Windshield wiper fuse.
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    1. #1
      Junior Member
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      Windshield wiper fuse.

      When I restored my 544, I rewired it with a Painless Wiring fuse block. So, I now have a separate fuse for each component. I've had it this way for 20 years with no problems. Today, I blew the wiper fuse. The original wiring had the wiper motor running thru the 25A fuse along with other components. The Painless Wiring instructions suggested using a 15A fuse for the wiper motor. I've been running it with a 10A fuse. Should I put a 15A in instead?
      Last edited by gearbasher; 02-07-2019 at 07:09 AM.

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    3. #2
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      gb;

      Fuse 1 was rated 25A because it supplies a number of loads, and when adding up their current to get "max normal current", its that high...now that you have a different harness, with presumably somewhat separate wiring for each or almost each load, fuse values should be individually determined (by measuring normal current in circuit, and doubling it for most loads) and installing that value fuse...the 15A they suggested was just a recommendation, and you decreased that...again, proper way to determine fuse rating is to measure load current, and be aware, motor current is a function of motor load...so a heavily loaded wiper motor, will draw a lot more than a lightly loaded motor.

      If you don't want to go to the trouble of doing it right, and measuring motor current, and basing fuse rating on that...if fuse blew during normal operation, or just because motor was loaded (with dry windshield, or moving snow for instance), you can try a 15A fuse...and try lubing motor and linkage and pushing through cowl to keep running resistance low...if it blows again, some service is in order...

      Cheers
      Last edited by Ron Kwas; 02-23-2019 at 11:02 AM.

    4. #3
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      First off: Ron, thank you for the info.

      Now, my findings:
      All measurements were with a dry windshield.
      With the key in the accessory position and the wiper running just on 12.5 VDC battery power: On initial start the wiper motor drew 9 amps. While running, the draw fluctuated between 7 and 8 amps.
      With the engine running and the alternator (yes, I converted it to an alternator) putting out 14+ VDC: The wiper motor was drawing 4.5 to 5.5 amps while running. I didn't catch the initial start draw.
      I threw in another 10A fuse and it was fine. So, I guess I'll say the blown fuse was a fluke and I'll stay with a 10A fuse. I'll just carry a spare around.
      Last edited by gearbasher; 02-07-2019 at 03:55 PM.

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    6. #4
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      gb;

      At 12.5V startup current of 9A, running 7-8A, and at 14+V Starting I ?, running 4.5-5.5...that can't be...because current is a resultant which can be calculated by I = V/R...so if V is higher, so must Current be...I expect the discrepancy is a result of how, or where you made the measurements...not a huge deal, but if the 10A fuse blows again, replacing it with a 15A wouldn't be unreasonable (it would also be in the range of 7-8A X 2, the rule of thumb).

      Cheers

    7. #5
      Junior Member
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      Yes, I found my results strange, also. All measurements were taken across the blown fuse. When running on just the battery, the wipers are very slow and seem to struggle. With the engine running and the higher voltage, the wipers are much faster and smoother. So, I'm guessing the "bog" at the lower voltage is what's causing the higher current draw.

      Thanks again for your input.

    8. #6
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      gb;

      "Bogging" is a sign of a Voltage Drop (some of the source voltage is not making it to the load)...likely on the supply side (but it can also occur on the chassis side!)...if you are measuring 12.5V at the battery, you should measure no less than 12V at the Wiper Motor (measured under load, and when current is flowing, because ONLY then is when VDs occur!)...if less, locate VD and eliminate (or at least minimize)...a particular area to look: Fuse 1 and associated connections at Fuseblock...see: http://www.sw-em.com/gastight.htm I expect you are also experiencing a glowing AMP Indicator. ...so it's likely time to rework your FB! See: http://www.sw-em.com/AMP_Indicator_ON.htm

      Good Hunting!

    9. #7
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      As I said, I have a modern after market fuse block with blade type fuses. And since I get the wiper motor "bog" only when the key is in the accessory position, there will always be a glowing AMP light (and Oil Pressure light). With the engine running, everything is good. There are no indicator lights, the wipers run fast and smooth, and I can run all accessories with the system voltage still over 13.5 VDC. So, I'm just gonna leave it as is and see what happens. This is my only car. You can call it a "Daily Driver", but I only put on around 600-800 miles a year. I do between 11,000-14,000 miles a year on a bicycle. My go to choice of transportation.
      Last edited by gearbasher; 02-08-2019 at 07:11 AM.

    10. #8
      Global Moderator tmtalpey's Avatar
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      You may want to clean up your battery posts and the insides of the clamps. And also, pay special attention to the ground connections at the body and especially between the engine and frame. Marginal return current can cause these types of anomaly, particularly in a car that gets driven only occasionally (i.e. battery may cycle low and alternator works hard to recharge it).
      2005 V50 T5 AWD 6MT w/Heico tune, Heico exhaust, Bell intercooler, IPD TCV, Viva Forge CBV, Elevate rear swaybar and torque mount, Snabb shift kit, etc.
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    11. #9
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      Happy to report, I drove for an hour in a snowstorm and did not fry the new (10A) wiper fuse.

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