Posted this on VS last month; not too much interest, but maybe someone here could find this useful.. FYI.

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 Volvo Adjustable Delay Intermittent Wiper Relay IPD 103226 9140663 3515279 3523609 5531

Most modern cars just have a **** at the end of the wiper stalk that you twist 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 **** 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.
Caveats:

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... 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 from the resistor pads 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 have questions. I can add more details if requested..
https://www.dropbox.com/sh/8cvhfgho6...bmYqoI-ia?dl=0

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.


NOTES:
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 which 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 pin-outs too. If you're going to go to that much bother you may as well create your own PLD..

 

NICE!! I live in th Pac NW too (on the coast) and this will come in handy for our 850 and S70!

 

Very clever. Thanks for posting that!