In general a problem with the ABS module can often be the cause of an ABS warning light that stays on. (Other possible causes include wiring damage, ABS pump fault (less often) and ABS wheel sensor failure.) High power 12V pins can develop solder joint cracks when they are soldered directly to circuit boards. This also happens with high power relays such as fuel pump relays. Your dealer can do a scan and give you an idea of where the problem lies, faulty sensors are relatively cheap and easy to replace but the dealer will not fix faulty modules - the replacement charge may well be more than the value of your vehicle ..
Grand Cherokee ZJ 1998 - ABS module repair Edit
(originally posted as a comment on YouTube )
This repair took me around two hours and it saved my 1998 Grand ZJ (90k miles/145k km) from certain inspection/CT/MOT failure and premature 'enterrement'. I used a 4mm hex socket (Sunday..), access was awkward but the bolts were not tight. Scratched forearms and tier 1 swearing only. I didn't loosen the pump housing bracket - didn't want to risk a fractured brake line .. Three bolts are accessible - but the inner corner isn't so simple, although it is much easier if you shine a light down past the ABS pump bracket - helps you line up a (small) socket wrench for little 1/8 turns right in the corner!
So then I just sawed off the corner above the 12V plug with a multi-tool (carefully) rather than the whole lid and then epoxy/silicone to put back. Removed the transparent protective silicone layer - one of the 12V pin solder joints was cracked (zoom in and you will see) - I am learning this happens often on cars over 10/15 years when high current/high use pins are soldered direct to circuit boards - eg. a porsche 911/993 fuel relay* Please use a minimum 100W soldering iron so you can resolder quickly - a weaker iron will require you to pause and risk heating everything up further away and damaging the plug pins and their plastic shielding - don't get everything hot - that's not the idea of soldering.
ABS wheel sensors EditI checked the 4 sensors first - they were all reading 1000-1100 ohms. I even rolled at 15 mph and checked the AC (NOT DC) voltage output (0.3 - 0.5V). Easier to take the feeds via the multi connector than go searching for the sensor cables .. so I worked out which sensor feeds the ABS module contacts.
The 4 sensors multi plug pinouts are [RL FR .....] / [RR FL .....] / [RL RR FR FL ...] reading left to right / top to bottom looking at the connecter underside (3 rows of 7 small contacts - yes they are all using the leftmost pins - ignore the big 4 pins to the right) This way you can also check continuity all the way to the wheels :thumbs: - not going to scrap my Jeep for a cracked solder joint :) . Good luck.
FL = Front Left , FR = Front Right , RL = Rear Left , RR = Rear Right
*It's a kind of fatigue I guess but I never understand why hard-working physically and power stressed components (eg. 12V plug socket pins or other power line terminals) are DIRECTLY physically attached to a delicate circuit board which relies ultimately on a thin copper film for electrical integrity. Surely they should be solidly terminated and then bridged to the board with flexible straps?
*UPDATE: The problem returned intermittently after this repair.
- I opened the ABS module across the entire end to also access the pins of the multi-connector, since this supplies the 12V feed to the module. Many of those, including the smaller wheel sensor connections were also fatigued/cracked. I re-soldered them all.
- Still the problem remained and finally I isolated it to bad contacts at the multi-connector itself - just removing and reconnecting the connector was making the ABS come on or stay off. In fact with the grey shroud removed the light stayed off, then came on when I replaced it. I carefully bent in a little the contacts of all the gold connectors to make them a tighter fit on the pins in the module (you have to unclip the pale grey shroud). For now the problem is fixed!