Electrical Gremlins! Wiper switch causes engine knock, backfire! (1 Viewer)

[FishNinJay]

1971 FJ40, with recent SBC350, running great and smooth idle.

I was cruising along the highway at around 60mph, and flipped the wiper switch.
The engine faltered, knocked, and 1 loud backfire, before I could switch the wipers off. The driver side motor on the wiper has been bad, and never worked right. The passenger side seemed to work better.

What's going on here? I have to try and find the '71 wiring diagram, but I'm not sure if the prior owner did something funky with the wiring. Or, if the wiper motors are short or grounding out something? Any ideas?

As always.. thanks.
-Jay

 

[3jvj]

Sounds like your backfeeding power into the system. I'd get that motor gone. Check for a loose wire at the switch too.

 

[bikersmurf]

You're not getting a spark. Either not getting power to coil or distributor isn't grounded.

I'd guess a faulty ground. Check grounds to engine block, body, battery, and wiring harness x2.

 

[Combat Chuck]

What kind of distributor does the SBC have? If it's a large cap, sometimes careless PO's will wire accessories onto the coil, maybe your wipers are tied into that?

 

[FishNinJay]

What kind of distributor does the SBC have? If it's a large cap, sometimes careless PO's will wire accessories onto the coil, maybe your wipers are tied into that?

Yeah, that's what I was considering. I don't recall this happening before, so i also want to check grounds. This started after a new
crate SBC was installed. If the wiper switch is off, the engine runs great! Luckily, not much rain or snow in my area!

I have an AC Delco HEI, pretty standard distributor, rotor, cap.

 

[FishNinJay]

Sounds like your backfeeding power into the system. I'd get that motor gone. Check for a loose wire at the switch too.

Sorry, not exactly sure what "backfeeding power" means, but I think I am seeing the wiper circuit coming off the ignition side of the fuse box. When the wiper motor is OFF, the engine runs fine, very smooth, low rpm idle. If the ground at the wiper switch or the wiper motor/arm assembly went out, would that divert back through the ignition? Would that mean power diverting backwards thru the fusebox then to the coil? So the wiper circuit then is grounding via the ignition/distributor ground?

Interestingly, I do not know if I have a washer motor, and in fact, I'm pretty sure I do not. According to the 1971 wiring diagram, the washer motor also comes off the ignition side of the fuse box, and presumeably grounds via the switch. I only see 2 grounds, one at the wiper arm motor(s), and one at the wiper/washer switch. I'll go look for those 2 grounds first.

To test ground with Multitester, how do you use Resistance? You want to see a drop in resistance, right?

Bikersmurf:
It seems like the distributor ground should be ok. The timing and engine seems to run perfectly all the other times, when the wiper switch is OFF. Is my reasoning here flawed?

Chuck,
So if they did that, something would be wired directly off the ignition coil? Basically, unfused since it bypassed the fuse box altogether? Wow, that does seem careless..
In my mind, and looking at the diagram, I can see how that would disrupt or draw down the power going to the IC and distributor and screw up the timing and/or spark. I'll check that out as well.

Ok.. sorry for the questions.. and THANKS!!

-Jay

 

[bikersmurf]

If there is a "floating" ground (bad ground), then the electricity will find a new route to the - on the battery. Sometimes it will find its way back through lights, motors, and other weird things. This is why such weird things happen when there is a bad ground. If the distributor was somehow finding a ground through the wiper motor, then when the wiper motor was turned on the ground would be interrupted. (confused yet) Perhaps a more likely sinereo is that the wiper ground through the wiring is poor and the power from the wiper is then completing it's circuit through the ignition circuit.

If you haven't already cleaned them, check: block to frame strap, harness to frame (under gas tank on inside of frame), harness to inside right fender, and any othe grounds you can find.

Does anyone know if there is a ground wire inside the distributor to ensure good grounding of the points? Is it points or HEI?


If in doubt use jumpers to add extra redundant grounds to see if anything changes. If something does then you have identified the problem.

 

[FishNinJay]

If there is a "floating" ground (bad ground), then the electricity will find a new route to the - on the battery. Sometimes it will find its way back through lights, motors, and other weird things. This is why such weird things happen when there is a bad ground. If the distributor was somehow finding a ground through the wiper motor, then when the wiper motor was turned on the ground would be interrupted. (confused yet) Perhaps a more likely sinereo is that the wiper ground through the wiring is poor and the power from the wiper is then completing it's circuit through the ignition circuit.

If you haven't already cleaned them, check: block to frame strap, harness to frame (under gas tank on inside of frame), harness to inside right fender, and any othe grounds you can find.

Does anyone know if there is a ground wire inside the distributor to ensure good grounding of the points? Is it points or HEI?


If in doubt use jumpers to add extra redundant grounds to see if anything changes. If something does then you have identified the problem.

I see what you're saying.. the distributor ground may be faulty, and it is by default grounding via the wiper ground all the time. It only gets disrupted when the wiper is ON. Makes sense...

It's HEI.

The crate SBC 350 is brand new, just installed. So, I'm presuming the engine block ground is new, so should be ok. I'll check out as many grounds as I can find and make sure they are all tight and good contacts. Thanks again, and please keep any tips coming!!


-Jay