Notices
928 Forum 1978-1995
Sponsored by:
Sponsored by: 928 Specialists

Who's in control here?

Thread Tools
 
Search this Thread
 
Old 06-29-2002, 01:47 PM
  #1  
John Struthers
User
Thread Starter
 
John Struthers's Avatar
 
Join Date: Jul 2001
Location: Midland, Texas
Posts: 3,291
Received 2 Likes on 2 Posts
Question Who's in control here?

What is the controlling device in the world of mileage/mph?
I always thought, right or wrong, that if the odometer went 'down' my speedo wouldn't work.
But after a year of intermittent speedo operation I am starting to believe the reverse is true... speedometer 'down' = odometer 'down'.
Checked the speedo sender - cleaned, tightened loose wiring nut -, soldered in a waterproof, rubberized, 4 pin connector in the tire well. I'm thinking a new board is in order when I pull the pod. BTW Who is the Guy who posted the "just clean the board contacts by cleaning with a pencil eraser"? Please send pictures of your hands - do they resemble E.T.'s? I'll bet that light on the fingertip dealie comes in handy!
Snow, don't even say it!!
It's really nice to drive 1500 miles and only put 286 miles on the clock but it can be a bummer come belt retentioning time.
What's the relationship between the odo and speedo?
TIA
John S and Pattycakes
Old 06-29-2002, 01:53 PM
  #2  
WallyP

Rennlist Member
Rennlist Site Sponsor

 
WallyP's Avatar
 
Join Date: Jun 2001
Location: Acworth, GA
Posts: 6,469
Likes: 0
Received 11 Likes on 11 Posts
Post

The speedometer and odometer are independent devices in a common housing, fed by a common input.

No signal in, neither works. Good signal in, broken orange gear, speedometer works, odo doesn't - this is a common failure.
Old 06-29-2002, 03:12 PM
  #3  
John Struthers
User
Thread Starter
 
John Struthers's Avatar
 
Join Date: Jul 2001
Location: Midland, Texas
Posts: 3,291
Received 2 Likes on 2 Posts
Question

Thanks Wally,
I was browsing the archives when I came across your reply to a back-up harness problem, good stuff! Dennis wilson, check it out!
For clarification... the sending unit on the transmission signals, both, speedo and tach?
The orange gear is refering to the little bugger in the odometer probably related to reset on the trip meter?
In you experience, the odometer would or would not affect the the speedo- bouncing/pegging needle?
My tach is a happy camper but the speedo needle will bounce, then peg if I go over 80mph. Also,
if I romp on it and attack the 70mph barrier the speedo goes straight to peg. In your experience, does this sound like a board problem or a sending unit problem?
I soldered in a new connector - orig. post - so I think we can eliminate that as a problem.
TIA
John S. & Pattycakes <img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" />
Old 06-29-2002, 03:38 PM
  #4  
Mike Schmidt
Racer
 
Mike Schmidt's Avatar
 
Join Date: Jul 2001
Location: Chicago
Posts: 395
Likes: 0
Received 1 Like on 1 Post
Post

The sending unit on the transmission does provide the signal for both the tach and speedometer.
The orange gear is the one in the odometer, and it may or may not be messed up due to resetting the trip odometer.

The odometer should not have anything to do with the strange speedometer behavior.

The speedometer problem sounds most like a connection problem at the board or elsewhere. The sending unit works by picking up pulses from a ring with magnets that spins inside the transmission. It's just a Hall effect device, and should generally be pretty reliable. The latest cars used the signal from the ABS sensors instead of the ring with magnets in the transmission method.
Old 06-29-2002, 03:43 PM
  #5  
iantdopps
Instructor
 
iantdopps's Avatar
 
Join Date: Jun 2002
Posts: 223
Likes: 0
Received 0 Likes on 0 Posts
Post

Hey John! If everything quits, it is a systemic problem, like a bad motor that controls BOTH the odometer AND the speedometer, or a lack of proper current. Both are controlled by the same motor, but after the motor, they have seperate gears that control them seperately. So, the gear that connects the motor to the odometer is old/broken/subluxated. So, open it up and look at how the gears condition and position are between the motor and the odometer. Just be careful with the speedo needle....they are very fragile....and don't loose the screws that hold the speedometer face on :-) from your fellow Weissach owner!
Old 06-29-2002, 03:49 PM
  #6  
John Struthers
User
Thread Starter
 
John Struthers's Avatar
 
Join Date: Jul 2001
Location: Midland, Texas
Posts: 3,291
Received 2 Likes on 2 Posts
Talking

Thanks, all...
Let the surgery begin!
John S. <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />
Old 06-29-2002, 03:50 PM
  #7  
Dozman
Addict
Rennlist Member

 
Dozman's Avatar
 
Join Date: Aug 2001
Location: Land of many Potholes, Michigan
Posts: 1,047
Likes: 0
Received 1 Like on 1 Post
Post

John,

I couldn't find the archive your talking about from Wally. How can I find it ? Would like to read that. I am still having problems with my tach.

John D.
'85 928 Auto, Black
Old 06-29-2002, 04:41 PM
  #8  
John Struthers
User
Thread Starter
 
John Struthers's Avatar
 
Join Date: Jul 2001
Location: Midland, Texas
Posts: 3,291
Received 2 Likes on 2 Posts
Post

Doz,
Go to top of the page click page 9, scroll down it should be right before the post on John B's engine swap.
We are talking about the back -up harness here, Right?
Imbedded in the quetion was a query as to what some of the switches and sending unit's sprouted off of the Tranny, O.K.?
John.S <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
Old 06-29-2002, 06:09 PM
  #9  
Snowball the 81 white 928
Registered User
 
Snowball the 81 white 928's Avatar
 
Join Date: Jun 2001
Location: Arms Cache, WY
Posts: 373
Likes: 0
Received 0 Likes on 0 Posts
Post

i believe al haig is in control here.
Old 06-29-2002, 08:22 PM
  #10  
Dennis Wilson
Drifting
 
Dennis Wilson's Avatar
 
Join Date: May 2001
Location: Owasso, OK
Posts: 2,747
Likes: 0
Received 3 Likes on 3 Posts
Post

John,

Thanks for the tip. I received a new reverse light switch yesterday. That should fix the backup light problem if I can remember to put the fuse back in.

Thanks,

Dennis
Old 06-30-2002, 12:02 PM
  #11  
Steve Cattaneo
Three Wheelin'
 
Steve Cattaneo's Avatar
 
Join Date: May 2001
Location: Hudson Valley NY
Posts: 1,641
Likes: 0
Received 0 Likes on 0 Posts
Post

The electronic sensor on the differential cover only serves the speedometer, its not a tach input.


The tach signal is from the ignition-timing sensor (rear of engine black). Not to be confused with the TDC (top dead center sensor, test sensor) on the engine bell housing. The ignition-timing sensor has a duel function. It serves as a reference for engine speed (RPM) and crank position sensor (for timing). The tack input signal is generated from the EZK controller via the ignition-timing sensor, on later vehicles. On the early 928’s the tach signal is form the ignition control unit. Pin # 8 tach feed wire.
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
Old 06-30-2002, 12:58 PM
  #12  
Mike Schmidt
Racer
 
Mike Schmidt's Avatar
 
Join Date: Jul 2001
Location: Chicago
Posts: 395
Likes: 0
Received 1 Like on 1 Post
Post

[quote]Originally posted by Mike Schmidt:
<strong>The sending unit on the transmission does provide the signal for both the tach and speedometer.</strong><hr></blockquote>

Ooops. What I meant to say was that it sends the signal for the speedometer and ODOMETER, not the tachometer. As Steve said, the tachometer signal is a whole different thing.
Old 06-30-2002, 05:13 PM
  #13  
John Struthers
User
Thread Starter
 
John Struthers's Avatar
 
Join Date: Jul 2001
Location: Midland, Texas
Posts: 3,291
Received 2 Likes on 2 Posts
Red face

Mike,
Caught it, thanks. But then promptly bum-steered Doz ...just thinking of the archive location and not content. <img src="graemlins/oops.gif" border="0" alt="[oops]" />

Doz,
Sorry!!!
May have some help with the following question.

Steve,
Thanks much...
A question, if I may?
Is there an amplifier for either the speedo, or, tach.? If so, where does it, they live?
TIA

Snow,
I thought Al only takes over after a lung shot...
Anyway he's the point man for the Army of One.
Iraq or bust...you'd like him he's like the rest of those Talking Heads on Fox - better a million dead Arabs than a one penny raise -or less- in gas pricing. What can we blame them for, today?
Terroristic breathing with felony intent to survive? Yuk, yuk, yuk, hey Moe!

John S.
Old 06-30-2002, 07:13 PM
  #14  
Snowball the 81 white 928
Registered User
 
Snowball the 81 white 928's Avatar
 
Join Date: Jun 2001
Location: Arms Cache, WY
Posts: 373
Likes: 0
Received 0 Likes on 0 Posts
Post

you must have been the only one listening to my political rants at the dinner table. <img src="graemlins/roflmao.gif" border="0" alt="[hiha]" />
Old 06-30-2002, 10:14 PM
  #15  
Steve Cattaneo
Three Wheelin'
 
Steve Cattaneo's Avatar
 
Join Date: May 2001
Location: Hudson Valley NY
Posts: 1,641
Likes: 0
Received 0 Likes on 0 Posts
Post

John,
The tech input signal on your car originates from the distributor. The transistor ignition control unit processes it.

The speedometer signal is processes by a solid-state circuit in the mileage counter (speedometer head-). If your odometer is not working it probably has a striped gear.

On Dozmen car the AC signal from the RPM and reference mark transmitter (magnetic inductive sensor), Is fed into the EZF controller, there the AC signal is converted into a digital signal.

If Dozman were having a problem with his tech I would change the RPM, Rm sensor. It’s a magnetic sensor. When the magnet gets weak it affects the signal, the only way to check it is with an oscilloscope. If the tech still doesn’t work with a new sensor substitute the EZF controller with a good donor one. If he has access to an oscilloscope, connect the positive lead to terminal # 7, and negative lead to terminal #19 of the disconnect EZF control plug. You should see a sine wave oscillation of at least 2.5 volts.
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />


Quick Reply: Who's in control here?



All times are GMT -3. The time now is 10:15 PM.