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Old 09-12-2018, 06:08 PM   #1
petebee
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Default How to probe ECU to test voltage and continuity

I am trying to diagnose a stalling/poor running condition on my 91 245. I would like to test the ECT by measuring voltage and continuity at the LH ECU, but I am struggling to figure out how to probe the terminals. Below is a snippet that I copied from Brickboard:

"With the control unit connected, connect a voltmeter across LH ECU terminals 13 and 5 (ground) for Bosch LH 2.4 systems or terminals 2 and 5 (ground) for LH 2.2 systems. This unit is in the passenger side footwell, under the plastic cover. Remove the cover of the large electrical connector to access the backs of the pins for the test. Turn the ignition switch ON."

- with the control unit connect: I assume this means with the large connector plug remains connected to the ECU, correct?
- remove the cover of the large electrical connector to access the backs of pins: do I somehow remove the white plastic cover on the connector? It doesn't appear to have a method other that prying the thing off...doesn't seem right to me
- turn the ignition switch ON: key position with accessories on (warning lights on dash illuminated), but car not actually running, correct?

Thanks!
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Old 09-12-2018, 06:25 PM   #2
hiperfauto
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There's a screw holding the cover on.

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Old 09-12-2018, 08:05 PM   #3
bobxyz
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^^^ This. You'll need to unplug it first, and the shell slides off the end where you remove the screw. Careful, there are some wicked sharp plastic edges under the cover. You may need to remove some electrical tape from the cable end side if the shell is taped to the lower housing.

Even though they say not to do it, I usually probe the connector pins directly (connector unplugged) but am very careful and use an exceedingly light touch with the meter probes to not damage the pins.
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Old 09-12-2018, 08:05 PM   #4
ZVOLV
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Only rookies front probe without the proper test lead.

Needle probes work if you have to front probe, but you can still damage the terminal.

I made some front probing pins out of a spare module and soldered a length of wire to them.

You can pop that cover off and backprobe with grandma's "T" shaped pins. (Thanks Scanner Danner for the tip). Or I use professional needle probes to backprobe:

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Old 09-12-2018, 08:28 PM   #5
petebee
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Okay thanks that makes sense. Mine does have some tape on the end that I'll have to remove. Should I simply back probe (with cover off) without reconnecting the plug? Seems like it would be easy to mess things up trying to reconnect it to the ECU with that cover off.
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Old 09-13-2018, 08:41 AM   #6
bobxyz
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If you're just trying to test the coolant temp sensor, measure the resistance between the ect pin and a ground pin, at the connector, with connector unplugged and key off, and compare it to the chart in the manual. There's also a separate temp sensor that goes to the EZK box.

If you decide to probe the pins directly, without removing the shell, just lightly touch the probes to the sides of the pins. Do not shove the probe into the connector or else you'll damage it. Think about holding a probe to your closed eyelid and that's about the gentleness you need to prevent any damage.

You can also check continuity and behavior of the TPS idle/wot switches.
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