Well there's $225 bucks for nothing

Bought the pulse sensor today for my speedo issue since there were no obvious wiring problems anywhere, got it installed and I the speedometer/odometer still doesn't work. Called the shop that works on it and they said by what I am describing, 98% of the time the new sensor would be the fix. I seem to always live on the 2% side.

 

You must be 1%, I am the other 1% - this happens to me all the time. What is your next step?

 

Well, considering I've checked the connections in the instrument module multiple times and inspected the the wire connections at the sensor and at the connector at the left rear strut, I'm going to have to take it in to the shop. Knowing my luck, not only will I be out for the sensor, now I'll be out for an instrument pod.

I guess if it is something in the instrument pod, I can put the old pulse sensor unit back on the car and sell the new one to whoever has that problem next.

 

This is hardly in the same league as you, but I have similar frustrations being in the 2% with my BMW MINI's steering/suspension gremlins. So I can sympathise with your frustration.

All the best,
Andrew.

 

It does suck to replace parts that are not the problem ....... ........ but at least you have a new 'sender' ................ though if it really bugs you ...... you can always put that new sender on ebay and recoupe most of the cost .......

 

Check the bullet connections on the main battery cables (there are 2 of them). i would guess a bad ground connection. Check the connection with a voltmeter / ohm meter...
It does suck to be that 2%, but I think you will get the problem fixed.
BT

 

Send it back to whomever you bought it from. You'd be amazed what I get 'convinced' to take back into inventory.

 

Could you elaborate on this a little more? I'm not sure what you mean by the bullet connections. I do need to get a volt/ohm meter with all the DIY work I've been doing lately so I am curious what I need to do and how to test.

 

May need the speedo sensor from you. My speedo reads at least 10mph fast. Which doesn't bother me. But if the mileage on the ODO is also high, I may need to replace it. I'm assuming that the speedo and ODO get there data from the same place. Will have to take the GPS along and compare.........Lou

 

Hey Lou. In talking with my shop about my problem, they indicated that the pulse sensor does run the speedometer and ODO so, just let me know. After lots of calls to the shop and lots of trying "this and that", I am having to take the car into the shop Monday where they will do the tests to see where the problem is. If it turns out the pulse sensors were still good, then I plan to just put the old one back and I can send you this new one if you eventually want/need it.

Mike

 

The speedo is basically a 3 part system on the 348...

One part is the sensor which produces a square wave form based on the sensor wheel on the main-shaft. This waves form can be analyzed with an oscilloscope on the green and white wire at the connector. To analyze you need to put the car in neutral and roll it slowly back or forwards at least 2 feet, you should see a swing in the reading as the voltage pulse is formed when the sensor is triggered (4 times per shaft revolution).

This pulse is then read by a small circuit board (a simple AD/DA circuit) at a given rate for metric and another rate for imperial. The little board then converts the pulses to an output voltage which powers a DC motor connected to a magnetic disc. The DC motor also runs a drive with two worm screws that power the two scales for the odometer and the trip counter.

The magnetic disc driven by the motor spins inside an iron cup, which is connected to a tension spring and the needle which reads out the speed.

The tension of the helical spring is critical as it is what the magnetized disc has to work against to then deliver a speed reading.

I found this out as mine was consistently over-reading by some 3-4 MPH below 40 mph, and over 10 MPH over 120. Now I have it tweaked against my GPS to read the exact speed below 70mph and about 15 mph over 120, with city speed control being the more critical.

My spring was quite corroded and I am now trying to change the system with a totally digital one, with a stepper motor directly calibrated to read a given speed for a given range of pulses (calibrated at every 5 mph interval from 0 to 160)...

Hope this helps you figure it out... You can test the wiring by these simple tests:

1) Sensor test - Take your suspect speed sensor, connect a simple ammeter on the millivolt scale. Pass a magnetized screw driver at the sensor front and look for a swing in output. If you get one, it is fine.

2) Wiring loom test - Put your ammeter on continuity, and find which each connector point corresponds to which pin in the dash connector for the speedo. At the same time test them against earth to ensure neither is shorting. If each matches one behind the dash and there is no shorting to earth, the circuit is OK... You can further test it by putting a paperclip as a jumper at the sensor end, then measuring for continuity between the two pins in the dash end of the wires. If you read continuity and then read no continuity when you remove the paperclip you have good wiring.

3) Combined test - If the two above tests pass, you can plug the sensor into the wiring harness and repeat test one but reading it from the dash connector.

If the above 3 pass then your speedo is suspect.

4) Check if it the two counters do continue recording the distances - if so you have good wiring and motor, then the problem is mechanical, probably a seized pin (very rare)
5) Open up the speedo (three screws at the back) and visually inspect the wiring and mini connectors - you can take the little circuit board off. Look at the connector solder points and underside of the board for scorch marks etc. (I have a spare imperial circuit board and can do component level fixing on these if needed)

My $0.02 worth and wishes of good luck!

 

It simply is amazing how much info there is on this website. So it sounds like if the ODO mileage is correct but the speed isn't, then it's a spring adjustment. I'll try this today and see where I stand.........Lou

 

Thanks Marco. I will look at doing these test this weekend before I take it down to the shop Monday.

 

Yep, Marco is a cross between Albert Einstein and Walter Middy!

 

Had to take it to the shop and they called today to confirm it was the speedometer module. The pulse sensor and the wiring all checked out ok.

 

Just to answer your question, the main battery cables have two snap together connections. One is under the air filter box, the other is close to the battery. They are red plastic connections that allow you to disconnect the power without removing or disconnecting the battery. Glad you found the problem though!
BT

 

Ahhhhh. Yeah. I thought you meant something else. I replaced the pulse sensor first so yeah, the connection was good. I also had to replace the battery in the past 2 weeks also, so that one was connected too.