Strange WUR problem

[Pkaaso]

Ok, I have an 81 SC that keeps cooking the inside bimetal element of the WUR. I starts with the connector slightly melting at the female end. Then the WUR fails and when opened, the wiring is cooked and now is a junk core (unrebuildable).

I have checked the voltage from the Alt, at verious RPMs and found a good solid 13volts. The battery is good so I don't think it is a voltage/amp issue.

What the heck is going on?

Thanks for any/all input

Paul

[cmonref]

Sounds like a short in the wiring that wraps around and heats the bimetal element. Any battery/alternator voltage will do -- all you need is a short.

Search on Mysterytrain and "AAR Revisted" (Misspell "revisited" just like that -- that's how the thread is titled.) In that thread, Ron documents how to rewire the bimetal strip in his AAR. It is the same process for the WUR -- it just looks different.

[450knotOffice]

Here's the link Brian mentions, Paul. It looks like he's on to something.
AAR Revisited

[Pkaaso]

Thanks Guys... I figured out the problem. Anyone want to take a guess. It's as simple as it can be. Hint "trust no one, and suspect everyone."

[450knotOffice]

Uh....

I give up. What was it?

[Pkaaso]

The previous mech had switched the connector wires going into the WUR. We (idiots) replaced bad WUR and killed it also. Waiting for replacment to be 100%

Thanks for the imput.

Paul

[Mysterytrain]

That doesn't make any sense to me. Please explain?

[patkeefe]

subscribing for the follow up details

[450knotOffice]

I walked into Paul's shop and looked at the offending WUR. It seems that the positive and Negative leads to the WUR had been switched at some previous point in the car's maintenance history and this is what caused the overheating problem in the wires internally. Paul and his buddy simply didn't notice that the wires were crossed when they first started working on the problem, so when they got the first replacement WUR and plugged the leads in (identical to the way they found them beforehand), it overheated the wires again. They took a good close look at it again and also looked at another SC in their shop and noticed that the leads on the other car were plugged in oppositely. That was their "duh" moment when they suddenly realized that the leads on the offending car had been plugged in incorrectly all along.

[patkeefe]

I don't think the leads make any difference on either the WUR or the AAR.

[Mysterytrain]

The WUR and the AAR are connected in parallel. Flipping the internal wires on the WUR heating coil should have no effect. I don't recall the exact internal wiring but I don't think they use the case as a ground. I'm pretty sure the ground is provided thru one of the terminals on the connector. Anyway, this still doesn't make sense to me.

[jonbot]

Most diagrams I've seen of the WUR make it look like one side of the coil is hooked to case. Take this one for example:



It may be possible that one of the pins does hook to the case to ground it, and the other hooks to the coil. You can use a multimeter across each of the pins to the case to find out. I just can't believe a fuse wouldn't pop or something before it got to the point of melting the wires.

[Pkaaso]

I'm still in the "Huh" stage on this one guys. I have always thought the same that the reverse wire on a heating coil won't make a difference but, several others (including one rebuilder) have suggested that this is the problem. I CAN NOT make any other sense of the problem at this time.

What else could cause a (two) perfectly good WUR('s) to cook inside? The voltage and amps look OK.

????

[450knotOffice]

The wire inside the case only got hot enough to burn through the insulation, but the metal wire strands leading to the bi-metal strip remained intact, as far as I could tell. As far as the windings wrapped around the strip, not sure what happened to them. A meltdown to ground (onto the bi-metal strip) may have occurred or a meltdown to an open may have occured also.

Paul, I can't remember, how many wires were down there in the case? I thought I saw three, but maybe I was wrong.

[patkeefe]

I fried a WUR not long ago. The WUR and AAR ground via D- on the alternator. In my case, I suspected that there was a bad ground at D- (I had previously retapped the D- threads on the alternator, as my new alternator had a bad set of threads). After replacing the electrical elements on the WUR, I tested it and the AAR on a spare battery; neither was polarity sensitive. My theory became that the heating element tried to "hunt" for a ground to complete the circuit.

What I ultimately did was put in a dedicated chassis ground for the WUR and AAR, and have not had a recurrance of the problem. I suspect that the WUR and AAR are wired this way from the factory to ensure cold start ability, so cold control pressure will not start to risw until the engine is actually running.

link:
Alternator D- ????


Pat

[450knotOffice]

Very interesting Pat (this is all academic for me anyway, as I don't even have a CIS car - mine's Motronic).

[911quest]

How is your fuel pump wired? Does it come on with the key on or does it operate correctly? I would do some contunity checks on the wiring I would check the Red/white wire for contunity to ground to start...the wire in these cars is getting old and brittle from the extreme heat back there a bad ground at D- could caused the wire to overheat and fuse it self to another wire that it shouldn't have causeing the short to ground.

[Mysterytrain]

Ok, I pulled out one of my spare 78 WUR's and the Fluke meter. The ground is thru the connector and not at all thru the WUR casing. When you think about it there would be no nice way to attach the wire to the aluminum housing anyway. Couple of thoughts...I've seen WUR's with an internal switch. The switch appears to be a thermal 'make or break' contact I've never understood its purpose. I think it appears on the later SC's. A bad connection from the contacts might create a problem. The other thing to check out is D- as Pat mentioned. I would do this before I connected a new WUR and watched it cook.
Paul..is this a 3.0 liter transplant in a 74?

[450knotOffice]

It's actually a customer's car.
(keep it up...I'm learning here)

[jonbot]

I'd be interested to see how many amps the WUR is pulling, and compare it to another car. If it's generating enough heat to melt the insulation on the wire, then I'd imagine the current is pretty high. I'd also check to see how many volts are coming in to the WUR.

Edit: I'd check how many volts are coming in to the WUR first before anything.