Problem with semiconductor-driven, automotive starter control circuit - Page 1
Forum section: electronics / beginners

author: Agent 0
author_rank: Newbie
timestamp: September 28, 2025, 05:45:26 pm
content: This heavy duty vehicle I'm servicing has a problem with its starter control circuit. The problem (intermittent at first) propagates after two to three months of field work. Triggering ignition leads to "click no crank" condition: you can hear plunger inside of starter solenoid move, but it doesn't travel far enough to bridge "BAT" and "MTR" terminals. The OEM resolved the problem by replacing the original starter motor with an aftermarket alternative. Electrically, the only major difference between the two is an additional starter relay (aka integrated magnetic switch) mounted on top of the new replacement starter solenoid. This fact lead me to believe the cause of the problem is BTS555 power switch (used as the solenoid driver) degrading due to EMF kickback from the original starter solenoid (as there appears to be no dedicated flyback diode/snubber in the starter control circuit). I know that voltage transients degrade both Power MOSFETs and BJTs by increasing threshold voltage or base drive current requirements, as evident by these two research papers from NASA Technical Reports Server:1.https://ntrs.nasa.gov/api/citations/19660030238/downloads/19660030238.pdf2.https://ntrs.nasa.gov/api/citations/20110014343/downloads/20110014343.pdfCurrent consumption of pull-in and hold-in coils of original starter solenoid is 49.3 (±3.3A) and 6.8A (max) respectively, both at 20V, as was measured by the starter manufacturer's customer care staff at my request. This seems to be much lower than BTS555 current carrying capability, so I think an overload scenario is unlikely. BTS555 data sheet:https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/1086/BTS555.pdfAs electronics isn't exactly my field of expertise, my question is: are principles behind the IRF520NPbF Power MOSFET used in one of the abovementioned research papers and the BTS555 power switch similar enough to warranty applicability of my back EMF theory?IRF520NPbF Power MOSFET data sheet:https://www.infineon.com/assets/row/public/documents/24/49/infineon-irf520n-datasheet-en.pdf

author: floobydust
author_rank: Super Contributor
timestamp: September 28, 2025, 07:09:27 pm
content: Infineon BTS555 Profet is not protected against the-vevoltage spike generated by an inductive load. The Profet specs look all marvelous - except for that.They use the MOSFET's avalanche to tolerate that, but it may not be enough. Datasheet at 40A 12V system is 3mH max. and 2mH at 50A, single switch off.It could easily degrade the MOSFET as you propose but you'd have to see what the BTS555 support circuit looks like. It might be poorly done. Need a diode+TVS or at the very least a resistor across the solenoid coil to attenuate the back EMF.I can confirm this next week and if the part is tough enough for 24V systems. A common trap is Infineon parts for 12V systems but look like they would be OK on 24V use... and they don't last.Chevrolet had problems with early Delco Remy starters- the solenoid's plunger would not move.It was terrible- when they were hot especially, nothing happens and you had to wait until the starter (engine) cooled off.I measured 40-50A draw. Can't remember the exact fix. The wire gauge was too thin #14AWG, they upgraded to #10AWG (1972). Then they added an assist spring. You can confirm the voltage drop total - on the Neutral safety switch and Start switch and wire etc. (although it should be low current for OP though).Then they have this dual-coil solenoid with a pull-in and hold coil. It can be a mechanical or electrical problem in the first place.

author: Agent 0
author_rank: Newbie
timestamp: September 28, 2025, 09:12:59 pm
content: Quote from: floobydust on September 28, 2025, 07:09:27 pmThe "problematic" starter is CAT 435-1240 but, considering the CAT customer support doesn't have any technical documents for it (and that's why they had to measure the solenoid current consumption), I think it's likely that CAT 435-1240 is a Delco Remy MT50 rebrand manufactured in Mexico. Reason being, I contacted Delco Remy customer support and they relayed my request for the MT50 electrical data to Delco Remy Mexico. I was told it might take awhile to get a response.The BTS555 is located inside of Body Control Module (BCM) of the vehicle and I don't have access to the PCB schematics, as this is considered to be outside my scope of work. I managed to get info on the BTS555 from one of the company's electrical engineers though, so its a fact it acts as a driver for the starter solenoid. However, I do not know if the BCM contains a zener/TVS diode or a snubber circuit. What I'm sure of, is that outside of the BCM, no such protection provided, as I have physically inspected that part of the system and have access to the related wiring diagrams.It's also worth mentioning that some of the vehicle operators tried loosening the starter's pinion clearance adjustment nut, which helped to relieve the problem, albeit temporarily. I think loosening the adjustment nut reduces plunger spring tension, which allows the weakened magnetic field of the power-starved solenoid to push plunger just far enough to close the motor power supply circuit. Of course, as a side effect, loosening the nut may cause the pinion gear of the starter to smash against the pinion housing when fully extended.Additionally, I tried searching for someone on the Internet experiencing the same or similar CAT 435-1240 starter issues, but found nothing (which reinforced my belief that the problem is isolated to the solenoid control circuit of this particular vehicle). This starter model and the Caterpillar engine combo is used on a variety of specialized machinery. Surely, if anyone else had problems with it, I would find some mentions of that on the Internet, no?As for the wiring gauge — good call, I will make sure to check on that once I have physical access. Because, weirdly enough, I found no mention of it in the supporting vehicle documentation.Lastly, I personally removed three starters exhibiting the click no crank behaviour and pulled out the pinion gear/rotated it by hand with no noticeable difficulty. I'm an industrial maintenance electrician by trade (not a mechanic), so you may take this with a grain of salt.P.S.: I retrofitted an additional starter relay on top of one of the CAT 435-1240 solenoid. The issue here is, the vehicle with the modified starter only saw two weeks of use. Needless to say, this isn't enough to draw any conclusions.

author: mikerj
author_rank: Super Contributor
timestamp: September 29, 2025, 01:14:58 pm
content: Are you able to measure the voltage on the solenoid terminal with a "click no crank" condition and the starter activated?

author: Agent 0
author_rank: Newbie
timestamp: September 30, 2025, 09:01:37 am
content: Quote from: mikerj on September 29, 2025, 01:14:58 pmNo, not at the moment. It may take quite some time before I get another service call, or we'll get one of the vehicles sent to our company.

author: floobydust
author_rank: Super Contributor
timestamp: September 30, 2025, 05:02:43 pm
content: I checked and manufacturers use an intermediate starter relay so the BCM does not see such a high current load, or get the high back-EMF spike. I think this is your best solution.It is a common mistake for a vehicle manufacturer and ECU maker to not put the two together properly. The BCM connector and wiring and mosfet current ratings are what get missed. To be cheap, let's leave out that extra relay.You can bench test the starters, see if the "click no crank" is just due to low voltage appearing at the solenoid or it's mechanical issue.I also had that happen due to the wrong grease/lube on the plunger. At low temperatures it became too thick.Some starter manufacturers want no lube on the plunger, others it was graphite powder. Check again what is needed there.

author: Agent 0
author_rank: Newbie
timestamp: September 30, 2025, 05:32:15 pm
content: Quote from: floobydust on September 28, 2025, 07:09:27 pmToday I searched through support documentation once again and found that a 35mm² cables for the starter MTR & GRND connections are used, along with 10mm² cables for the solenoid positive (signal) wire and GRND.No noticeable correlation with increase in ambient temperature was reported, although seeing how copper resistance raises along with temperature, I can see why the problem you described was exacerbated when the car was running.

author: Agent 0
author_rank: Newbie
timestamp: September 30, 2025, 07:14:37 pm
content: Quote from: floobydust on September 30, 2025, 05:02:43 pm1. That's what I meant by saying I retrofitted an additional starter relay to one of the CAT 435-1240 starters. These IMS are usually mounted on top of the main starter solenoid, so that's what I did. It was a made in Korea solenoid type starter relay. And I think that adding a snubber or a flyback diode in parallel to the coil contacts of the additional starter relay is still the best practice. I did not add any to my test subject only because I was researching what kind of snubber will achieve the lowest contact opening lag (i.e. which snubber dissipates back EMF the fastest) at the time.2. Another reason why I think the issue lies within the starter control circuit: I tried powering three of the 'problematic' starters using a cheap made in China battery charger as a power supply. The charger is rated at 40A (24V) output, so the idea was to use it as a power limiter for safety (heavy duty starters develop quite a torque in order to spin engine flywheel at sufficient RPM). Now, the interesting part: despite being power starved and spinning at about 30% of the rated RPM, the solenoid and the starter both worked as expected. However, after putting these starters back on the vehicle, the click no crank behaviour observed earlier continued. Of course, such setup cannot be fully representative, as the starters worked in no load condition, but I think that was somewhat compensated by the fact they weren't receiving their rated power input. Therefore, I assumed, that if there indeed were mechanical reasons for the clink no crank condition to occur, the setup would have replicated these also.3. I disassembled multiple CAT 435-1240 solenoids and I don't recall seeing any kind of grease there. I was focused on cleaning internal contact points mainly, so there's a small probability I haven't noticed the grease, because I wasn't looking for it.4. I will check for voltage drop as soon, as I'll get an opportunity to do so.

author: amyk
author_rank: Super Contributor
timestamp: October 01, 2025, 06:14:57 am
content: Quote from: Agent 0 on September 30, 2025, 07:14:37 pmThat sounds like a weak battery or bad wiring on the vehicle is the fault, not the starter itself. A thermal camera is very useful for locating the latter fault, as a lot of power will be dissipated at a high resistance and things will get hot very quickly.