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Manually bleeding the HCU

Old 03-02-2019, 12:08 AM
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Default Manually bleeding the HCU

Service manual says you need a DRB tool to do some fancy stuff with the HCU motor and solenoids. Those tools apparently cost a couple thousand or more, and I haven't found an OBD tool that will do it yet. Could you manually activate the motor and solenoids in some sequence to achieve the same effect?

I rebuild my HCU a few days ago and it seems fairly simple. You have the motor that is on or off and six solenoid valves that are open or closed. It's simple enough to activate them with an external power supply. I used 12V for each with just enough current to get the solenoids to click, and even then only for a few fractions of a second. Anyone have specs for these things to avoid burning them out?

The six solenoid valves work as mostly-isolated pairs. On the 8 pin connector coming from the HCU, the 2 larger pins are neutrals for the solenoids, and the 6 remaining pins are arranged as vertical pairs corresponding to the solenoid pairs. The three pins furthest from the Christmas tree fastener thing only serve to cut off normal (non ABS) flow through the HCU, so activating those alone shouldn't accomplish anything. Activating the other 3 individually, and also possibly in combination with their pairs, should open flow through the accumulators that does not occur during normal driving. I don't think the motor or any components could be damaged by being on for ~5 seconds at a time during this process, although I could see it potentially stalling if pressure isn't relieved from both the rear and a front circuit after it's build up - all depends on how some of those valves work.

Anyway, don't really have a solid plan yet, but just wanted to put the thoughts out there and see if anyone had experience here. I've got one more brake tube to replace, and after that's done I'll do some experimenting. Picked up some Tygon 2375 so I can see what's going on as I bleed the brakes. Figure I'd just bleed them back into the MC reservoir since it's a clean system and new fluid.
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Old 03-02-2019, 10:44 AM
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So you already have a Jeep Antilock Brake System FSM? With diagrams, troubleshooting procedures, etc.? or your reading from the main Jeep FSM?

Last edited by Muddz; 03-02-2019 at 10:52 AM.
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Old 03-02-2019, 03:02 PM
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Originally Posted by Muddz View Post
So you already have a Jeep Antilock Brake System FSM? With diagrams, troubleshooting procedures, etc.? or your reading from the main Jeep FSM?
Oh, didn't know they had that, I was just going off the main FSM.

Now to find one that isn't a moldy copy from eBay haha.

Last edited by Hatsuwr; 03-02-2019 at 04:48 PM.
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Old 03-03-2019, 06:37 AM
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Yes I have a couple of them for different years, which are very similar ... for Cherokee, Grand Cherokee and Wrangler.

Here's one for 1996-1997 Jeep Cherokee Teves Mark IVG

https://www.ebay.com/itm/1996-1997-J...j51Y:rk:1:pf:0

I have one for 1997 - 1998 Teves Mark IVG and Teves Mark 20

Fairly informative some discussion for the entire operations of the anti-lock brake system plus diagnostic procedures, diagrams, pinouts, etc. does integrate a DRB scanner into diagnostics which you should be able to circumvent.

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Old 03-03-2019, 03:31 PM
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Originally Posted by Muddz View Post
Yes I have a couple of them for different years, which are very similar ... for Cherokee, Grand Cherokee and Wrangler.

Here's one for 1996-1997 Jeep Cherokee Teves Mark IVG

https://www.ebay.com/itm/1996-1997-J...j51Y:rk:1:pf:0

I have one for 1997 - 1998 Teves Mark IVG and Teves Mark 20

Fairly informative some discussion for the entire operations of the anti-lock brake system plus diagnostic procedures, diagrams, pinouts, etc. does integrate a DRB scanner into diagnostics which you should be able to circumvent.
Ya that's the moldy one I saw haha. Got one on Amazon for a few dollars more that seems to be that same one. Will see in a few days!
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Old 03-03-2019, 06:45 PM
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Yeah, after a closer inspection that one is moldy.

I purchased mine for an XJ and a ZJ in sets of five and six, no mold but the main FSM for the ZJ has some dirty folded pages of which I have a PDF version.

I've been able to obtain information from the various supplements that can't normally be found on the internet.

Unless you can find someone who has repaired and reconditioned the entire ABS I think the ABS FSM is probably going to provide the most details. (without a DRB scanner)

I have an older Snap-On MT-2500 that works with pre-obd1, obd1 and some obd2 Jeeps but I haven't tried connecting it to ABS which may require other connectors to see what additional information about ABS it may provide. I connected it to a Renix Jeep, OBD1 ZJ and OBD2 XJ and was able to get live readout with a few tests and diagnostics plus a fairly thorough troubleshooter. When I have time I'll see if I'm able to connect to a 95 ZJ's ABS.

There is an online Mitchell manual however I've never used one for ABS. It does have some good electrical schematics. The only problem is it can only be accessed through a subscription.

https://eautorepair.net/Marketing/Default.asp

95 ZJ electrical schematic ABS





Last edited by Muddz; 03-03-2019 at 07:04 PM.
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