually I did more experimentation than engineering; I can get into more details if anyone is interested but suffice it to say I did figure out a nice mod to add variable intermittent wipers to our cars.


  1.  This mod will only allow you to vary the time interval between 4 and 16 to 17 seconds.  I wanted to make it longer than that but that would require more extensive mods to the wiper relay.
  2. The mod is not plug and play.  This was disappointing to discover.  i calculated the max resistance necessary to be around 700K ohms IIRC from the U2640B data sheet (link inside below link) but on my 855 it actually came out to 370K.  And on my wife's S70 it came out about 520K.  I.e. you will have to tune each installation a bit.
  3. Couple more items I forgot about...  when in delay mode above about 10s or so the wipers will park a bit higher than normal.  Not a big deal; if you go from intermittent delay mode to off, just do a single wipe and they'll return to the normal park position.
  4. If in the max delay intermittent mode and you pull the stalk toward you to wash the windshield, the time that the wipers run after the wash will be extended.  If this becomes an issue for you just turn the pot to the shorter time position and will return to normal operation.

Here's a link to some pics illustrating the mod.  I did it to both our cars now and it's one mod the Mrs definitely approves of!  Basically there's a 200K resistor in your wiper relay.  Remove it, and run two wires to a pot.  You could install the pot in the dash for simplicity but it's very handy if you install it in the backside of the wiper switch; a small enough pot will actually fit.  I twisted the two wires (to help noise immunity) and ran them up the left side of the steering column; be careful with your routing so they don't get pinched when the steering wheel is moved.  I have an inline two pin connector to ease install.  Like I said you may need to tune the resistance range a bit before finalizing the install.  On my car the pot resistance range is 0-370K ohms but on the S70 it came out 100K - 520K range.  I don't think it was because of different models; I think it's just because of the tolerances in the different relays.  YMMV.  I have a bunch of resistors I used for trimming but you could use trim pots too to make it quicker.  The first pots I bought turned out to be junk (couldn't handle the soldering heat) but the part label in the pics worked well.  Let me know if you questions.  I can add more details later..



Oh yeah; getting the back off of the wiper switch can be a biaaaaatch; the bone tool and/or a thin dull blade screwdriver works well.  Just go slow and heat may help too.  I can draw up a simple schematic if that would help.

Edited by gdog
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Interesting work.  I was thinking you might have installed a different stalk with an adjustable dial.  The one on my Honda Odyssey is perhaps the best one I've ever used across the many cars I've driven.

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10 hours ago, Burn-E said:

Interesting work.  I was thinking you might have installed a different stalk with an adjustable dial.  The one on my Honda Odyssey is perhaps the best one I've ever used across the many cars I've driven.

Yeah I looked for compatible stalks/parts on bmw/vw/audi/etc but didn't find anything; not saying it's not a possibility, just not one I ran across in my limited search.. 

Most other systems that I looked at use much lower resistance value pots than this circuit; makes sense as this would give you better noise immunity (though I haven't run into any issues so far).  Inside the wiper relay assembly is an atmel U2640B chip which is the logic chip.  It's a dedicated automotive wiper control.  There's a comparable chip Temic U846B that i discovered out there that is more configurable than the U2640, but the only source for it I could find was alibaba so who knows what the quality is like..  it had different pinouts too.  If you're going to go to that much bother you may as well create your own PLD..

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Maybe related to this.

I also happen to own a Saab 9-5 Aero as a DD and the PO installed a Bosch aftermarket rain sensor relais. It comes with a rain sensor which you put somewhere in the area of your inside rear view mirror and the relais plugs into the stock relais socket. It works great actually. So I might be doing this to my Volvo 850 as well, tbh.


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18 hours ago, German850R said:

... Bosch aftermarket rain sensor relais. It comes with a rain sensor which you put somewhere in the area of your inside rear view mirror and the relais plugs into the stock relais socket. It works great actually.

Do you have any links showing this product?  Sounds interesting..

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  • 7 years later...

Some have pinged me for more details so adding below..

Basically you de-solder that 200K ohm stock resistor on back of the wiper relay, run two wires from those points out to your wiper stalk and hook them up to, one outside terminal, and one to the pot middle terminal, of your 500K ohm pot.  There's a pic that shows the pot part number that I used.

Then you will need to experiment to find the max ohm setting you can use.  What happens is the wiper won't park and just keep running (i.e. not intermittent), if it's too many ohms.  You'll need to note that ohm value where that threshold is, and then setup your particular circuit so the max ohm setting is a bit less than that max value.  From my experiments on two cars, that max ohm correlates to about 16 to 17 seconds, between wipes.  Remember, the goal is to get it to wipe once, then park, until the next cycle.  If your ohm setting is too high, it won't park.

If you find your max ohms is less than 500K, then you can add a shunt resistor (back in the stock R location on the wiper relay) to make your max ohms what you need (google how to calc parallel resistor networks if you're not familiar).  If you need more than 500K, then just add necessary resistor value into one of the wires.

Note this is RC tuning circuit so any currents will be very low; i.e. 1/4 or even 1/8 watt resistors and pots will be fine.  Same w/wires; they can be small gauge, but use stranded (not solid copper) wires.  I twisted the wires together (to make them twisted pair) running from the wiper relay to the stalk, to hopefully add some noise immunity.

Biggest pain is finding a good route through the steering column so it doesn't snag on anything that moves; remember, not only have the turning steering wheel shaft, but also note column telescopes too. 

I did this mod on our two P80s (96 850 and 98 S70) but looks like the same wiper relay is used through 2000 MY, so should work for those too.


Hopefully that's all clear?  Ping me if you have any other questions.

Edited by gdog
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  • 11 months later...

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