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Speed sensor for Cruise Control

I have a long thread about my efforts to fix my non-working cruise control, but since I have narrowed things down, I felt I should start a new thread to ask a more specific question.

First - a little bit of background - I have verified the brake switch, clutch switch, stalk functions, the cruise module, and the actuator, all work.

The one test that never passed is the test that shows the speed signal. Most writeups say that as long as the speedometer is working, this is not likely to be the problem. But in my case, its the one thing that I have not been able to rule out.

So, does anyone have any tips on what to do next?

Specifically, the speedometer works, but there is no speed signal making it to the cruise wiring harness. I have done the test where you connect a multimeter to the appropriate pins and turn the rear wheels, and this test fails.

Here is other thread:

Clutch Switch for Cruise Control
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1988 911 Carrera, M491 Cabriolet
Old 09-10-2018, 08:57 AM
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I once had some very minor speedometer problems, it was reading correctly but it was a little slugish. I ended up cleaning the magnets on the transmission and it seemed to help. Just a thought.
Old 09-10-2018, 08:10 PM
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I'd check the continuity of those wires in the cruise harness with a multimeter to rule out the wiring.
Old 09-11-2018, 07:39 AM
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Since my only solid lead on my non-working cruise is that the speed sensor signal is not reaching the cruise module, I thought I should remove the speedometer and see if there are any loose wires.

I didn't see anything loose, but perhaps someone can help by seeing if this looks right.





The wiring diagram I found said that a red/white wire goes from the speedometer to the cruise. From left to right, in the 5 positions, I have

Red/white
blank
red with a plastic connector
green
brown

I was a bit surprised that in the first position, there are 3 separate wires feeding into that one location.

Can anyone confirm if this all looks correct for a 1988?

Reminder: the speedometer and odometer work fine, I am trying to determine if there is a problem with the speed sensor signal output from the speedometer going into the cruise. I have already done the test to check the speed signal at the cruise wiring harness and that test failed, which led me to the speedometer.
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Old 10-10-2018, 07:58 PM
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The A terminal should have the White with red trace wire.
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Last edited by john walker's workshop; 10-11-2018 at 04:14 AM..
Old 10-10-2018, 09:20 PM
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By A terminal, do you mean the one on the left in my photo? It has 3 wires, all of which appear to be white with red trace. I would assume that one of those leads to the cruise wiring receptacle.

Anyone know of a wiring diagram or marked photo, for a 1988 speedometer?
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Last edited by megerian; 10-11-2018 at 04:17 PM..
Old 10-11-2018, 07:07 AM
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Posting for comparison, this photo is not my car, its from a 1987 that I found in another thread.

They do not match my setup, which either means that 1987 and 1988 have different wiring, or that one of them is wrong, and given that my cruise is not getting a speed signal, I am hoping that mine is wrong.




In particular, this has a single wire going into the terminal marked with a + sign, where mine has 3 wires.
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Old 10-11-2018, 08:12 AM
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One conclusion I could draw, based on John Walker's comment, and the 2 photos, is that on my speedo, the red wire and the white wires are reversed. While that may explain the speed sensor output not reaching the cruise, would that make sense given that my speedometer is working correctly?
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Old 10-11-2018, 03:18 PM
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Here is the back of my 88 speedo. I think you have two connectors switched.


Last edited by old man neri; 10-11-2018 at 06:07 PM..
Old 10-11-2018, 06:05 PM
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Well, that could not have gone any worse. Emboldened by the wiring diagrams, and the two photos, I decided to switch the two connectors (reversed the red with the white/red) turned on the car to try it out... snap! puff of smoke... fried the wiring!!!

I don't know if I also fried my speedo, or if I just need to rehab the fried wires and reconnect back to the way it was before. I also don't know why it happened, either due to my mistake of switching those connectors, or if by messing around with the wires and pushing the speedo back in I caused some wires to be crossed. CRAP!!!

Obviously I shouldnt have messed with it - given that I don't know exactly what I am doing, but after the switch, it did match exactly to the pic from Old Man Neri's post, and they are both 1988.
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Old 10-12-2018, 03:31 PM
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how did you end up - did mixing the wires up do any damage and what was the solution?
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Old 10-22-2018, 08:40 AM
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I had to leave on a trip shortly after, so I haven't tackled it yet.

I was hoping some other Pelicans would chime in with similar experiences and help out.

For instance, I am not exactly sure on the proper wires to use, if I end up having to replace portions of the wiring that got fried in the process.

Another issue I had was that the wires that lead into the bulbs did not seat very well and they seemed to easily pop out whenever I moved the speedo around, so its possible that was related to the problem and caused the spark.
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Old 10-22-2018, 08:55 AM
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I would suggest removing the speedometer and testing the connections with a voltmeter. easy to test is the lights - once you confirm those two you should be able to isolate the speed sensor easily - I am not certain if you can use a voltmeter to test the signal from it but I assume you could, just have someone else drive while you are testing,, this should confirm if you have a problem with wires or something inside speedo - I don't think there is a fuse but would be interested in knowing that for certain - not sure what serviceable components are inside the speedo,,,
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Old 10-22-2018, 09:02 AM
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We have cruise!! For the first time in the 7 years I have owned the car. Here is a recap of my long, occasionally stupid, but informative and ultimately successful saga, for your entertainment:

1) Bought car, cruise didn't work but it wasn't at the top of my to-do list. Most people said to replace cruise brain as the most common failure point. Searched and searched, no brain, no wiring harness. I eventually took apart the the passenger side floorboards, and found the elusive cruise wiring harness, dangling loose, no cruise brain.
2) Bought a cruise brain module from a fellow Pelican who said it worked "as far as I know", I installed it... cruise still didn't work.
3) Others suggested the clutch switch, so I took apart the center console and driver side floorboards, found what appeared to be a bent metal arm on the clutch switch, so I lined it up properly... cruise still didn't work
4) Eventually got suspicious of the brain module, so I sent it out to an expert that reported good news, it failed the bench test, but by resoldering some joints, it now passes the bench test! Installed it... cruise still didn't work.
5) Took advice to get a multimeter and validated using the trouble shooting guide in Bentley. Cruise stalk tests all passed, then this test failed
"Connect ohmmeter to ter.7 and 10, reading should be approx 2-6 ohm >>> actual reading 610 BAD"
Since this directly goes to the cruise servo - I felt I needed to replace that, so I found one on eBay for 30 bucks, and installed it... cruise still didn't work.
6) Ran another test which involves raising the rear of the car and spinning the wheels and connecting an ohmmeter to the wiring harness. This test also failed. Now you may ask why I didn't run this right away before replacing the servo. My reasoning at the time was that I already had a test that failed, with a relatively simple solution, plus from what I read, this test is unlikely to fail if the speedometer is working, and mine was working fine. But I did do the test, and it failed, so now I knew the speed signal wasn't making it from the speedometer to the cruise.
7) Now we come to the steps described at the beginning of this thread, pulled out the speedo, compared the wiring to the diagrams in Bentley and to photos shared by others, to determine that speedo didn't appear to be wired correctly. In the process of swapping wires and testing, I shorted a wire and fried it. Now I have no cruise and no speedo.
8) Got some help from Auto Edge, who replaced the fried wire, and reattached the wires to the speedo, the right way, and drumroll.... CRUISE WORKS!!

Had a lovely long drive today (20 degrees out but clear) and it was nice to use the cruise on a few long stretches of highway. I know, its not a big thing, but it was the last non-working thing on the car, so that feels good. Please don't ask how much time and money I wasted getting it to work :-)
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Last edited by megerian; 11-14-2018 at 11:14 AM..
Old 11-12-2018, 11:03 AM
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Glad to hear it.

I fixed my cruise last winter. I use it almost every day on my commute to work...even used it today.
Old 11-12-2018, 03:04 PM
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