Switch pitch keeps blowing fuses

Last Saturday night I had a little electrical issue. The wire I had setup to engage the SP shorted out, and melted, and took out a few other wires in the process. I've got everything re-wired and sorted out, however, now anytime I try and engage the SP (I'm running it on a toggle), it blows a fuse. The circuit I had it on first was 30A, and the only other things on that circuit were my tach, 2 electric gauges, and 3 gauge lights. I then tried it on it's own 15A fused circuit, and it blew that. I changed switches, same problem. I KNOW that this wire is not shorted out any where, as I just ran it yesterday. Any thoughts? Did fry something related to the SP in the trans when the wire shorted? Thanks guys.

 

Do you have an ohmmeter handy? I have a vp solenoid in good shape I just tested on the bench, it reads 18.5 Ohms resistance across the coils. It would be interesting to see what you get in comparison.

Start by taking readings in the wiring furthest from the trans and work your way closer.

Devon

 

I wouldn't go that far on the first step. If you get high resistance readings at your switch but you get low readings (around 20 ohms) at the connector on the case, you know the issue is most likely outside of the trans. If it's a true short to ground somewhere along the line, the ohmmeter will go offscale, "infinite" resistance.

Devon

 

You are correct.

I, on the other hand, am an idiot tonight! Time to stop multitasking. Chewing gum and giving electrical advice is too much for me.

Devon

 

Then I have a head full of gas alright! :spank:

Thanks, Larry.

Devon

 

Ok, so across the top and bottom tangs I get 19.1 Ohms.

Stupid question. To get readings at the swtich, obviously one probe from the ohmmeter goes to the open terminal on the switch (which would be used for 12V in), and the other probe would just go to a good dash ground?

Have I mentioned I hate wiring and trouble shooting electronics?! :af:

 

Andy, I'll dive in again...

When you say "across the tangs", are you placing the ohmmeter leads on each of the male terminals sticking out of the plug on the left side of the transmission case?

If so, I don't think that will tell you much; one terminal leads to the detent (downshift) solenoid, the other is the variable pitch terminal. They are two completely different circuits. The variable pitch terminal is the upper one on the plug. Put one of the ohmmeter leads on the top terminal (did I remember right Larry?), the other to ground somewhere, like the trans case itself.

Devon

 

Crap, yup, dunno what I was thinking. The top tang is the SP.

So doing things the correct way (between the top tang and the trans case), I get what appears to be an open circuit condition. The reading on my ohmmeter didn't change. Not sure what that means?

 

Then Larry's first reply tells you about your next step; there may be a short to ground between the terminal you tested and the vp solenoid. It may also be a problem with the solenoid itself.

Dropping the pan, you'll see the wire running from that terminal towards the front pump. If I remember right, there's steel p-clip holding the wire along the way, it may be the culprit. Age, heat and trans fluid takes its toll on the wire's insulation, so I think some inspection there is what's next.

Devon

 

Aww hell. It's never as easy as you think it'll be. Thanks for the help Devon & Larry!!! :beer

 

Glad we can help and glad people here can keep everyone on an even keel as far as advice goes!

In my car I've seen the original vp wire inside the transmission showing cracks in the insulation. That in itself isn't an issue as long as the affected area isn't close to any metal inside the case, but I'm thinking it's typical considering the age.

I hope you can find the problem with the inspection; if the solenoid is dead, the front pump has to come out.

Devon

 

Man, not having high stall sucks!!! I'm waiting on my new trans pan to come in (extra capacity, finned, with a drain hole) before I dig into the trans. I'm hoping it'll be an easy fix, we shall see.

If I had to replace the solenoid, how hard and or difficult is it to remove the front pump? I've never really been inside an auto trans aside from your basic valve bodies and shift kits.

Also, just for reference, I could use a fixed stall TH400 converter, correct?

 

Andy, to answer your last question first, no you can't use a fixed-stall converter unless you convert the rest of the transmission.

If you're ready to pull the front pump to deal with the solenoid, you'll need that front pump gasket at the least. You may have to buy a rebuild kit to get there.

Devon

 

Ended up pulling the pan today. The SP wire was wedged between a valve body bolt and a dimple in the trans pan. It was flatened pretty good, insulation was cracked, etc. Definitely the culprit. Must of happened when I re-assembled it last year. Such a dumb problem, but oh well. I'll re-wire it tomorrow and test it out before I re-assemble.

Oh, and I never did order a new trans pan. I realized I'd have to convert to a later style TH400 filter. How hard would that be to do?

 

Glad it worked out!

To convert the filter, you'll need the new pan, new style filter (obviously) and the tube + o-rings and filter bolt from a donor trans. If you have a transmission shop nearby they should be able to set you up.

Devon

 

If you have a custom valve body, the new style filter should bolt up and make finding filters a whole lot easier. An original switch pitch valve body has no thread for the new style filter bolt. It IS POSSIBLE to drill & tap for that bolt, but you don't want any of those metal pieces in your trans. The newer style pan/filter is about 1/4" lower.

Just about all the switch pitch trans solinoids by now have falling apart insulation on their wires. I try to rewire them with Tefl*n insulated wire to deal with the harsh environment. A couple original clips keep the wire out of trouble.

The first 400s had 19 ohm solinoids. But more recent solinoids are 10 ohms, I avoid them. Bruce Roe