All, Would anyone like to donate a 308 style speedometer sender unit (a good or bad one) for surgery? I would like to cut this open, determine exactly how it works, post great pics for all you guys and a quick check method using common tools. I am hoping to dispell a lot of the frustration of troubleshooting this part and speedo systems in general. Someone's gotta have a bad one in a box somewhere. I will publish your name in the writeup with all the kudos and honors that I can muster. Please PM me if you are interested. Thanks, -Rjay
Tom, Thanks for your reply. I didn't know there were two. The one that I know of is mounted on the right rear tucked in next to the differential. I would be glad to work on both types but the one I have is for a 78 308. Can you explain the two types? -Rjay Image Unavailable, Please Login
Hi RJay I can post pics of one opened up. Or do you want the unit itself? I was surprised/disappointed to see the complexity of the circuit inside. And to add to the challenge, it is coated in a sealant (much like PC boards on navy ships) Gerrit
Gerrit, Yea! go ahead post the pics. Have you reversed this thing? If not, I would be glad to take a look and we can compare notes and then publish together. Let me know. -Rjay
Gerrit, I would like to get into one of these if you want to send one over. If not, the pics are a good start. I'll PM my address if you want to proceed. -Rjay
Hi Bob I'll post pictures first. And then send the sender to the requestor who is requesting the sender to eventually be sent back to the sender (you can guess what kind of day I am having Seriously, I will send it out over the weekend, Monday at worst. Inside it gives the impression of being a reluctance pickup with an amplifier built in running from a 4 pole 'wheel' . My reverse engineering skills aren't great, I'm useless with colour codes. The later style pickup looks very much like a reluctance unit with no amplifier inside. Which is why those don't seem to be as prone to failing. Gerrit
I do have one of those but the customer hasn't picked up his car that it came out of yet but if he doesn't want it, I'll be glad to send it to you.
My Audi 5000TQ has a sending unit very similar to the 308 (most senders from pre computer cars are). I have had the Audi unit apart many times due to a broken plastic part that requires regluing. There isn't much to them. The square drive stem has a disk on the end. A small magnet is molded into the disk, offset towards the edge. The upper part of the sender has a winding that creates an electrical pulse each time that the magnet passes under it. The speedo head reads the time between the pulses, long time = slow MPH, short time = fast MPH. The speedo needs a significantly short time between pulses to determine the speed, hence the speedo begins to read at something like 10 mph. Please excuse the crude discription, I am not a "lectrision".
Here are pics of the unit, on the detail pic the pickup/coil is at top. Gerrit http://dino308gt4.com Image Unavailable, Please Login Image Unavailable, Please Login
Gerrit, Good pics. Now we're getting to it! It does look like a small inductive pickup with an amplifier - at first glance. I can draw up the schematic if you would like to send it. Of course, I will send it back. Do you know what the difference is in the two types? I would like to try and work on both if I could. Tom is also seeing if he can send one too. Maybe between you two, I can get a look at both types. -Rjay
Rjay, Does anyone have scope traces of a functioning early sender? --I later found Rjay's post that describes sender function here: http://www.ferrarichat.com/forum/showthread.php?t=101159 Thanks! Chris
My understaning one generates electricty the one in your drawing part number 106324 (new number 124146) and the speedometer is a voltmeter. Part number 116987 is a pulse sender. They are not compatable with the others speedometer's nor mounts. From what I have been able to find out with my 81 GTSi. Parts book said it took 116987 mounted on the bottom. Took it out from the TOP and it was a 106324. Don't have a bad one or good one (I am using it in my car) but thought I would tell you what I have found out. stephen
I believe the cadence of the DC pulses is converted to an approximate AC voltage / current amplitude & frequency and the Speedo reacts (i. e. , as a meter) to this voltage / current output. Essentially then, the circuit is an inverter / rectifier (there may also be an amplifier to boost a clearer driver signal). This device / system is identical in theory to all inductive pick-up reference sensors which send info to a circuit within an ECU which performs the same operation on the signal and outputs an AC voltage (or) current as needed for processing (with this sender unit, the pick-up and processor circuit are integrated into the single device). If you can determine the specs / values of the various capacitors, resistors, transistors (an possibly the Op Amp, if there is one) that are in this thing, you can probably source all the replacement components from Newark Electronics for a total of about 3 bucks --- only problem is some of the solid state components may have proprietary p/n's and you'll have a tough time trying to cross reference them --- unless you take one from a healthy circuit and put on a scope to measure its specs. If you figure it out, you'll have a great sideline business rebuilding units from every import car made between 1975 and 1990!!! Good luck
Hi Dave The chip used in the earlier speedos is a SAY115Y, it is officially NLA. This chip converts pulses to current for the meter and divides the pulses by 32 to drive the odometer with stepper pulses. The data sheet for the SAY115 i attached. While the discrete components can be had from Newark the chip must be sourced from a surplus dealer. Last year someone found a bunch and some of us bought some. I think it was US$28 per chip. One of the previous threads has the website name. All of the senders generate pulses, but the charactersitics of the later ones are different. They are more like reluctance pickups while the early ones generate pulses by a sensor and internal amplifier. RJay posted the speedometer circuit in a previous post. Even the circuit changed somewhat over the 1974-198x time frame but fundamentally it was the same until the sensor changed. When my speedo broke a second time I built a replacement circuit using a LM2917. Works great except that I never built the odometer piece. For that I was going to use an 8 pin PIC controller since it was the simplest way to divide by 32. If someone wants to go that route I can publish the circuit. None of this is particularly challenging if you can read data sheets and solder. Gerrit http://dino308gt4.com
A blast from the past! I have my dead speedo sender out. It is different than what is pictured above, but very similar. Did anything come of this thread? Has anyone fixed one of these? I am going to fix mine. I have ordered a few parts and I have been playing around a little. I used a hall-effect sensor and that works, but it I will have to glue magnets to the arms. I just ordered an optical sensor that will require no mods to the arms, and I'm planning on just replacing the whole circuit board with my own. I'm not quite sure how I'm going to reseal it yet. Will I just RTV the thing closed or make up an aluminum collar of sort that presses on? Not sure yet. Mine had some water in it and a bunch of foul smelling white goo, so I would like to better than the factory seal. I would love to hear/see other's solutions. Image Unavailable, Please Login
Just for the record, this is from a 1975 Series 1 GT4, and the arms do not have any gearing like the later sender as mentioned above. The arms rotate once for each input rotation, not twice.
