I'm troubleshooting the dead speedo (+odometer) in my 87 300sdl and could use a bit of guidance. I'm trying to verify that the hall-effect (speed) sensor (on the tranny) is working. There are 2 leads coming from it which I assume are power in and signal out. Does anyone know if this correct and if so, where said power should come from? I see no reference to the sensor ANYWHERE in the W126 schematics, including power and ground distribution(s).

As for the speedo itself, I found a shorted electrolytic filter cap and supply resistor on the speedometer board. The speedo chip (UAF2115) isn't shorted but they're supposedly notorious for the stepper motor drivers (odometer driver) dying. I replaced the components and the chip appears to have believable voltages on its pins, but pulsing the input pin does not cause any needle deflection ;( Does anyone know if there's a bench test to verify basic speedo operation?

thanks!

__________________
1987 300SDL 216k
2003 F350 7.3L + Banks - 315k

Speedometer & Odometer Repairs — Overseas Speedometer

Rick repaired the odometer on Coda. I will probably be sending him the Odometer\Speedometer out of the 87 Champagne since the trip odometer wants to display the ONES place in half face.

Hope this helps you.

__________________
daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
Cluster Needles Paint
New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more

Hi and thanks for the reply. If I was convinced it was the speedometer, by eliminating wiring / sensor issues, I'd probably let someone fix mine as well.

__________________
1987 300SDL 216k
2003 F350 7.3L + Banks - 315k

Look in the driver's footwell. On the center console side...there is a multi-pin connector. This connects the sensor to the cluster. On my buddy's 560SEL this wasnt connected.

Additonally, if you monitor the frequency of the signal output, 113 Hz should be about 60 MPH. I think around 25 MPH you'll see 6VAC

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Excellent. Thanks for that, James Dean. I found several connectors where you mentioned but none of the pins in them buzz-out to the speedometer input. Also, its an open circuit from the lead going to the speedo input, to ground. Does that sound right for a hall-effect sensor (suppsoedly without an internal amp) ? It wreaks of a missing sensor ground but I can't find out if and where the sensor grounds...

With the key on, I read about 4.52VDC on the the lead going to the speedo with it pulled from the speedo (yes the same one that's open tp ground). I get around 137mV AC, though I haven't scoped anything to look for pulses yet. Since that vehicle speed signal goes several places, I wonder if something else is clamping it to 4.5vdc.

__________________
1987 300SDL 216k
2003 F350 7.3L + Banks - 315k

Picture of transmission harness attached.

Instrument cluster, Odometer, Speedometer, Tachometer links

.

Attached Thumbnails

 

Thanks much. It looks like it may have been working ever since I repaired the speedometer circuit board. I had the car on Jackstands and was chicken to punch it enough get the wheels moving fast enough to get a signal out of the sensor. I finally just held the throttle at about 2000 rpm and voila... 20mph! Don't know about the odometer yet... Thanks to everyone for the info!

__________________
1987 300SDL 216k
2003 F350 7.3L + Banks - 315k

Good news: Cruise works!
Bad News : Odometer not so much
Guess I'll try replacing the UAF2115 speedo chip since you can buy them from ebay for < $10

__________________
1987 300SDL 216k
2003 F350 7.3L + Banks - 315k