RPM transducer question

[project 911]

This is starting to make me crazy!

I have a 2.2T with Zeniths, MSD and Crane optical points replacement.

A while back I started to have problems with backfiring and some popping through the carbs. I checked for exhaust leaks , etc. I tested and found that the RPM transducer had quit (had no power activating the enrichment solinoid on the carb). I replaced it with another one I had and everything seemed better till the other day. Well, the replacement is toast too. I think I probably damaged both of them trying to hook up my tach to the MSD box.

So, some people say you can just toss the whole setup and be fine which I would love to do at this point, but I'm not sure how you "tune out" the popping.

The other option is to repair one or both of the transducers. I'd like to do it myself if possible, but I know there is someone on this board that can do it (companzer?).

I tried running a jumper wire (without the tranducer) that powers the solinoid when in the closed throttle possition, but still seem to have problems. I'm also wondering since I don't have and tach or points signal going to the transducer if that makes the whole thing not work anyway.

I'm having a terrible time trying to get the car tuned and I want to eliminate this part from the picture before I move on to all the other possible issues.

The main problem right now seems to be right at the point that the microswitch is activated (closed throttle). When I press the gas it pops through the carbs (just when I first touch the gas). It wants to backfire on decel unless I completely lift off the gas to activate the switch.

The car starts fine, accelerates fine, pulls hard, but this popping and banging is keeping me from driving comfortably.

Any advice? (I did go through lots of info in the archives about the transducer)

[Early_S_Man]

Whatever signal from the Crane that triggers the MSD, or tach, should trigger the Speed Switch/RPM Transducer just fine. Just to be sure ... test the input signal at the RPM Transducer with a 12 Volt test lamp, and it should blink during cranking.

[jstobo]

I did not hook it up, but on my Zenith conversion I ran the tach output from the MSD to the RPM transducer where the black/purple wire plugs in. This is a nice square wave 12 volt signal. I tried this signal because in wiring the MSD I disconnected the signal that goes to the transducer from the stock CDI box as my tach needed the RPM adapter from MSD to work. You could probably tie in the crane trigger also, in my case another wire from the points and tach adapter, but the MSD tach signal was so easy to wire up. It's funny. The MSD tach signal would not drive my tach but it worked on the RPM Transducer and also on my handheld Tach and Dwell meter. Go Figure.

You need to wire the transducer to a RPM signal because the 12 volt output to the microswitch occurs at 1350 RPM and higher. Without an RPM signal the transducer thinks it is less than 1300 RPM and will not send 12 volts to the microswitch. I went through this wiring testing because I thought I was getting too much backfiring. Turned out I had a vacuum leak on one of the carb hoses. Tough to see. Now all tuned up and no backfiring. I kinda miss it though. Check for some vacuum leaks. It may be most of your problem. Hope this helps.

[rs dreamer]

Just curious .. I know what the RPM tranducer does for an MFI pump to prevent backfiring. Why is it used for a setup that has Zenith carbs? TIA Gary

[project 911]

I think it has the same function for both. To alter the mixture on deceleration, etc. There is a small solenoid on one carb that is activated when in the closed throttle position. I still don't completely understand it...

[project 911]

I have two part # 0 336 611 006, both seem to be dead at this point. I have tested many of the resistors and all seem ok except one, but it doesn't have any kind of color code for me to identify what the part is. I'm not much at reading schematics, but is there some way to identify the part and maybe get something at Radio Shack?
I have one schematic that someone posted of an MFI transducer, but it has a different part number.

I'm putting in a Autometer tach shortly, so I hope that will solve that problem, but it sounds like if I still plan to use the transducer, I'll need to send a tach signal to it. I figure that's how I burned these out though. Still confused. I guess I'm hoping someone will just say definitively that I can just take all that stuff out...

[project 911]

As I said, I tested most of the stuff with a meter, but the part labeled #1 tests as open(?). I don't know if that means it is the part that is burned out or what. The part labeled #2 was corroded, and one of the connections to it broke when I touched it. Can anyone tell me what #1 is?

[Early_S_Man]

Steve,

Just ran accross this thread, again, during a search ... hopefully you haven't given up on the repair and thrown out the two dead Speed Switch units!!!

The schematic I posted on other threads is the same for all of the 1969 and later Speed Switches [aka RPM Transducer/Speed Relay] regardless of the application on carbed or MFI engines ... just the values of C4, R5, and R6 changed!

D7 and D8 [your device #1] are both protection devices or surge suppressors of the MOV, SIDACtor or bidirectional Transzorb type ... here are a couple of comprehensive databooks that can be downloaded:

http://www.onsemi.com/PowerSolutions/search.do?searchString=DL150-D.pdf&searchType=others&tabbed=Y&clearFilters=Y

http://www.littlefuse.com/cgi-bin/r.cgi/en/allsearch.html?DoSearch=Y&WhichDB=&textfield=sidactor+databook

Even though I don't have a part number or specs for D7 or D8 [haven't put one on a curve tracer, yet] ... it is safe to say that around 15 - 16 Volts is the effective range they need to be in to protect the 16 Volt capacitors from blowing up when transients occur with a failing alternator!

Your device #2 is T2, a Siemens BCY45-IX transistor, no longer available, except for NOS parts (new old stock) that can be replaced by a 2N2222A transistor [selected for hFe > 200] or 2N5088 transistor.

[project 911]

Thanks so much Warren-

I still have both units and have just been running without them. I ended up with an aftermarket tach that solved that problem.
I'll take a look at the links for more info.

[HKZ Bob]

Steve are you satisfied with your MSD.

What is the advantage out of your view.

Bob

[stlrj]

To help you understand how it works or what it does, the rpm transducer activates the solenoid which introduces air to weaken a too rich mixture that results when the throttle is closed on overrun where high vacuum conditions cause too much fuel to be introduced for the plugs to fire and foul your plugs which causes popping/back firing.

[project 911]

Hi Bob,

I've been very happy with my MSD setup, but I don't have any great stories like some others regarding performance improvements. I didn't notice any difference in performance, so it probably wasn't worth all the effort to install. I do like being able to set the rev-limit though.

Steve

[POKDUB]

I have just repaired my RPM transducer over the weekend. Simple fix.

The spade connectors had become loose from the pcb. This was very hard to see. The solder joints had broken on the underside (reverse of your two pics). Have a look again, wiggle the connections, see if there is any breaks in the solder connections.

If so, you should heat solder, and if possible use a solder vacum pump to suck off old stuff. Emery paper the connector points and resolder.

On another point, I dont think the RPM Transducer is really an aid to tuning, it is really geared towards fuel cutoff. The popping / backfiring needs to be cured elsewhere (IMO).

Rgds
Peter

[mhunter]

Originally Posted by project 911

Hi Bob,

I've been very happy with my MSD setup, but I don't have any great stories like some others regarding performance improvements. I didn't notice any difference in performance, so it probably wasn't worth all the effort to install. I do like being able to set the rev-limit though.

Steve

So was the RPM transducer the problem with popping or backfiring on decel? I have 71 911T 2.2 Zenith carbs too. It backfires on decel, and sometimes at idle. My old spark plugs indicated some richness.