98 B4000 with you guessed it a bad speedo :P

[falcon1620]

Hey Guys,
Fighting with a 98 Mazda B4000 with no speedo. The Mazda B4000 is 4 wheel drive, RABS.
-Speedo would drop to 0 and bounce around while hitting a bump / with everyday driving. Car would surge a little bit.
-Speedo eventually cut out entirely, working sometimes with bump / breaking with standard non-crazy driving. Car would surge somewhat, noticeable but not super bad.
-Speedo is now completely dead (no life) No more surging though. ABS light was on, after “dealer help” its now “off” maybe the ABS gave up the ghost completely. Just an ODBII code for speed sensor seems to be broken

ABS light on dash came on, check engine came on (code in ODB2 for no speed data essentially, check speed sensor). Car has no ABS relay / fuses, according to the manual populated, has a tiny fuse for RABS module, and that’s good, so power from that side feeding to it, its tight and making good connection from the main box.
I took it into a shop, Shop A, they looked at it, checked fuses, those were OK, checked the rear diff speed sensor tested OK. Said “electrical nightmare” and returned the truck to me. Said they felt confident speed sensor was OK, ohmed it out, checked on some of the frame rail wiring, and said they could not help, recommended taking it to the dealer and said they didn’t have tools to talk with the RABS module.

Dealer (Shop B) kept it 5 weeks, here is what they said. Checked the speed sensor again, it tested OK, swapped it anyway, and said both worked. Checked the connectors going to the speed sensor and wiring, up to the cab with a meter, said no community issues found, and felt confident there was no break in the wires going there. They said that they checked it to the RABS module and said that they thought the RABS module was bad. Said they checked parts, and could not located one, and returned it. Called back next day and said they found a junkyard RABS module. Took it in and they replaced it, and then said that was a bad unit, did not fix the issue and returned it keeping it for a total of 5 ½ weeks, when I had it for a day to run errands.

SO now I have it back, we’ve paid for a bunch of diag info on it and here is what I got. I asked the mechanic about the RABS module, he said, it’s by the fender well (or foot well) I forget which one. Apparently, he was annoyed, because he would not point out WHERE it was, just like even come over back here and look at this and its right here. I asked, do you have a part no for me? Nope, can you obtain the part no for me? NOPE? Uh ok then thanks… I appreciate it… He asked if I could track it down the RABS module myself… I said a part no, some details about where it’s at… and maybe or at the very least a picture of it to compare parts would be super helpful for me. But nope. Service advisor tried to look it up, gave me a part no that pointed to a Ford PDF from … … with just a list of part no’s no diagrams. Part references this part no or this part no… kind of a thing… “super helpful documentation…” LOL I Googled a few of those in the laundry list of parts and came up with no info. Tossed the sticky note with that part no, no help at all. He apologized for the mechanics attitude and tried to help. I’m thinking based of my research that its one of two things. I found an ABS module it’s a black plastic box, that you can find on Ebay and some part stores as a re-manufactured unit, but unsure if that’s the one they are referring to. Nothing turns up for RABS module, but the little 2-line solenoid by the driver side fender under the hood as the replacement RABS actuator. I don’t see how that could handle speed data, since there are like 3 wires going to that, and one single line in and out. I unplugged that module, and plugged it in, and it did not affect speedo readout when I drive it while the problem was intermittent, meaning that it didn’t have an effect on the speedo when it kind of worked about the same regardless if it was plugged in or not, so that suggests that maybe there is a computer controller in here somewhere to look at. Only thing it did was prevent the RABS from kicking on (as expected) and throw the ABS code on the dash, when the speedo dropped out. Now it has no ABS light, and a ODBII code for speed sensor only.

I called Ford and Mazda to see if I could get some factory tech info to help (not worth it), Ford said, oh no it’s a Mazda so call them (yea a Mazda built by FORD LOL come on guys). Someone did answer at Mazda and seamed to find the RABS module that I was talking about, based on my description but said, we don’t have those available, and gave me a number to call “classic/vintage auto parts” or something to this extent. Of course, they couldn’t give me any details with out a part no, so I called Mazda back, and they didn’t return my call LOL. OK. Guess they were closed? In any case, I should have asked for the part no, or something, like WHERE THE HECK IS IT??? Its maybe called something else. Seems to be like a control module of some sort, but maybe not the general electronic module? I don’t really have a good grasp about what module where I guess in the next “chain of things”. I’d like to prove that input and see if its getting speed data, could scope it I do have a scope for other expensive hobbies… so that might be helpful. I could also check continuity and shorts up to that point, and probe the module for power too, but not sure RABS module seems to not really be the actual name of the module, kind of a thing.

SO… question? Reading here some people fixed the speedo by replacing the GEM module? Or did I just make that up misread it on a thread? I would assume they said the RABS module was the culprit, but where is it? I pulled the glove box and poked around a bit, but could not find any modules, just the HVAC blend door actuator, some vacuum lines, the GEM module from behind the radio, and a few other dashboard bits, no harness or connectors module like. I’ll have to dig again. SO driver side / passenger side? RABS is presumably different than ABS so maybe they share / have separate modules? I mean I guess I could call around to yet another shop and ask them for help?

Seams like its just unfixable. Been driving it around with a cell phone for the speedo (an those adds for those apps are annoying) haha.

 

[falcon1620]

Alright, it got fixed, it was a short in the wiring harness near the fuel tank area. So shes good to go. RABS module was still good, just getting no signal. Thankfully a shop here had some better expertise than the dealer did She's all fixed up.

 

[RonD]

Welcome to TRS

Good stuff, glad you got it working


I think your original post scared most people off, a LONG READ, lol

Nothing wrong with that, good to get the details, but...................

I would just post the highlights next time
Year and engine, manual or auto, 2wd or 4x4
The problem, "speedo not working", and Odometer if its also not working, they share a signal
Then "been to a few mechanics and problem not solved"
And any new parts installed


Yes, in 1998-2000 Rangers the GEM module was used for speed pulse conversion
In those years the rear axle ABS sensor(VSS) was used by ABS module AND the GEM
The rear axle VSS sends out 25,000PPM(pulse per mile), the Ford speedometers/odometers need 8,000PPM

So the VSS signal was sent to GEM module and it converted the 25,000 to 8,000 and sent that corrected signal to speedo, computer and cruise, if so equipped

The fact the speedo dropped out and ABS light also came on would mean VSS issue or wiring issue from VSS to ABS and GEM
"Hitting a bump" and seeing a drop would lean it more to wiring issue

But to answer the question yes, GEM module in those years can effect speedo


I too post more info than needed, lol

 

[Dirtman]

Thanks for posting the fix.