Problem -- Correcting the speedo for large tires with 4W-ABS.

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Would someone post a source or infor on abbott Box? Thanks

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Denny

 

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well i did switch from 245's to 265's on my 98 gasser and they used the drb tool to change it , i also only had rear wheel abs , the 285 is now an available size , but that doesn't help those with larger tires ... oh well.

 

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Originally posted by Glen Pratt:
DixieDawg, Divide 65mph - your actual speed by 54 mph - your speedo, 54/65=1. 203703703. Then multiply this number by your odometer/trip meter reading and you will get your correct mileage

With the inaccuracy I've seen in the speedo, I would use the highway mile marker method. Set the correction as close as you can, then see how many miles accrue over 100 miles of yardsticks. As near as I can tell, my odo is *quite* correct, while the speedo is rather non-linear.

 

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I have not been to the site in a while... My 1999 Dodge 2500 4X4 with 4 wheel ABS does have the Abbott box attached with no problems. when I first hooked it up someone on this site told me the wrong wire to hook it up and it would not work. It is now hooked up to the red wire W/blue stripe underneath the truck right behind the front tire. Since the hookup I have never had my ABS light come on except when I first start it of course. When I took the truck into the dealer 1 time to get a reflash on the ECM the Abbott quit working. Abbott sent me another 1 and I have not had a problem yet.

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1999 QC 5 Speed 4X4 3. 54LS White/Driftwood Agate with Almost every option, 285 BFG's 16X8 custom rims W/ Aboott speedometer fix, 3500 tailgate lights, Amsoil Air Filter, Nerf Bars W/ much more instore.

 

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Rob,
Blair at Parker Motors says they can change the speedo readings, i am sending a guy up there next week to have one done. That is obviously too far for you to drive but you might want to contact them to see how they do it. I have got a phone # 1-877-511-5599.
Blair is also a TDR member so you can probably look up his e mail address. I know he is on holidays this week.

 

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backwudz, I had my Abbott ERA installed when I had the "low power" reflash and did not experience any adverse effects. #ad

 

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I took it in for the reflash and when I went to pick it up they had my dash apart. I asked them what they were doing and the tech said my speedo was not working. He said that he has checked everything out and everything was ok but it still was not working. He said he was fixing to replace the computer and I told him about the ERA. He didnt have a clue as to what I was talking about so I got under the truck pulled the wires off and hooked up the original wires and it worked (just 10 mph off) When I put the new one on I hooked it up with a quick plug so now if it goes to DC for anything I disconnect it. I figured maybee there computer equipment seeks out aftermarket products and fry's it. I even disconnected it when I had the trailer hitch fixed.

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1999 QC 5 Speed 4X4 3. 54LS White/Driftwood Agate with Almost every option, 285 BFG's 16X8 custom rims W/ Aboott speedometer fix, 3500 tailgate lights, Amsoil Air Filter, Nerf Bars W/ much more instore.

 

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Originally posted by DixieDawg:
If anyone finds a solution to this dilemma, make sure to post it conspicuously. ... Why have I found that my speedo reads 54mph when I'm doing 65?

Also, I haven't yet figured out the formula for correcting the odometer reading. Any ideas?

I have some news about the Abbott box at the end of this missive. First:

To correct both the odo and the speedo, divide your new tire's *loaded* radius by the old tire's *loaded* radius. That is, measure from the (vertical) center of the axle to the ground. That ratio will correct the speedo. It will correct the odo provided you subtract the mileage at which you installed the different tires from the current mileage, multiply by the ratio, and add back the previously subtracted mileage.

If you know you are traveling 65 when the speedo reads 54, then you seem to have a tire that is 20% larger than the OEM tire. If this seems bogus, then perhaps your speedo is not accurate. Since the speedo and the odo are 'driven' from the same signal from the CAB, there should be no discrepancy.

Since you do seem to have a discrepancy, try taking a 100+ mile drive on an interstate. At some mile marker, note both the marker and your odo reading. Then, 25, 50, and/or 100 mile markers down the road, note both readings again. Divide your mile marker count by your odo reading and you should have a fairly accurate ratio. If this, too, results in a 20% difference, I might suspect your CAB is faulty.
I've noticed that my speedo is none too accurate; but my odo is dead on.

Now to the Abbott box. Over the past couple days, I bench tested a new Abbott ERA box. I discovered that it will work with a sine wave input from around 5Hz to around 4. 5KHz (this corresponds to about 0. 25MPH to around 200MPH) PROVIDED the voltage from the rear wheel sensor (RWS) remains above about 0. 9V. Below that, it is below the signal the ERA uses to compare, thus the ERA will report zero velocity.

Next, I hooked a meter to the output of the RWS and drove around, measuring the voltage at various speeds. The voltage would max out at around 2V at around 35 MPH or so. The voltage would drop to 0. 9V at around 5MPH, and to around 0. 4V at around 3MPH. Thus, when you are braking and cross below 5 MPH or so, the ERA box, seeing 'no' signal from the RWS, sends no corrected signal to the CAB. The CAB, having previously seen a nice signal from the ERA, is suddenly seeing zero Hz, which it interprets as the rear wheels locking up; thus it activates its anti-lock function. The PCM, seeing zero velocity from the CAB, tells the speedo to indicate 0 MPH.

I also measured the signal voltage from the CAB to the PCM; it's about 2. 5V.

The ERA puts out about 1. 7V.

In layman's terms, the signal from the RWS drops too low for the ERA to read, and the ERA then sends zero speed to the CAB. On *most* vehicles, this point is under the CAB's 'ABS turn-off' point, so the brakes seem normal. On *some* vehicles, the velocity is still higher than the CAB's shut-off point, thus the CAB activates.

I'm working on two possible solutions. First, I am attempting to boost the ERA's output to 2. 5V (just a resistor change). This may be enough to allow the ERA to correct the CAB's speed signal to the PCM, thus eliminating all messing with the CAB and anti-lock brakes. I shall see when I solder in the new resistor.

[Technically, the CAB sends a square wave to the PCM. The CAB-to-PCM signal is about 1/4 to 1/8 the frequency of the RWS signal. On the bench, the ERA was happy with a 2. 5V square wave input, from about 2HZ to 8KHz (about 1 MPH to over 1000MPH, assuming stock tires). I've got the in-circuit voltage to about 2V, which appears less than needed to drive the PCM's input. ]

[More technical: the ERA uses an RS-232 (computer serial) chip to drive the output. RS-232 uses +/- 10V for its signalling, so I believe I can drive the voltage high enough to satisfy the PCM. ]

Another solution which would work for RWAL vehicles, and possibly for AWAL trucks would be to
replace an IC in the ERA with a slightly different one. This would allow me to amplify the input signal high enough to register to down below 2MPH.
But this solution would not necessarily work on 4WAL systems.

[Technical: The ERA circuitry clamps the input to about 0. 6V and compares this signal to a set 0. 4V
signal. When the input is higher than 0. 4V, it outputs 'high'; otherwise it outputs 'low'. The ERA uses an LM393, using only one of the comparators. This could be 'replaced' with an LM392, which has a comparator and an op-amp. The input could be re-routed to the op-amp, which would boost it 3x, then route its output to the comparator, which would then see a sufficiently strong signal to properly correct. ]

In a sentence, the sensor Dodge chose seems to put out a signal that's too low for the ERA to read at speeds under 5MPH or so.

I installed the new ERA box Dan gave me to test, and my problem with the anti-lock activating seems to have disappeared. But I shan't quit just because *I* don't have a problem any more. I'll post again when I have more news.

Fest3er

[This message has been edited by fest3er (edited 08-09-2000). ]

 

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Okay Fest3er,
While I appreciate your input, and hope that you continue with your testing, I fail to see how this relates to the 4 wheel ABS dilemma. Did I miss something?

 

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Originally posted by DixieDawg:
Okay Fest3er,
While I appreciate your input, and hope that you continue with your testing, I fail to see how this relates to the 4 wheel ABS dilemma. Did I miss something?

Perhaps you did. More likely, I could be less obtuse... .

If I can modify the ERA to correct the signal between the CAB and the PCM, the inputs to the CAB will be untouched. The CAB will have unmodified signals from the front wheels and the rear axle, which allow it to properly determine wheel speeds and which ones, if any, are locking up when the brakes are applied. Since all three sensor signals are unmodified, ABS operation will not be affected.

The CAB autonomously handles ABS functionality. In addition, it takes one of the wheel signals (should be from the rear wheel sensor), computes an MPH figure, and sends a square wave to the PCM that indicates vehicle speed.

The PCM then interprets the signal, determines the MPH, and sends messages to the instrument cluster telling it what speed to display. It also computes (and permanently stores) miles driven and tells the cluster what to display on the odo. The trip meter is only temporary storage. If you remove power from the PCM, the trip meter appears 'reset' to zero.

I can see the ERA working on RWAL-only systems. But I don't know why the ERA works at all work 4WAL systems; it shouldn't, because the CAB is seeing the front wheels moving at a speed that differs from the rear wheels.

...

Oh. 4WAL works because the CAB only activates the ABS when the difference between any two signals is more than 20-30%. This means that, under normal braking, a user of a 4WAL truck will never notice anything unusual. Under panic braking, the driver of a 4WAL truck will notice that the rear ABS will be activated sooner if he has the ERA attached to the rear sensor. *If* he has tires that are more than 20-30% bigger (or smaller) than the OEM tires, the driver would then notice the ABS activating under normal braking.

Since *very* few people use tires that are more than 20% larger, 4WAL systems will work fine under normal braking, but will activate sooner than expected under hard braking.

FWIW, a tire 20% larger than OEM would be about 36" tall. A tire 30% larger would be about 39" in diameter. Tall tires indeed. Much taller than are usually used with Dodge pickups.

This all assumes that the ERA is getting a strong enough signal from the sensor to be able to read it. If the signal is weak (as happens when the vehicle is moving 0-5 MPH), the rear ABS could *still* activate on 4WAL trucks when coming to a stop, similar to the problem I had with my RWAL truck.

If I can get the ERA to correct the signal between the CAB and the PCM, ABS operation will be completely unaffected. (Well, almost; more below). The CAB will see all tires turning at the same speed. And the PCM, which does all the mileage storage and speedometer operation, will be getting the 'correct' vehicle speed and distance 'numbers.

As I said, ABS operation is *almost* unaffected. Bigger tires will cause the CAB to 'think' the vehicle is moving slower than it is. The CAB is designed to disengage ABS at 'under a few MPH', according to the service manual. I assume 'few' to mean '3-5'. 10% larger tires cause the truck to be moving at 3. 3-5. 5 MPH before the ABS cuts out. Thus, operation is *almost* unaffected. The difference is just about negligible.

Now, as I was reading the service manual, I came across a tolerance spec that may just explain why some trucks have a problem with the ERA box, whilst most don't. This is technical! The gap between the sensor and the 'tone wheel' is allowed to vary from 0. 005" to 0. 05". The farther away from the ring, the weaker the signal it generates.

Now combine this tolerance with electrical component tolerance. The ERA has diodes and such that, whilst marked with a 'nominal' value, actually have a value 'range. If these components make the ERA unable to read 'weaker' signals, and the truck has a sensor that is close to 0. 05" gap, it could result in the ERA reading '0' from the sensor, thus sending '0' to the CAB at a vehicle speed greater than the CAB's low-speed cut-off point, making the CAB think the wheels locked up and engaging the ABS.

I would estimate the probability of receiving a truck with a large gap under the sensor *and* receiving an ERA box less able to read weaker signals is very low. Probably less than the 1% Dan at Rickson has has found. Other factors may combine to raise the probability to 1%; one of these factors may well be a tone ring at the small end of tolerance.

Liken these tolerances to the 'matching' that Cummins use when building our engines. They match a 'short' connecting rod to a 'long' piston, so that the overall actual length is much closer to the desired length. Also liken these tolerances to the OEM matching Rickson use when mounting tires on 19. 5" wheels: matching the high spot of the non-round tire to the low spot of the non-round wheel; together, they produce a rather round combination that rolls smoothly.

Has this helped at all? Or have I further befuddled my fellow TDR folks? Please see the school nurse if you need aspirin.

Fest3er

 

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The speed can be adjusted with the DRB scan tool. The software stored into the OBD computer( the one on the firewall) has two different ways to do that.
With the tire sizes, this allows to enter only factory used tire sizes. The tallest tire is 245/75/16.
For any other size of tire, they use the revolution per mile method.
Before you have the speedo adjusted check it's readings.
Mine readed 10%high from the factory. I've checked it with my GPS.
When I switched to the 265/75/16 tires, it was 7%off.
After calculating the new revolution per mile rate, it's now precise within 2%.
Hope this helps.
Marco.



[This message has been edited by Marco (edited 08-10-2000). ]