Stamford,

The diode symbol on the meter makes me think it is of a design that could miss a failed tranzorb when measuring resistance. Since your armature reading is spot-on at 0.5 ohms, I think it's very likely that the tranzorb has failed and is the source of fuses blowing.

Smoke! Yikes. At the risk of butting in uninvited, that's scary. With such testing you run the risk of a healthy armature becoming a burned out armature, at which point things get expensive. A good safe test here would be to snip one transorb lead, getting it out of the circuit. Very likely the motor will run normally then. If not, all we've lost is a component worth about a buck.

Hang in there ...

 

Guess I'll know soon enough. Hoping for a little bad news followed by a bunch of good.

 

The device I've been calling a tranzorb ... it turns out that's a brand name used by Vishay. The more generic term would be TVS (transient voltage suppression) diode. A bit of a misnomer in our case since it's actually a diode-pair, having no polarity.

My suggestion to Stamford, to snip it out of the circuit, is for testing only. It would have to be replaced before re-installing the motor into the car. The TVS protects the car's electronic$ from damage.

TVSs come in lots of different flavors. Ours is designated as 1V5kE27CA. These are so cheap I bought 5 of them for $3.00, so if anyone ever needs one let me know.

 

Well, got my motor back today. The shop said the short is in the circuit board; the wiring's okay. May be shopping on eBay next!

UPDATE: Just looked at the TVS using a loupe. It looks to be split in half and there appears to be some dark colored oil(?) under it. I guess that's my problem. Soooo, how would I go about replacing it externally? Thoughts? Thanks!

 

Still willing to take a crack at it if you want. Not much on that board to short, except our friend the TVS (aka tranzorb).

Do you know if they messed with that component at all?

 

Dennis,
I just updated earlier message. It is the TVS. I'd welcome any help.

 

Pretty sure that smoke referenced earlier was the TVS being fried (it was already bad).

If nothing else is damaged, this can work out. You need to get the TVS out of the circuit. Snip it in half, snip a lead or whatever (tight quarters in there, but doable, I think).

Then test the motor, but using a stand-alone battery if possible. Or, if the car battery, be sure to have the ignition off. (Without the TVS the motor can put out voltage spikes and we don't want such things reaching our fancy electronics.)

I've got some TVS thingys coming so if the motor runs we'll just put one of those in and you're good to go.

 

I will try to snip it and check it on a 12v motorcycle battery and see what happens.

 

TVS out and motor runs like a champ! No smoking. Now, what's the best way to add a new one without disassembling further? Thanks!

 

It can be soldered between the wires outside of the motor body, but for proper suppression of high frequencies which can interfere with the electronics it needs to be close to the motor body (within a couple of inches should be OK so you have some wire slack to work with). The TVS is not polarized, so it does not matter which wire goes to which side of the device. Be sure to insulate the connections so it does not short anything.

 

Let's prepare a site inside the motor bell casing. Scratch some insulation from the black and white wires near where they exit the casing. They are pretty rigid and so will prevent any shorting. As soon as I have a TVS to send you I'll do so and you can solder it into that spot and get back in business.

 

I think I can manage that. I really appreciate the help. I've already missed some beautiful Spring days, so can't wait to go topless again.

 

Congratulations to both of you for being so persistent in finding the cause. That's got to save a bunch of money to those that will go that far into it.


Well done, Wayne

 

The next one (there surely will be another) will be so much easier.

I need to improve the "Motor Repair" link to include diagnosing a TVS failure. It's a little tricky: depending on the resistance meter in use, it might show up or might be completely invisible, as Stamford's was.

 

It was sort of a shared learning experience. That's a good thing!
Could not have done it without the Forum.

 

RESOLVED!

The TVS (Transient Voltage Suppressor) was the culprit. And the fix was pretty easy.


The TVS had actually split in half and the small amount of oil it contained burned whenever current was applied, causing a burn mark as shown in the photo between the two leftover stubs from the TVS leads. Since the TVS was already split, it was a simple matter of bending the two halves up and snipping them close to the board.

This is where I skinned off some insulation from the wires, close to the motor body. I shortened the leads on the new TVS, jammed them into the wire strands and soldered them in place.

I wrapped the repair with electrical tape and reinstalled the motor.

I am back on the road with my top down just in time for some spectacular weather!
Thanks to Dennis, Gus and everyone else who helped with advice. This Forum rocks!

 

I've updated the "convertible top motor repair" page (link in sig. line below) to include:
- Stamford's material on removing and replacing the TVS, aka transient voltage suppressor, and ...
- New troubleshooting steps to help identify a failed TVS.


Note to Norri: I know you had put in place a sticky on this topic a couple of times, but it appears to have been lost again. Thought I should mention ...