Trans Program - Pulse Generator Testing

[Dan Harjung]

My son's 94 540i shows "Trans Program". I did some preliminary testing and found that both the turbine and speed sensors show infinite ohms. I'm not sure what type of sensors they are, so I don't know what resistance value I should expect to see. Have any of you tested resistance across the pulse generators? I tested continuity through the harness and it tests okay.

Thanks in advance.

Dan Harjung

[whiskychaser]

I'd be looking for about 325 ohms. If your meter just keeps scrolling up and up then I'd say they are faulty. A bit unusual for them both to go though. NO disrespect meant, but have you got the right TCM plug pins? The pin numbers are in the socket but you are of course looking at a mirror in the plug.

[Dan Harjung]

I'm pretty sure I have the right pins. I checked several times. I opened up the connector and verified that the wire colors are correct. I also measured at the transmission connector, just be be sure. I agree, it does seem strange, that's why I'm concerned. I intend to monitor the output of the sensors on a scope this evening. Does the speedometer use one of these, because the speedo works fine?

Thanks.

[whiskychaser]

I'm pretty sure I have the right pins. I checked several times. I opened up the connector and verified that the wire colors are correct. I also measured at the transmission connector, just be be sure. I agree, it does seem strange, that's why I'm concerned. I intend to monitor the output of the sensors on a scope this evening. Does the speedometer use one of these, because the speedo works fine?

Thanks.

No, they have nothing to do with the speedo. You have an input (turbine) speed sensor and an output speed sensor. (They are both the same part number so should read the same). They just 'report' to the TCM and IIRC, you use pins 14/42 and 16/44 to test them. I would certainly do as you are doing - test them at the TCM end. That way you are proving both the wiring and the sensor

[Dan Harjung]

Yup, those are the pins I'm using. So after some testing I'm even more confused. I hooked up both sensors to an oscilloscope and took a drive. The turbine sensor is showing a nice 20Vp-p sine wave at 1,219Hz at ~50 mph (3rd gear?). The output sensor is showing a distorted 1.5Vp-p waveform at 1,163Hz. I checked resistance again and measured 25K on the output and 22.5K on the turbine. I must have had the meter on the wrong range the first time. It seems like the output sensor is certainly not working correctly, but I wonder why I'm measuring such a high resistance on the turbine. Also, the ratio between input and output seems wrong for third gear, but pretty close for fourth. Maybe it uses fourth for failsafe mode? Should I just go ahead and replace both sensors?

[Dan Harjung]

I think those resistance measurements are for the input circuit of the TCM module, I still had it connected. When I disconnect the module I get infinite resistance on both sensors.

[whiskychaser]

I think those resistance measurements are for the input circuit of the TCM module, I still had it connected. When I disconnect the module I get infinite resistance on both sensors.

I don't use a back probe to test the sensors - just disconnect the plug at the TCM and probe the 'holes' in that. I cant explain why but for some reason the output sensors seem to fail more than the inputs. But they should both read about 325 ohms and a duff one may have your DMM keep scrolling up and up. You get 4th and reverse when it goes into failsafe (transprog). Of course, when it does that, a code is stored in the TCM and you can soon spot which one is causing the issue by having the codes read. I would have done that first and then double checked the sensors as you are doing. You don't want to drop the pan and change the sensors only to find it doesn't fix the problem.

BTW. I just checked the fault codes list and the pins read 16/44 and 13/42 (not 14)

[Dan Harjung]

That makes somewhat more sense. Between 16 and 44 I read about 350 ohms, and about 20K between 14 and 42. 13 is unpopulated in my connector. Is 16/44 the turbine sensor?

I need to have the dealer read codes, right? I'm about an hour and a half from the nearest dealer. Is it okay to drive that far in this condition?

[whiskychaser]

That makes somewhat more sense. Between 16 and 44 I read about 350 ohms, and about 20K between 14 and 42. 13 is unpopulated in my connector. Is 16/44 the turbine sensor?

I need to have the dealer read codes, right? I'm about an hour and a half from the nearest dealer. Is it okay to drive that far in this condition?

Just checked the wiring diagram. Yes, 16 and 44 are for the turbine sensor. 14 and 42 are for the output sensor. 13 is not used. I should have trusted my memory :-) So it looks like your output sensor has had it. And a duff sensor will always trigger failsafe mode.

No, you don't need to go to a dealer. If there isn't a member near you who can run a diagnostic then try an independent. But stress that it is an E34 as they are not OBDII. If you cant find one, you could always take a punt and try a new sensor. I would only use a genuine ZF/BMW part. It does mean draining the ATF and dropping the pan and you need to follow a short procedure for refilling.

[Dan Harjung]

So it looks like the Bentley manual has a mistake in the wiring diagram. Thanks for the correct pin numbers. Everything makes sense now.

I think I'll take a chance and replace the sensor. I replaced the trans filter and fluid a few years ago, so I'm somewhat familiar with the procedure, but any tips would be appreciated.

I'll make sure to get a ZF/BMW part, thanks for the advice. I'll let you know how it turns out.

Thanks for all of your help. I couldn't have gotten this far without it.