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ABS Speed Sensor Testing '90 GT

 
Old 06-18-2017, 11:04 PM
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buccicone
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Default ABS Speed Sensor Testing '90 GT

Having an issue with ABS light/warning/PSD disabled once I start rolling. I am testing the sensors through the harness at the ABS/PSD/RDK control unit under the driver's side dash. Found the terminals to test each individual sensor on the wiring harness with the control unit disconnected. All 4 sensors pass the resistance test within a range of 1020-1070 Ohms. I hook up a digital volt meter to test output of the sensors. Front sensors put out around the same 500 milivolts (AC) spinning the wheels by hand. The rears even though I get resistance reading of 1060-1070 ohms, produce no voltage on either side. Checked the connections in the spare tire well and all seem good. Am I coincidentally having 2 bad sensors? The sensors have been cleaned at least two times. Can't say I have cleaned the toothed wheels in the rear. I will remove the wheels next and pull the sensors again and check them individually. Any other suggestions? Thanks everyone..............
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Old 06-19-2017, 01:46 AM
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Mike, when you pull the rear ABS sensors, rapidly pass it across metal with your meter on it. See if it responds to metal. Then look at your tone rings.
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Old 06-19-2017, 09:04 AM
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Seth:

I got one pulled this morning. Seems to respond according to your test. Will see if mounted and using the toothed gear and directly connected DVM meter to the sensor will see if there is activity. Did not peer down at the toothed ring to see if it is dirty.
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Old 06-20-2017, 07:14 PM
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Everything checks out at the sensors (at least what I can tell on the DVM). Took it out the first time. After rolling and getting to speed the ABS light returns. Drove it a couple of miles, shut off the engine for a few seconds, restarted and got rolling. ABS light stays out all the way home. Any ideas....Thanks
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Old 06-20-2017, 07:24 PM
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Hmmm...
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Old 06-21-2017, 05:32 AM
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Sounds like one of the relays on the ABS unit.
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Old 06-21-2017, 08:26 AM
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Originally Posted by John Speake View Post
Sounds like one of the relays on the ABS unit.

That was one of my next steps. I will be driving it today. I will see if the same happens. If so, it could be heat related. Thanks John, will do.
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Old 06-21-2017, 02:09 PM
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Drove it on initial startup a mile today before the ABS light came on. Immediately turned off and restarted....No light the rest of the morning all the way home 80 miles worth.
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Old 06-23-2017, 01:04 AM
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Have you checked continuity between the sensor wires and the sensor body - There should not be any i.e. resistance should be infinite.

I have seen at least couple of sensors where the resistance across the sensor wires was as expected but one or both wires (or more accurately the coil inside the sensor) was shorting out to the sensor body due to physical failure. And yes it was intermittent to start with and could be heat related.
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Old 06-29-2017, 11:08 PM
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Switched out speed sensors from the '88 S4 one by one. No change. All original sensors back in. Switched out the ABS relay on the CE panel with one from the '88. In the process something got disturbed around the CE panel? The car won't start!���� Ran 2hrs. earlier. Fuel pump relay jumped and fuel flowing. Got spark when cranked. Cranking fast. PO said he replaced the CPS and LH rebuilt. Hall sensor? Will check on on those..........
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Old 10-04-2017, 10:27 PM
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I believe I found the problem. I pulled the PSD relay in the spare tire well and tested it. Intermittent or dead and heat related operation. I put a hair dryer on the relay after I opened it up. Jumped the 85-86 terminals with 12 volts. Works when heated up. Took the GT out and the relay still warm. NO ABS light! The relay is hardly working and intermittent at best. Part number 964-615-135-00
superseded by 964-615-135-01 . Will order one and report back.
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Old 10-04-2017, 10:48 PM
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Excellent!
Good troubleshooting.
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Old 06-23-2018, 12:19 PM
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I've slowly got back to the GT ABS issue and wanted to update. I've cleaned almost every connector I believe with Deoxit 100 on the fuse, relay, grounds, power supply to ABS unit next to power steering reservoir. I also replaced the 2 relays under the fender with brand new ones. The relay in the tire well was replaced last Fall. Last week I cleaned all the toothed wheels especially the fronts. The rears of course are inside the bearing housing but I shot them with brake clean and compressed air. Still no joy. I drove to SITM and the ABS light was on most of the trip. That is a new thing because before once the car was driven for 30 minutes and restarted it would go away indefinitely for the rest of the day. Now it does not do that. It is on all the time.

Now.....today I had it on my quick jack lift to do some things and I thought I would spin the wheels and see if the light appears. With rear wheels spinning engine running (PSD), NO light except the green PSD light come on which I understand is normal. With the rear wheels spinning, I spun each front wheel one at a time by hand as fast as I could manage. Drivers side: No ABS light. Passenger side: Light come on. OK, does this tell me anything of which side may be the issue with a speed sensor? I was thinking it may be the driver's side since it is not being sensed when I spin the passenger side?
Any more suggestions?

Thanks everyone
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Old 06-24-2018, 01:15 AM
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Bump?
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Old 07-20-2018, 12:12 AM
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I finally got down to the last piece of the puzzle: The ABS/PSD controller (computer). I switched out the controller from my '93 GTS. Same part number. Took it out for a drive. NO ABS light. Got a couple of leads on a replacement (used). New these controllers are $2K new.
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