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Hydraulic control unit ?

reflex26

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August 29, 2009
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City, State
Deposit,NY
Year, Model & Trim Level
1996 xlt
I have a problem with my brakes on my 1996 explorer 4x4.My left rear caliper leaked all the brake fluid out.I replaced both rear calipers and rotors,and pads as well the front rotors and pads and bled the rear brakes.The brakes are fine until I stop completely.They work great slowing down but when I come to a complete stop I hear a motor or pump run for 4 or 5 seconds and I can feel a pulsation in the pedal.I heard that if air gets into the brake line the hcu has to be bled with a Ford special tool. Can anyone tell me if this problem is air in the hcu or something else and if I would have to have it bled at a dealer or a repair shop?Thanks for any advise.
 



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I believe air only gets into the HCU if it replaced or maybe with master cylinder work such as replacement. Air in the HCU is demonstrated as a spongy brake pedal.

Bleeding the HCU is a big deal. You need the uber special adapter T90P-50-ALA and probably the pigtail OR I *think* you can do it with a Snap On
MT2500 with the correct ABS TSI cartridge. The expensive bi-directional scan tools should be able to do it.

Keep the reservoir topped up and bleed the brakes in the normal way. Air downstream of the HCU is bled off and you get a solid system IF no air has been introduced into the ABS pump suction (the 2 lines coming off the master to the top of the HCU).
Bleed sequence is RR, LR, RF, LF. (Haynes Manual) Bleed sequence is shown different in other places for some reason.

Disclaimer: I'm not a tech, and I hope this is right.
 






Thanks for the reply Fixt.What throes me off is I have great brake pressure,not at all spongy but when I come to a complete stop the abs pump [ I think] starts for 4 or 5 seconds and the pedal pulsates just as the truck completely stops and the brakes work fine if I don't completely stop. I read on this site that the HCU has to be bled with a special tool.Does this mean only a dealer can do it?Thanks again for the help.This has me puzzled because the brakes aren't spongy at all.
 






Well, I don't completely understand everything I know about the ABS system. I've done a lot of reading and I'm trying to piece it together. I should say that up front.

For the HCU bleeds, it's a specialized routine that cycles the HCU isolation and dump valves in a way that returns fluid to the reservoir, in combination with running the pump.

I checked into the tools, and the Snap On MT2500 would be the way to go for the home DIYer, depending on the year of your vehicles. Snap On has dropped support for the tool, you still have to find the correct cartridge, but they are all over ebay for not unreasonable prices. With the correct plugin, it can function as a Ford New Generation Star performing bidirectional routines. I've had a hard time finding the true capabilities of the tool. New tools are in the $4k range and up, more or less.
The alternative is the T90P-50-ALA, highly specialized and good for pretty much one thing only.
A good shop should be able to do it, but I would ask what they have first. If they don't want to say, keep looking.

Since I don't even like new vehicles past 2000, I've been eyeballing the Snap On instrument as having more utility for my purposes. I have an Innova 3160, good tool but limited capabilites. It does read ABS and can erase stored codes, and most enhanced codes. You have to look up ABS codes. It's good to know fault codes before going to a shop as some charge just for hooking up.

For the starting of your pump, my guess is that the system is seeing different pressures and is trying to maintain line pressure much as if wheel speed actuated the ABS. The only switch input I can see is the brake switch on the pedal input to 4WABS module at pin 35, and feeds the PCM. I have no idea what happens inside the magic boxes. A real tech would be of more help here. Again, guessing on my part.

I would do the conventional brake bleed paying attention to maintaining reservoir level and see what that did, since you don't seem to have air in the HCU. With all the replacements you did, it might take a lot of bleeding.

Does your ABS light come on when the pump starts, or any time other than self test?
 






Thanks for the reply Fixt.What throes me off is I have great brake pressure,not at all spongy but when I come to a complete stop the abs pump [ I think] starts for 4 or 5 seconds and the pedal pulsates just as the truck completely stops and the brakes work fine if I don't completely stop. I read on this site that the HCU has to be bled with a special tool.Does this mean only a dealer can do it?Thanks again for the help.This has me puzzled because the brakes aren't spongy at all.

you have a bad speed sensor most likely causing abs to go on.
 






Thanks for the help.No my ABS light does not come on.I'll check into the speed sensor.Is there any way to test it?
 






Forgot to say I'll bleed the system like FIXT recommended.Thanks
 






Thanks for the help.No my ABS light does not come on.I'll check into the speed sensor.Is there any way to test it?

just cause you dont have an abs light does not mean the sensor does not mess up at slow speeds,
 






Thanks WASKLY.It makes sense.I'll check that too.I've fixed so much on this truck since I got it that I'd like to get this fixed too.Love the truck and this site.
 






Thanks for the help.No my ABS light does not come on.I'll check into the speed sensor.Is there any way to test it?

You can check resistance. Values provided by Swetrid from a service manual.

FR Wheel Speed Sensor Resistance (Ranger) Off 23 + 36 k Ohms 1.0-1.4 k Ohms
FL Wheel Speed Sensor Resistance (Ranger) Off 24 + 25 k Ohms 1.0-1.4 k Ohms
Explorer 4x4 Only
FR Wheel Speed Sensor Resistance Off 23 + 36 k Ohms 0.270-0.330
FL Wheel Speed Sensor Resistance Off 24 + 25 k Ohms 0.270-0.330
RA Speed Sensor Resistance Off 21 + 22 k Ohms 0.8-1.4 k Ohms
Sensor Output

(note: I don't know what the + values mean)

I have a bad LF sensor with code C1155. Resistance check shows open circuit. I unplugged the harness at the front. The hard code was a big help in my case.
 






Thanks for all the help guys.My plan is to bleed all the brakes just make sure air isn't in the system and check all the speed sensors. You've steered me in the right direction.
 


















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